NZ621959B2 - Quartet breeding - Google Patents

Abstract

Discloses a method for the production of a set of seeds which are genetically identical to the male gametes from which they arise, comprising: a) placing a limited number of paternal gametes that have the form of tetrads or dyads on the stigma of a flower to fertilise maternal egg cells to obtain a number of zygotes, wherein the number of paternal gametes is limited to be equal or lower than the number of egg cells contained in the female reproductive organ carrying the stigma; b) inducing the loss of maternal chromosomes from the zygotes to obtain a seed set containing a limited number of seeds in which the maternal chromosomes are absent. number of zygotes, wherein the number of paternal gametes is limited to be equal or lower than the number of egg cells contained in the female reproductive organ carrying the stigma; b) inducing the loss of maternal chromosomes from the zygotes to obtain a seed set containing a limited number of seeds in which the maternal chromosomes are absent.

Description

QUARTET BREEDING J:' ill.) Oh' THi'. NViiNT ON This invention relates to a method for the production 0: a set 0: seeds which are genetically identical to ,he male gametes jrom which they arise. This invention jurther relates to a set 0: seeds containing a limited number 0: seeds in which the maternal chromosomes are absent, which set is composed 0" pairs 0" genetically mentary seeds which when plants grown from the seeds are crossed result in essentially the same hybrid. The invention also relates to a method for providing a se, ol parent plants for the production 0: a plant 0: which the genetic tution is ially identical to the genetic constitution 0: its male grandparent.

BACKGROUND Plant breeding corresponds to the domestication o; plant species for the bene it o" humans to obtain jood, feed 2O and ‘iber o su t y and quantity. Plant breeding is a very old occupation o: mankind and only in the course o: the 2OUlcentury the practical dge received a scienti lC "oundation. Plant breeding was originally based on selecting and propagating those plants that were outper‘orming in local selection ‘ie'ds. With the rediscovery of the genetic laws and the development or statistical tools plant breeding became based on knowledge o: genetics and was technologically supported by methods such as d haploids (DH) — see e.g. Haploids in Crop ement eds; Palmer C, Keller W, and Kasha K (2005) in: 3iotechno'ogy in Agriculture and Forestry 56 -—Idds; _. Nagata T, Lorz H, and Widholm J. Springer—Verlag Qerlin leidelberg New York, S%N 3—500—22224—3 — and molecular markers — see e.g. De Vienne ed. (2003) Molecular Markers in Plant Genetics and Biotechnology. Science publishers nc. finfield, NH USA. S%N l—57808O. ?lant breeding delivers c concepts tailored to a ic environment which allows its exploitation in an economic manner. This objective 0' p'ant breeding is achieved through the e ”icient ation 0: genetic variation which exists within the germplasm of plant species. Such genetic concepts comprise combinations 0; genes which lead to a desirable phenOtype in a particular environment. This means that the plan, parts which are harvested are maximised in yield and quality, at the lowest possible cost ed to grow the plants and t the product. When plant breeding is applied at a commercial level, seed produCtion is also an important issue. Seed production aims at the multiplication of plants by means I] sexual reproduction, in which the genetic composition is ved.

In addition, the commercial seeds need to be 0: 2O su "icient quality to allow e "icient germination. The preservation o : c constitution through sexual reproduCtion is however a paradox, because sexual reproduction fundamentally exists to create 0 "spring with new combinations 0" alle'es. The genetic mechanisms that act during sexual reproduction have evolved to increase genetic variation, in order to enhance the chances O" al 0' a species in a changing environment. Meiotic recombination, independent chromosome assortment and the mating system are main contribut ing factors in this respect. Uniformity in 0 "spring thro Jgh sexual reproduction can ore only be achi v d wh n th par ntal plants are fully homozygous.

Combining game ,es of such plant will lead to the exact reproduction o: the genetic composition 0: the parent in each subsequent generation.

In many crops, the commercial seeds result from a cross 0: two homozygous al lines. This approach ensures that the F1 hybrid is heterozygous for several loci, which " can result in hybrid vigour and uni ormity. a breeder wishes to improve an existing F’ hybrid variety or an inbred variety, he will traditionally need to ma<e crosses and go through several rounds O" empirical selection to e this objective.

As the knowledge 0: gene function in relation to plant growth and development is still d, rs still largely depend upon phenotypic ion. As during inbreeding many genes are in a heterozygous state, especially during early generations, the allelic variants I) the genes responsible for the phenotypic value assigned to some 0: the individual plants can be lost easily. This is due to the fact that during sexual reproduction and inbreeding zygosity and specific gene interactions are 2O lost. Therefore, in plant breeding these mechanisms may act counterproductively, especially in those cases where cally heterozygous plants have been identified with high agronomic, horticultural or ornamental value. Sexual uction will result in the segregation of the desirable alleles.

Therefore there is a strong need for technology that e ”iciently allows the preservation o: the genetic constitution during sexual reproduction o:_ plants with a high agronomic, horticultural or ornamental value.

One possibility to perpetuate pl ants while preserving the genetic constitution is by vegetative propagation. This allows a complete preservation o: the genetic composition, as multiplication occurs exclusively 2012/069191 throagh mitosis. Plants have evolved natural mechanisms 0; vegeoaoive ation, which allow them to swiftly occupy ts. For example, vegetative propagation can occur throagh the formation 0' tubers, bulbs or rhizomes. An alternative is to use in vitro or in vivo culture technology to prodJce cuttings. A commercial disadvantage of vegetative propagation technology, when compared to propagation through seeds, is the fact tha it is labour-intensive and therefore costly. Furthermore, i is di" ”icult to store plants :or longer periods ol dime, which poses logistic problems, and the risks 0 in ec,ions o: the plant material with pathogens like viruses is considerably larger as compared to a situation in which plant material is propagated through seeds.

Alternatively, vegetative propagation may be achieved through ,he "ormation O" l seeds, which is genera"y referred to as apomixis. This phenomenon occurs "y in a number 0:: species, and it may be induced in ly propagating plant species by genetic engineering. 2O In theory, this can be achieved by making use 0: specific genes which lly induce the three di "eren, steps or apomixis, i.e. apomeiosis, par':henogenesis and autonomous endosperm development. In prac:ice, however, the genes responsible for ,he di "erent steps have not yet been fied, and their interaction may be quite complicated.

On the other hand, artificial engineering 0" lhe apomixis components may be quite feasible. For example, by modifying di "erent steps during meiosis it has been shown that meiosis can be essentially converted into mitosis. This so-called “MiMe approach” makes use 0: a ation 0; mutations which suppress double strand break formation -1), induce sister chromatid segregation during meiosis l (rec8) and skip the second meiotic cell division 2012/069191 (osdl). Combining this approach with parthenogenesis and autonomous endosperm formation may tely result in engineered apomixis (d’Trfurth et al: Turning meiosis into mitosis; PROS Qio'ogy 7009; WO/2010/07943 2). gh since long the potentia' 0' apomixis technology for plant breeding has been widely re cognised, proo o concept is still not available.

As yet another alternative, use can be made 0; reverse breeding technology (W003/017753). Reverse breeding is based on the suppression 0'' meiotic recombination through genetic engineering or al interference, and the subsequent product ion of doubled haploid plants (3 ls) derived from spore s ning unrecombined parental chromosomes. These DHS di" "er with t to their genetic composition solely as a consequence o: the independent parental chromosome assor :ment which occurs during meiosis.

Therefore, it is s U ”icient to make use 0 : one co-dominant, polymorphic marker per chromosome to determine which of the DHs or lines derived thereo: should be combined through crossing to reconstruCt the genetic composition of the original starting plant. As such, application of reverse breeding technology allows c preservation of any fertile selected plant h seeds, even i: its genetic composition is unknown.

A disadvantage of ,his Lechnology is the fact that complete suppressi OH O" meiOtic recombination results in the absence 0: ata. This may lead to inappropriate chromosome segregation during meiosis I, which can aneup' oidy O" the gametes and thus to reduced gamete viability and per: ormance. When no chiasmata are formed during meiosis I, every chromosome has an independent 50% chance to move to either one o: the poles . This means that the theoretical chance to make a spore with a full chromosome complement is (%)n, wherein n represents the haploid chromosome number. The frequency 0: balanced gametes therefore decreases with increasing haploid chromosome number. Although many crop s have a relatively low chromosome number (e.g. cucumber has 7 chromosomes per d genome; spinach has only 6) there are also economically important species with relatively high chromosome numbers. A good example is tomato, economically one of the most important vegetable crops, which has 12 chromosomes per haploid genome. This technical constraint significantly reduces the e ”iciency 0: reverse breeding technology.

As another alternative ch use can be made plants regenerated from unrediced spores. This technology has b n t rm d N ar R v rs 3r ding (WOZOO6/O94773). The unreduced spores are formed preferentially as a consequence ol ,he omission of the second meiotic division. This lly occurring phenomenon is known as Second Division Restitution (SDR), and it can occur in plants during sexual 2O reproduction concomitant'y with regular meiotic events.

Near Reverse Breeding technology exploits SDR events by regenerating plants from unrediced spores, produced through natural or ered SDR. Genes have been discovered which — when mutated — give rise to SDR, such as OSDl and TAMI. The resulting plants d SDR—O plants) are y homozygoas, and they can be subsequently used to produce traditional DHs. Molecu'ar markers which are polymorphic between the al and maternal s o: the starting plant can be used to identify ,hose SDR—O plants (and DHs derived thereof) that are largely complementary with respect to their genetic composition.

Crossing of these plants will result in the near— complete reconstruction o: the c make-up o: the 2012/069191 original starting plant. However, due to meiotic recombination during the formation " 0 ,he SDR—O events and during the formation 0' the DHs derived thereof, the mentarity will not be complete. The reconstructed hybrids will genetically di "er to some extent, both from each other and from the original starting hybrid plant.

However, this variation will be strongly reduced when compared to a ion in which the DHs are derived directly from a rnglar meiotic event. Moreover, these DHs are genetically fixed, which means there is no room :OI r selection.

The advantage 0: integrating an SDR event in this process is that the selec ,ion for genetic complementarity occurs in a two-step process. The firs a step is concentrated on the proximal s 0: the chromosomes, i.e. including the centromeres. The second step is directed towards the distal ends 0: the somes, i.e. those regions which were exchanged due to recombination. This d genetic fixation reduces the complexity and increases the chances I] 70 finding largely complementary genotypes, especially when molecular markers are avai "able for selection.

A further advanta ge O: this approach is the fact aha, SDR can occur natural:_y during sexual reproduction and tha, i a can be :ed as such without further need CO interfere with sexual reproduction processes. Methods ":0 further increase the normal ence of S DR events are {nown in the art, for example through stress treatment with N20 (as has been previously been bed in lily: Qarba- Gonzalez et al (2006), Euphytica l48: 303-309; and in tulip: Okazaki et al. (2005), Euphytica l43: 101-114).

It is an object o: the present invention to further increase the e ”iciency o: the above-described methods for preserving the genetic constitution o: a heterozygous plant with excellent agronomic, ultural or ornamental proper':ies.

The presen' invention takes advantage 0: the observation that speci:fic mutations can lead to a defect in the tion 0' microspores during pol' en formation. This results in the formation 0 " clusters 0 Jr pollen grains which remain physically at :ached together throughout their development. Although the individual pol' en grains in the clusters remain together in a tetrad a, maturity, they are fertile and can each per "orm sation and produce seeds upon pollination. The biological explanaoion "or this so- called quartet phenotype is the e to dissolve the pectin layer which is normally only pr s nt b tw n th microspores, in the early stages of pollen pment.

The invention thus relates to a method for ,he tion of a set 0: seeds which are cally identical to the male gametes from which they arise, comprising: a) placing a limited number 0: paternal gametes that have ,he "orm o ,etrads or dyads on the stigma o; a 70 flower to "ertilile ,ernal cells to obtain number I) ma egg a or zygotes; b) inducing the loss ol al chromosomes from the rygotes to obtain a seed set con':aining a limited number 0: seeds in which the maternal chromosomes are absent.

“Genetically identical” means that all chromosomes of on s d ar th sam as the chromosomes 0: the corresponding gamete and the combination 0: chromosomes within a seed is the same as in the corresponding gamete.

In the research leading to the present invention it was furthermore surprisingly found that combining methods available in the prior ar with ,he above specific mutations that lead to a de.fect in ,he separation 0' pores during pollen formation leads to a great increase in the e "iciency with which parent plants with an essentially complementary genetic constitution can be fied, which a‘ter ng will give rise to a heterozygous plant with essentially the same genetic constitution as its paternal grandfather. Such methods with which the invention can be combined are for example reverse breeding and near reverse breeding.

The quartet phenotype was for example bed in Copenhaver et a; (2000) Plant Physiology 124, 7-15.

Mutations in di "eren a non-homologous genes may result in similar quartet ypes (e.g. grtl, grt2 and grt3 in Arabidopsis thaliana). The QRT3 gene (in Arabidopsis: At4g20050)has been molecularly characterised, and it encodes a member 0: a divergent class 0" polyga'acturonases (Qhee et al. (2003) Plant Physiology l33: ll70—l'80). These mutations are use ul "or providing the tetrad jor use in the method according to the invention.

The individual po'len grains in the clusters 0; four pollen grains contain the products 0'' a single meiOtic cell division. When an individua:_ pollen quartet is used to ise a plant, this idea'ly 'eads to the ‘ormation O" 4 seeds, when the e ”iciency o "ertilisation is l00%. The jact Lhao ,he "our dua' po'len grains 0" a tetrad represeno ,he ts 0' a sing'e meiotic ce'l division has 75 interesting implications ‘or the technology which is subject 0: this invention.

At the start of meiosis, the initial process is the replication 0: the genomic DNA. Subsequently, during prophase homologous chromosomes align and synaps, to form the bivalents. During this stage double strand breaks (383s) are formed which are repaired ising the aligned non-sister chromatids. The chromosomal ction during repair leads to the ‘ormation 0' speci.fic soructures called double holiday junctions, which are resolved. This leads either to gene conversion or crossing over. The ng over (CO) events are ultimate" y visible in the form 0" chiasmata, which are structura' ly required for appropriate gue segregation during meiosis Wi th respect to this invention it should be noted that irrespeCtive O: the bution 0; the COs the products 0" meiosis are always fully complementary with e ach other, with respect to their allelic composition. During the second meiotic division the sis:er chromatids segregate to opposite poles, giving rise to the final meiotic products, i.e. four haploid ce' ls. Normally these four meiotic products become ed from each other during their further development, and they will mingle with the pollen grains derived from other meiotic events inside the same anther loca le. However, when the mother plant ts the quartet phenOtype, the four products or a single meiotic event will physically remain together.

The set 0 "our pollen grains in a tetrad can n variable degrees 0: genetic complementarity, which 70 is a function of the number 0: haploid somes. As the products 0" meiosis are “ul 'y mentary among each other, the four pollen grains held together in the tetrad contain at least 50% complementarity. The specific ou':come for each tetrad is determined by chance, and this chance follows a distribution which is given by the so-called ?ascal’s Triangle (Figure 1).

For example, cucumber has 7 haploid chromosomes.

The first meiotic di vision results in two fully complementary meioti c products. During the second meiotic division the somes can segregate randomly in each '1 the two products 0" meiosis ", which can result in 2D 27 l78 di" "erent products. In general, it can be stated that in case 0: n chromosome s, a total 0.: 7n di" "erent Letrads can be obtained from a single meiotic event. The number 0 Fully complemen':ary events is always 1, i.e. the first number on each row (Figure 1), while the second number on each row corresponds to the number 0: haploid chromosomes. The subsequent numbers in the same row correspond to the expected number 0: meiotic products having 2, 3, 4, 5 and 6 non—complementary chromosomes in a , respec:ively.

In the example 0: cucumber, with 128 di""erent meiotic products originating from any given meiotic event, these numbers are 21, 35, 35, 21 and 7, respec:ively. As the producos o meiosis are Sully complementary to each other, the extreme situation that none 0:5 the chromosomes would be complementary does not exist n a tetrad, e there will always be complementarity :o the chromosomes of the other meiosis i product. The number 0: events with one non— complemen':ary chromosome is there:fore 7 * 2, with two non— complemen' 21 * :ary chromosomes 2, etc. "“ e.g. 3 chromosomes are non—complementary, this implies that the other four are complementary. The probability (percent chance) 0" "inding a certain degree 0: complementarity in the four members or a pollen te':rad is given by the :able in Figure 2. The chance O: GDCOUH':ering meiotic produC':s having 0, 1, 2 or 3 non— complemen':ary chromosomes (and hence with 7, 6, 5 or 4 men':ary chromosomes, respectively) within a pollen tetrad of cucumber is thus ;.6% (=(l+l)/l28), 10.9% )/;28), 32.8% (=(2l+2;)/l28) and 54.7% (=(35+35)/l28), respectively.

Importantly, “non—complementary” in this context ly only re:fers to the telomeric ends 0: these chromosomes. we e.g. have a situation with 3 non— complemen':ary chromosomes and only 40% heterozygosity after ina:ion, 4 of the 7 chromosomes will be completely complemen':ary, while the other 3 chromosomes are still 60% 2012/069191 complementary. In essence, the tetrad constellation thus results in a situation in which the pollen grains are always pairwise at least entirely mentary for 50% O f the chromosomes - — I and as a result 0: recombination still lly complementary for the remaining chromosomes. This invention thus accomplishes the near-complete titution O: the geno :ype of a hybrid plant, while still all owing between 0 and 50% of variation compared to the original hybrid plan jrom which the pollen tetrads are derived to carry out the invention.

This method there:fore allows the reconstruction I) the identical or near—identi cal genetic constituti OH O: a hybrid plant. The near-identical reconstruCtion ha s definite advantages, as this allows the tion on the e ecu O__ additional gene':ic variation on the hybrid phenotype o; interest. This additional genetic variation may either prove to have an advantageous or disadvantageous e "ect on the hybrid phenotype, and this will allow jor the further c improvement 0: a superior hybrid phenOtype . ion Ol” "our chromosomes is graphically illustrated in Figure 3.

An important aspect of the claimed invention is the use of quartet microspores for the pollination o: a mother plant which eliminates the maternal genome from it s hybrid progeny. An e 0: such a plant has been recently described in Arabidopsis by Maruthachalam and Chan (Haploid plants produced by centromere-mediated genome elimination; Nature 464 (2010), 615-619; US patent application 7011008370? u I W070ll044l37), but this example is in no way limiting the application 0“ this invention, as the ion can also be carried out wi':h other haploid inducer systems.

The elimination of the maternal genome from hybrid progeny can thus be achieved by means 0: transgenic replacement of the endogenous centromere-specific histone n CENTB by a modified version. In practice, a modified version 1 of ,he CENHB n is overexpressed in a plant that lacks a functional endogenous CENH3 gene.

Alternatively, also the CENPC, M2812, NDC8O or NUF2 polypeptides can be used for the same purpose, when overexpressed in a plant that has a corresponding inactivated endogenous CENPC, M1812, NDC80 or NUF2 gene.

Suitably one or two alleles of the endogenous CENH3, CENPC, M1812, NDC80 or NUF2 genomic coding sequence 0: the plant is inactiva ,ed or knocked-out, and preferably all alleles are inactiva ,ed or knocked-out. The plant, when crossed with a wildtype plant, tes for example at least 0.1% haploid progeny.

Pre ferably the polypeptide is a recombinantly al tered CENHB polypeptide. The polypeptide may comprise a logous amino acid ce 0 f at least five amino acids (or alternatively O" at 1eas: ten amino acids) linked to a protein comprising a CEN {3 histone- fold domain, wherein 2O the amino acid sequence is ologous go she CENHB histone-fold domain. Suitably, the said heterologous amino acid sequence is linked direcoly Lo the CENTB histone—fold domain and the polypeptide lacks a CENHB tail domain.

Alternatively, the heterologous amino acid sequence may be linked to the CENHB histone—:fold domain via an intervening protein ce. This intervening protein sequence may se a CENHB tail domain or a non—CENHB e H3 tail domain (and the recombinant protein then corresponds to a tail-swap version of the CENHB protein).

Suitably, when the intervening protein comprises a CENHB histone H3 tail domain, the CENHB tail domain may be heterologous to the CENHB histone-fold domain. When the polypeptide comprises a CENHB histone-fold domain and a truncated CL-lJWH3 tail domain, the amino us ol the ,ail domain is trancated relative to the plant’s endogenous tail domain.

Pollinating such transgenicall_y complemented plants with pollen from a wild ,ype "ache r plant results in sterile progeny, due to the fac, thau ,he Fl progeny is haploid. ln fact each Fl progeny is ically identical to the pollen grain that was used O? "ertililation and from which it originated. Wild type and modi:fied chromosomes are apparently incompatible in kinetochore assembly in the zygote. Spontaneous or induced doubling o: somes leads to the "orma ,ion of DHs. CENH3 is a conserved and probably single copy gene in plants, and this system can ore also be applied in crop species. " seed formation would be problematic due to endosperm imbalance, embryo rescue can be performed.

In another embodiment, the maternal genome can be eliminated from the Fl—progeny by means ol O ,her haploid r systems, or through interspecies crossing (as described by e.g. 3ains & Howard 1950, Nature 166: 795). "n a further embodiment, the t mutation can also be combined with reverse breeding logy (WOO3/Ol7753; Dirks et al. 2009, Plant Biotech J 7: 837— 845), thereby greatly improving the e ”iciency o: reverse breeding. In this embodiment, the quartet phenotype is combined with suppression O" meiotic chromosome recombination in the :ather plant , by c, transgenic or al means. While the quartet mutation results in the physical attachment 0: the 4 products 0" a sing' e meiosis in a tetrad at pollen maouri vY/ the suppression o; recombination ensures that these pollen grains contain unrecombined parental chromosomes. The present invention thus ensures that DHs with complementary genetic composition WO 45616 can easily be identified among the four DHs derived from po'len from a single meiosis, with one co-dominant, pO' ymorphic marker per chromosome.

A drawback of reverse breeding may be the occurrence o: unbalanced (aneuploid) spores. It is r possible to morpho' ogically idenoi "y balanced tetrads (for example by visually selecting tetrads in which the four pollen grains are equal in size, which is tive 0' an equal bution 0“ all chromosomes, or by means 0: e.g. flow cytometry), and DHs regenerated from such tetrads will automatically be pairwise complementary. When balanced tetrads are used for pollinating a mOther plant which eliminates the maternal genome from its hybrid progeny, this will give rise to 4 haploid seeds which are pairwise complemen':ary with respect to their genetic composition.

Subsequent crossing of the complementary DHs or lines derived thereO“ will result in the truction of the genetic composition of ,he original starting plant. When the four progeny plants of ,his cross are nOt genOtyped prior to 2O crossing, the chance 0: obtaining a reconstruCtion of the genetic composition of ,he original starting plant by randomly crossing two 0: these four plants is 50%.

In another embodiment, the t phenotype can be combined wi':h Near Reverse Breeding (WO2006/O94773). In this embodimen "I the ence 0: second division restitution (S DR) in a plant that exhibits the quartet phenOtype will lead to the production 0'' pol'en dyads by the father plant, which ses two diploid po' len grains with a perfectly complementary gene':ic composition, including cal chromosome breaking . Pollinating a mother plant that eliminates the maternal genome from its hybrid progeny with such a pollen dyad will result in two diploid plants, and crossing these two plants with each other will result in the near-complete reconstitution o: the genetic composition 0: the original hybrid.

“Near-complete” re fers in this ation to the fact that together the two plants have the same genetic material as their father plant (as 1’10 DNA is lost or gained during a meiotic division), but the relative genomic position 0: chromosomal segmen':s may be di "erent, asa result 0; cross-over events during meiosis. The on I] chromosomal break-points is however identical in the ":WO plants, as they originated from a sing:_e meiotic event. The idenoi on O" complemen':ary SDQ—O lines is thus greatly facilitated by exploiting the quartet ype, and the omplete reconstitution 0: the genetic composition I] any given hybrid becomes much easier and e ”icient.

Due to meiotic recombination during ,he formation of the SDR-O events the recons litu ,ion will no t be 100% complete, since the reconstruC':ed hybrids will genetically di" "er to some extent, both from e ach other and from she original starting hybrid (fa lher) plant, ally at the 2O telomers as a result 0:f COs. This feature thus provides the additional advantage that additional genetic variation is being created in a pre-selected superior hybrid plant, by ing alternative and slightly di "ering c arrangements while maintaining most 0: the original hybrid tution. This may lead to a further improvement or a hybrid phenotype. "t is a further object o : this invention to provide an e ”icient method for obtaining a se, ol seeds, which seeds are genetically identical to the male gametes from which they arise, and which set 0: seeds is composed I) pairs 0: genetically essentially complementary seeds which, when the plants grown from them are crossed, give rise to essentially the same hybrid plant. This hybrid plant is genetically essentially iden':ical to the plant that produced the male gametes from which the said set 0: seeds arose.

“Essentially” as used herein is intended to mean that the genetic complementarity ol ,he pairs 0: seeds need not be 100%, as also the near-complete reconstitution o; a hybrid plant may be desirable, as explained above (because it may 0 "er the opportunity to further e a hybrid phenotype). Such a near-comple':ely reconstituted hybrid plant is only essentially iden':ical to the original hybrid plant and to other near-comple':ely reconstituted hybrid plants obtainable by the invention, because although it has the same or large: y the same genomic material as the original hybrid p— ant, there may be alternative or slightly di" "ering genomic ements present in their genomes, as a result 0 "di" "erent cross—over events, or some c regions may be gous as a result 0: recombination. When meiotic cross-over takes place, “essentially” thus relates to the degree 0: cross-over that occurred during the ‘ormation o: the pollen grains used for pollinating the haploid-inducer mother plant.

In another contex "I jor example in the absence I] meiotic recombination, “essen':ially” may also re:fer to the selection 0" pairs 0" seeds ( from among the set 0:: seeds obtained by this invention) that are not 100% genetically complementary to each other. In this embodiment, which is also intended ,0 al' within the scope of the present invention, for example a pair O: seeds can be selected ( from the said se, ol seeds) that are fully mentary to each other for n—l chromosomes (with n being the hap:_oid chromosome number 0: the species), and identica' ‘or the remaining chromosome. The hybrid plant resulting ‘rom the CIOSS O: the plants grown from this pair 0: seeds will then be cally cal to the original hybrid plant "01” 2012/069191 all but one chromosome, and di""erent (namely homozygous) for the remaining chromosome. Overall the plant is thus only “essentially” genetically identical. SJCh plants can for example be obtained when carrying out the present invention in a preferred embodiment, with suppression O“ meiotic recombination (“Reverse Breeding”). It is to be understood that the same t can be done for n—2 chromosomes, n—3 chromosomes, etce':era. This concept allows, for example, the genomic and pheno :ypic analysis 0' a p' ant while ng on only a subset o: its somes, and while leaving the rest O: the hybrid genome unchanged.

The invention thus relates to a method for the production 0: a set 0: seeds which are genetically cal to the male gametes from which they arise, comprising: a) placing a limited number 0: al gametes that have the "orm o tetrads or dyads on the stigma o; a flower to fertilize ma':ernal egg cells to obtain a number n zygotes; b) inducing the loss 0. macernal chromosomes from 2O the zygotes to obtain a seed set con':aining a d number 0: seeds in which the maternal chromosomes are .

In this method competition among pollen tubes for the "ertilization O" ovules should pre'ferab' y be minimized or prevented, to avoid that some of the pollen grains comprised in a tetrad or dyad ail to "erti' ize an ovule.

Therefore a limited pollination is pre'ferab' e, such that there are at least as many ovules present in the female reproductive organ 0: the pollinated _ ower as there are pollen grains deposi:ed onto its stigma. Each pollen grain should then be able to fer,ilize an ovule.

In one embodimen':, the limited number 0: paternal gametes is therefore equal to or lower than the number 0; egg cells contained in the female reproductive organ carrying the stigma.

The average number 0: egg cells or ovules per flower, being the preferred upper limit for the number of paternal gametes that can be successfully used in this method, is known so she skilled person who is familiar with a specific crop. "t is generally slightly higher ,han ,he average number of seeds present in a typical ”rui, o" that crop. For comato, for example, the average number of egg cells per flower is about lOO, for arassica species about , for Arabidopsis about 40—50, for elon about 200, for grape about 4, for cacumber about 250—300, for sweet pepper about 100, and for melon about 500. 3urd et al., Am.

J. Bot. 96(6), 1159—ll67 (2009) describes a study on ovule number per f'ower in ’87 angiosperm species.

A limited number 0: paternal s suitably comprises any number tha' allows to use the method of the invention in an e "icien': way. This means that too much c variation in the male gametes must be avoided, i.e. 2O the number 0: meiOtic events that gave rise to the male s deposited onto I) a single stigma should be kept low i; those meiotic events each genera :ed a large degree 0; genetic rearrangements in the genome o: the individual gametes that constitJte the dyad or tetrad (through some recombination).

In a preferred embodiment the limited number 0; paternal gametes is two or four (corresponding to the number O: gametes comprised in a single pollen dyad or tetrad, respectively, and hence derived from a single s in the absence OI presence 0 f the second meiotic division, respectively). This s tra uegy will lead to the formation I] four seeds (in case a d was used for poll ination) or two seeds (in case a dyad was used for pollination), which are geneti cally identical to the pollen grains that originated from a single meiOtic division. When the gametes have the f orm O" dyads, the limited number of pacernal gametes that allows to use the method of the invention in an e "icient way may be much higher than two, because the amount 0: c variation is greatly reduced, as the two s comprised within a dyad are genetically fully complementary to each other. Especially when using gametes that have the form 0" dyads i, is ,hus e ”icient to use any number 0; gametes that is smaller than the average number I) egg cells or ovules per flower.

The tetrad or dyad form 0" the male gametes is the resu't o inter"erence with microspore tetrad separation in the father plant. In one embodiment, the interference with microspore tetrad separation ses interference with one or more target genes involved in the break-down of the pectin layer n the microspores resulting from a single meiotic division. The one or more target genes can be selected from the group consisting of QRTl, QRTZ, QRT3, or 2O their functional homologues. Suitable mutations are the introducti OH O: stop codons or irame shifts in the target genes, ami no acid substitutions that t protein structure and/or function, and insertions 0: genetic elements such as T—DNA into the coding sequence, er or other rnglatory sequence of ,he gene. In Arabidopsis, the qrtl-4 musation results from ,he insertion of a T—DNA into an exon 0“ she QRTl gene, the quartet mutant phenOtype in the grtl—5 mutant is caused by a T9DNA insertion into the QRTl gene’ s promoter, and the grtl—6 mutation is caused by a T—DNA insertion into an intron 0: the QRTl gene. The qrtZ-l mutant in Arabidopsis comprises a Valine to Alanine amino acid subst itution on position 372 (a GTG to GCG mutation) in WO 45616 the QRT2 protein, which is the underlying cause of the quartet phenotype in this mutant line.

In another embodiment, interference with pore tetrad separation comprises inter:ference with the break-down O: the pectin layer between the microspores resulting jrom a single meiotic division by chemical means.

The QQT gene produC':s are enzymes that play a role in the breakdown ol the pectic ccharide (peCtin) layer that is pr s nt b 'ZW 1’1 th individual male gametes (microspores) that arise from the meiotic division 0: a pollen mother cell. " this pectin layer is not degraded, the four male gametes (microspores) remain physically attached to each other in a tetrad form. "nter‘ering with one or more target genes involved in microspore tetrad separation can be achieved by a number 0" di" "erent approaches. The inter:fering with a target gene can consist o: preventing transcription thereo . "n one embodiment transcription ol a Large t gene can be prevented by means 0:_ RNA oligonucleotides, DNA ucleotides or 2O RNAi molecules directed agains the target gene promoter.

In another embodimen transcription is prevented by means 0:f the expression O: a negatively acting transcription ‘actor acting on the target gene er.

Furth rmor r: rir wi , th int g th the target gene can also consist of destabilizing tt e targe t gene mRNA or ript.

This can be achieved by mea HS 0' Jcleic acid molecules that are complementary to the La rge t gene mRNA or transcript, selected from the group cor sis ting of antisense RNA, RNAi molecules, Virus-Induced Gene Silencing (V lGS) molecules, pressor molecules, RAA oligonucleotides or DNA oligonucleotides. "nter‘ering with the target gene can also consist O: inhibiting the target gene expression product, by means O: one or more dominant negative c acid constructs, OI by means 0: one or more al compounds.

In yet another ment, the interfering with the target gene t S O: the introduCtion 0: one or more ons into ,he target gene, leading to perturbation o; its bio'ogical "uncoion. The one or more mutations can ei ther be introduced randomly - by means 0: one OI more chemical compounds (such as ethyl methanesulphonate, ni trosomethylurea, hydroxylamine, profl avine, N—methyl—N— ni trosoguanidine, N—ethyl—N—nitrosourea , N—methyl—N—nitro— ni trosoguanidine, diethyl sulphate, e thylene imine, sodium az ide, formaline, urethane, , e thylene oxide) and/or physical means (such as UV—irradiation, fast-neutron exposure, X—rays, gamma irradiation) and/or insertion o; genetic ts (such as transposons, T—DNA, retroviral elements) - or speci -:-iC al' y, by means 0 : homologous recombination or oligonuc:_eotide-based mutation induction.

Chemical mean s to inter‘ere w ith microspore tetrad separation comprise the use 0: chemical inhibitors that 2O reduce the aCtivity (or the stability) oj she QRT gene products, OI the use 0; chemicals that — directly or indireCtly - reduce the expression leve l of the QQT gene products, resulting in the persistence or ,he peCtin layer between the tour microspores resulting ‘rom a single meiotic event. The enzyme activity o: QRT prote ins can be inhibited by means O: chemical tors, such that treatment or a flower bud during the stage 0" normal microspore separation (or a preceding stage) with the inhibit ing chemical leads to the persistence o: the pectin layer be':WGeD the microspores that are d from a single meiosis, and thus to the persistence o: microspore tetrads during later developmental stages and at anthesis.

In a preferred embodiment, the father plant, which produced the said male gametes (in the form 0 a tetrad or a dyad) exhibits — in addition to the quartet ype - suppression o: chromosome recombination, which abolishes chromosome crossing-over and leads to the intact transmission 0" entire chromosomes. In this embodiment the chance 0" identi"ying two genetically complementary genomes among the individual pollen grains 0" a tetrad is 50 t. When the father plant which produced the said male gametes exhibits suppression o: chromosome ination, this suppression o: chromosome recombination is achieved by interfering with one or more target genes involved in recombination.

In one embodiment the target gene is ed in double strand breaks, and it can be selected from the group consisting 0" SP077, MHKl, MHKQ, MRHQ, MH/4, KHClOQ, RPC704, RPC774, MFKl/MRR4, RPDl, HOPl, RAD50, MREll, XRS2, or their functional homologues.

In another embodiment the target gene is involved 2O in some pairing and/or strand exchange, and it can be selected from the group consisting of RHD54/TID1, DMCl, SAE3, REDl, HOPl, HOP2, REC8, MPRl, MRPQ, qui, HTPQ, MRT5, RAD51, RAD52, RAD54, RAD55, RAD57, RPAl, SMC3, SCCl, MSHZ, MSH3, MSH6, PMSl, SOLODANCERS, HIM6, CHKZ, or their onal homologues.

In yet anOther embodiment the target gene is involved in the meiOtic recombination process, and it can be selected from the group consisting o: SGSl, MSH4, MSH5, ZIPl and HTPQ, or their functional homologues.

In r embodiment the target gene is selected from the group consisting of BKDl, BKDZ, BKDB, BHSl, NBSl, COMl, MNDl, MEK3/KCK, uiy3, uiy4, ETD, SHOCl, ZYPl, MLHi, MLH3, or their functional homologues. "n,er "ering with one or more target genes involved in ina:ion " can be achieved by number a 0 di""erent approaches. The interfering with a target gene can consist 0: preventing transcription thereo;.

In one ment transcription 0: a target gene can be prevented by means 0: RNA oligonucleotides, DNA ucleotides or RNAi mo'ecules directed against the target gene promoter. In another embodiment transcription is prevented by means of the expression 0: a negatively acting transcription ‘actor acting on the target gene promoter.

Furth rmor th int r: rir g wi , uh ,he target gene can also consist of destabilizing tt e target gene mRNA or transcript.

This can be achieved by mea n S O" n Jcleic acid molecules that are complementary to the La rge u gene mRNA or transcript, selected from the group cor S is ting 0: antisense RNA, RNAi molecules, Induced Gene Silencing (VZGS) molecules, co-suppressor molecules, RkA oligonucleotides or DNA oligonucleotides. "nter‘ering with the target gene can also consist 0: inhibiting the target gene expression product, by 2O means 0: one or more dominant nega :ive c acid constructs, or by means 0: One OI more chemical compounds.

In yet another embodiment, the interfering with the target gene ts o: the introduCtion o: one OI more mutations into ,he target gene, leading to perturbation o; 75 its biological UHC ,ion. The one or more mutations can either be introduced randomly - by means 0: one OI more chemical nds (s JCh as ethyl methanesulphonate, ni trosomethylurea, hydroxylamine, proflavine, N—methyl—N— ni trosoguanidine, N—ethyl—N—nitrosourea, N—methyl—N—nitro— ni trosoguanidine, l sulphate, e thylene imine, sodium azide, formaline, urethane, phenol, e thylene oxide) and/or physical means (such as UV-irradiation, eutron exposure, X—rays, gamma irradiation) and/or insertion I] genetic elements (such as transposons, T—DNA, retroviral elements) - or specifical'y, by means of homologous recombination or oligonucleotide-based mutation induction.

In another pre:ferred ment, ,he father plant which produced the said male gametes exhibits — in addition to the quartet phenotype - second on restitution (SDR) during meiosis. In this ment, the father plant produces male game':es which are 2n and which have the form 0: dyads, because the second meiotic division does not take place. The two male gametes comprised within one dyad are genetically fully complementary, and the chance 0; idenoifying two genetically complementary genomes among the two individual pollen grains 0"_ a dyad is thus 100 percent.

When the father plant, which produced the said male gametes, ts second division restitution during meiosis, this second on restitution can occur spontaneoasly, without interference with the starting organism. In another embodiment, second division restitution is induced by means 0: genetic modification. This genetic modification can be transient, or it can be achieved by stab'e incorporation into the genome o: a genetic element (such as a transgene, mutation , oson, retroviral element, T—DNA) increasing the number 0: second division restitution events in the organism.

In yet another embodiment second division restitution is achieved by subjecting the father plant to nmental stress, such as temperature , NO2 I nitrous oxide (N20) I or combinations thereo: (Zhang et al. (2002) Journal of ultura 7 Science & Biotechnology 78: 84-88; wo 2006/ 094773; 3arba-Gonzalez et al. (2006), Euphytica L48: 303—309; Okazaki et al. (2005), Euphytica 143: lOl-ll 4).

The loss 0: maternal chromosomes from the zygote - to obtain a seed set containing a limited nmeer 0: seeds in which the maternal chromosomes are absent - can be achieved in di "erent manners. In one embodiment, a haploid inducer line can be used as the female. A haploid inducer line is a plant in which the chromosomes 0: one ol the parents are eliminated from the genome ol the Aygote formed alter "ertililation 0" an egg cell by pollen. The female can e.g. be a plan, 0" a di""erent species, as has been bed by e.g. 3ains & Howard 1950, Nature 166: 795. In anOther embodiment the loss 0: maternal chromosomes from the Aygote results from the use 0: a transgenic plant as the mother organism, which transgenic plant comprises a logous transgene expression cassette, the expression cassette comprising a promoter operably lin<ed to a cleotide encoding a recombinantly altered CTNHB, CENPC, M"S’7, NDC8O or NUF2 polypeptide, and having a corresponding inactivated endogenous CENH3, CENPC, M1812, NDC80 or NUF2 gene as described in W07011/O44137. 2O The present invention also relates to a se, ol seeds containing a limited number 0: seeds in which the maternal somes are absent, which set is composed I) pairs 0: essentially genetically complementary seeds which when crossed result in essentially the same hybrid, and which set 0: seeds is obtainable by the method 0: the invention. The seeds ol this set 0: seeds a" have the same lather, because they originated from the po"ination 0' a mOther plant with a limited number 0: paternal s that have the form 0 s or dyads, which paternal s had been collected from a single father plant. Because 0; this, and because of the elimination of the maternal chromosomes from the Aygotes, from a genetic point 0: view the seeds of the said set 0: seeds only have one male grandparent and one female grandparent, namely the s 0.- their tather. This is schematically represen':ed in Figure The t invention also relates to a set 0; parent plants for the tion " which 0' a p' ant o_ the genetic constitution is essentially identical to the genetic constitution o: its male grandparent, comprising growing plants jrom seeds 0: the se, O__ seeds 0: the invention, a fter or prior to doubling the chromosome number of the seeds, and iden oi Lying two genetica" y complemen':ary plan' as the parent plants. Such genetica' 'y complemen':ary plan' can be identi:fied by means 0.- molecu— ar ic) markers for which the jather p' ant (who produc d th mal gam t s) was heterozygous, and :or which the two paternal arents had di erenL alleles. These markers can be scored with a number 0 di "erent approaches, such as direct quencing o: speci:fic genomic regior Sr AFLP, RFLP, SSR, QAP3, KASPar (K3ioscience), InvaderTM OI Invader PlusTM (see e.g. De Vienne ed. (2003) Mo" ecular Markers in Plant 2O Genetics and Biotechnology. Science publishers DC. file-Id, NH USA. S %N 1—578080).

The invention further relates to a method :OI screening the set 0: seeds or the plants grown thereo 01” their gene':ic constitution, to identi:fy a plan, O j which the c consti:ution is essentially identical uO ,he gene':ic constitution of its paternal grand:father, and to identify another plant 0: which the genetic consti all ,ion is ially iden':ical to the genetic cons oi oution o: its paternal grandmO':her.

A plant 0: which the genetic constitution is essentially iden':ical to the genetic constitution 0: its paterna' grand a ,her can subsequently be crossed to another plant 0: which the genetic constitution is essentially identical to the genetic constitution of its paternal grandmother, in order to obtain progeny plants 0: which the genetic constiouoion is essentially identical to the genetic tJtion ol oheir own grandfather. With ‘their own grandla,her’ the (hybrid) plant is meant, which produced the po"en tetrads or pollen dyads that were used :or po"ination O" the haploid inducer line. This pedigree is graphically i'lustrated and clarified in Figure 4.

The invention further re'ates to the use 0: the said set or seeds, alter or prior to doubling the chromosome number ol ,he seeds, for the identification 0" two genetically mentary plants as the parenos for a cross.

Crop species on which this invention can be sed include for example o, poplar, sugar beet, oilseed rape, soybean, tomato, cucumber, gherkin, corn salad, spinach, pepper, petunia, potato, eggplant, melon, elon, carrot, radish, vegetable Brassica species (cabbage, lower, broccoli, kohlrabi, Brussels sprouts), bean, pea, onion, strawberry, table beet, 2O gus, and grape vine.

The present invention is further described in the following clauses.

CLAUSES This invention relates to: l. A method for ,he production 0: a se, ol seeds which are genetically identical to the male gametes from which they arise, sing: a) placing a limited number 0: paternal gametes that have ,he "orm o tetrads or dyads on the stigma o; a flower to fertilize maternal egg cells to obtain a number I) zygotes; b) inducing the loss of macernal somes from the rygotes to obtain a s eed set containing a limited number O: seeds in which the maternal chromosomes are . 2. A method according to clause 1, wherein the limited number of paternal gamete s is equal to or lower than the number 0: egg cells contained in the female reproductive organ ng the stigma. 3. A method according to clause 1 or 2, wherein the limited number 0: paternal gametes is two or four. 4. A method according to any combination 0: the clauses 1-3, wherein the paternal game ,es that have the form 0" tetrads or dyads are the result 0 inuer"erence with microspore tetrad separation.

. A method according to clause 4, wherein interference with microspore tetrad separat ion comprises interference with one or more target genes invo lved in the down of the pectin layer between the microspores ing from a single meiotic divi sion. 6. A method according to clause 5, n the one or more 2O targ t g n s ar s l c, d from the group consisting of QRTl, QRT2, QRT3, or their finctional homologues. 7. A method according to clause 4, wherein interference with microspore tetrad separation is achieved by chemical means 8. A method according to any combination 0: clauses 1—7, wherein the father plant exhibits suppression o: chromosome recombination. 9. A method according to any combination of the s 1-7, wherein the fa,her plant exhibits second division restitution (SDR) during meiosis. lO. A method according to clause 8, wherein suppression o; chromosome recombination is achieved by inuerfering with one or more target genes involved in recombination. ll. A method according to clause 10, wherein the target gene 2012/069191 is involved in double strand breaks. 12. A method according to clause ll, wherein the target gene is selected jrom the group consisting O" S?Oll, fiRl, MfiR7, MRO, M4. 4, 7, R'icio4, R'ic114, MiiK'/M+'.4, R01, HO?l, RAD50, {111, XRS7, or their tunctional homologues.

L3. A method according to cLause LO, wherein the target gene is ed in chromosome pairing and/or strand exchange.

L4. A method according to cLause L3, wherein the target gene is selected jrom the group consisting o: R{D54/TLDl, DMCL, SAi'.3, Razai, HOLDl, HOP2, REC8, *ZRl, MR0, '. pi, '. P7, Mi: 5, RAD5l, QAD52, QAD54, RADSS, RAD57, RPA, SMC3, SCCL, MSH2, MSiB, MSH6, PMSL, SOLODANCjRS, HLM6, CHK2, or their functional homoLogues.

L5. A method according to cLause LO, wherein the target gene is involved in the meiOtic recombination process.

L6. A method ing to cLause L5, wherein the target gene is selected jrom the group consisting of SGSl, MSl4, MSH5, ZLPl and H"P7, or their tunctional homoLogues. 17. A method according to clause 5 and/or 10, wherein the 70 inter‘ering with the one or more target genes consists o; preventing ription thereor. 18. A method according to clause 17, n transcription is prevented by means 0: RNA oligonucleOtides, DNA oligonucleotides or RNAi molecules direCted against the target gene promoter. 19. A method according to clause 17, wherein transcription is prevented by means of the expression 0: a negatively acting transcription tactor acting on the target gene promoter. 20. A method according to clause 5 and/or 10, wherein the interfering with the target consists I) one or more genes o; destabilizing the target gene mRNA or transcript. 21. A method according to clause 20, wherein the target gene mRNA is destabilized by means 0“ c acid molecules that are complementary to the Larges gene mRNA or transcript, selected from the group consisting of antisense RNA, RNAi molecules, Virus-Induced Gene ing (VZGS) molecules, co-suppressor molecules, RNA oligonucleotides or DNA oligonucleotides. 22. A method according to clause 5 and/or 10, wherein the ering with the one or more target genes consists o; inhibiting the target gene expression product. 23. A method according to clause 22, n the target gene expression product is inhibited by means of the expression product(s) or one or more dominant ve nucleic acid construc:s. 24. A method according to clause 22, wherein the target gene sion product is inhibited by means 0: one or more chemical compounds.

. A method according to clause 5 and/or 10, wherein the interfering with the one or more target genes consists o; 2O the introduction 0: one or more mutations into the target gene, leading to bation o i,s ical runction. 26. A method according to clause 25, wherein the one or more mutations are introduced randomly by means 0: one or more chemical compounds and/or physical insertion I) means and/or 0; genetic elements. 27. A method according to clause 26, wherein the one or more chemical compounds the I) are selected from group consisting or ethyl methanesulphonate, nitrosomethylurea, hydroxylamine, prO' avine, N-methyl—N—nitrosoguanidine, N—ethyl—N— nitrosourea, N-methyl-N-nitro—nitrosoguanidine, diethyl sulphate, ethylene imine, sodium azide, formaline, urethane, phenol and ethylene oxide. 28. A method according to clause 26, wherein the physical means are selected from the group ting 0: UV— irradiation, fast-neutron re, X-rays, gamma irradiation. 29. A method according to clause 26, wherein the genetic element is selected from the group consisting of osons, T—DNA, retroviral elements.

. A method according to clause 25, wherein the one or more mutations are introduced specifically by means of homologous lO recombination or oligonJcleotide-based mutation induction. 31. A method according to clause 9, wherein second division restitution occurs spontaneously, in particular without erence with the starting organism. 32. A method ing to clause 9, wherein second division restitution is d by means of genetic modification. 33. A method according to clause 32, wherein the genetic modi”ica,ion is transient. 34. A method according to clause 32, wherein the genetic modifica,ion is achieved by stable incorporation into the genome of a genetic element increasing the number of second division restitution events in the organism.

. A method according to clause 9, wherein second division restitution is achieved by subjecting the father plant to environmental stress. 36. A method according to clause 35, wherein the environmental stress is selected from temperature , N02, nitrous oxide (N20), or combinations f. 37. A method according to any combination of ,he clauses l- 36, wherein the loss of maternal chromosomes from ,he 2ygote is induced by using a haploid inducer line as the female. 38. A method according to clause 37, wherein the female is a plant of a di""erent species. 39. A method ing to any combination 0: clauses l-37, wherein the female plant is a transgenic plant that comprises a heterologous transgene expression cassette, the expression te comprising a promO':er operably linked to a polynucleotide encoding a recombinan':ly altered C:NHB, CENPC, MlSlZ, NDC8O or NUF2 ptide, and having a corresponding inactivated endogenous CENH3, CENPC, M1812, NDC80 or NUF2 gene. 40. Se, of seeds containing a d number 0: seeds in which the maternal chromosomes are absent, which set is composed O" pairs 0" genetically complementary seeds which when plants grown from the seeds are crossed result in essentially the same hybrid, and which seed set is obtainable by to I] a method according any combination 0; clauses l-39. 4;. A method for providing a se, ol parent plan ,s for the production 0: a plant of which the genetic cons ,ituuion is essentially identical to the genetic cons:itution " o: its male grandparent, comprising growing plan as from seeds 0; the set 0: seed according to clause 40, a fter or prior CO 2O doubling the chromosome number ol ,he seeds, and identifying two cally complementary plan':s as the parent plants. 42. A method according to clause 41, wherein the se, ol seeds or the plants grown thereo: are screened for uheir c constitution, to identi fy a plan, O f which the genetic consoiou ,ion is essential ly identical to the genetic constitution of its paternal grandfather, and to identi:5y another plant 0: which the genetic consti all ,ion is essentially identical to the genetic cons oi oution 0; its paternal grandmOther. 43. A method according to any ation 0 : clauses 40-42, wherein a plant of which the genetic cons oi ,ion is essentially identical to the genetic cons oi all ,ion 0; its paternal grand a lher, is crossed to another plant of which the genetic constitution is ially iden':ical to the genetic constitution 0: its paternal O':her, in order to obtain progeny plan,s ol which the genetic constitution is essentially identical to the genetic constitution o; their own grandfather.

The inventior will be ‘urther illustrated in the Examples that follow and that are not intended to limit the invention in any way. In the Txamples, re‘erence is made to she "ollowing tigures: Figure 1: the so-called Pascal’s Triangle, depicting the specific outcome 0: chromosome complementarity for each pollen tetrad, in on 0‘ the number 0; somes of the species. From top to bottom the number or haploid chromosomes increases, and the sum 0: the numbers on each row always equals 2“, being tt e total ntmber o; di""erent meiotic products that can arise from a meiosis in which n somes are involved. "5 we ta<e the seventh row as an example (for e.g. cucumber, with 7 haploid somes, n = 7), it can be seen that the number 0" "ully 2O complementary events is always 1, i.e. ,he jirst number on the row, while the second number on the row corresponds to the number 0: haploid chromosomes. The uent numbers in the same row correspond to the expected number 0: meiotic products having 2, 3, 4, 5 and 6 non-complementary chromosomes in a tetrad, respectively. The total number 0; events with one non-complementary chromosome is 7 * 2, with two non-complementary chromosomes 21 * 2, etc. "“ e.g. 3 chromosomes are non—complementary, this implies that the other four somes are complementary. Importantly, “non— complementary” in this context actual'y on'y re‘ers to the telomeric ends ol ,hese chromosomes. wee .g. have a situation with 3 non—complemen':ary somes and only 40% heterozygosioy ajter recombination, 4 of the 7 chromosomes will be completely complementary, while the other 3 chromosomes are still 60% complementary.

Figure 2: this table shows the probability (percent chance) 0 g a certain degree 0; mentarity in the four members 0: a pollen tetrad, as a ‘unction E 0' the haploid chromosome number. For e.g. er (with n = 7) the chance 0: encoun :ering c products having 0, l, 2 or 3 non—complemen :ary chromosomes (and hence with 7, 6, 5 or 4 mentary chromosomes, respectively) within a pollen tetrad of cucumber is thus 1.6% (=(1+1)/128), 10.9% (=(7n7)/128), 32.8% (=(21+21)/128) and 54.7% (=(35+35)/128), respectively.

The probability (percent chance) 0" "inding a certain degree of complementarity in the four members of the quartet is given by the table in figure 2. The chance of encoun':ering meiotic products having 0, 1 2 , or 3 non— complementary chromosomes (and hence with 7, 6, 5 or 4 complementary somes, tively) within a pollen tetrad of cucumber is thus 1.6% (=(1+l)/1 28), 10.9% (=(7+7)/l28), 32.8% (=(21+2l)/128) and 54 .7% (=(35+35)/128), respectively.

Figure 3: graphical representation 1 0: a meiOtic event with 4 haploid chromosomes (n = 4). One parent 0: the hybrid plant contributed blue chromosomes, while the Other paren: contributed red chromosomes to the hybrid. "n a ‘irst step the genome doubles from 2n to 4n, and subsequently crossing-over can take pl_ace between the gous regions Ol sister chromatids, as illustrated here with a single cross-over event per Chromosome. During the first meiotic division two dip:_oid daughter cells are produced, which are genetically completely complementary (i.e . if both their genomes are taken together the 4n genomic composition from be fore the division is obtained). "n this ‘igure only one 2012/069191 possible example of he s " t e 0" is shown.

During the second meiotic division the s (in this context: microspores or pollen grains) are ed, and the chromosomes can randomly s gr gat to ith r daughter cell.

This leads to 7“"1 di "erent pairs 0: daughter cells (gametes). For the diploid cell on the lej, the jigure shows one possible pair 0: gametes, while for the diploid cell on the right all 8 possib'e pairs 0" gametes are shown (= 23 = 211—1) .

When uently doubled haploid plants are regeneraoed jrom the di "erent gametes (for example by the method 0: the invention, by generating haploid plants and doubling their genome), these plants can be crossed to each other. The numbers just below the di "erent chromosome sets correspond to the number 0: chromosomes that are complemenoary to the chromosome set on the tar le t o the tigure. "or example the plant containing ,he jour chromosomes that are ed on the tar le a (i.e. the :our entirely lee chromosomes) would be crossed to a plant 2O containing ,he jour entirely red chromosomes jrom the jirs, ol ,he eight chromosome pairs on the right, then all 4 somes would be complementary. This cross would result in the exact reconstitution o: the original hybrid plant that had produced the gametes. Similarly, crossing the same plant with the four blue chromosomes with plants containing the other possible chromosome sets displayed on the right (more precisely: the left chromosome set 0: each of the eight pairs) wil' either resu't in the complete reconstitution ol she origina' hybrid plant (when all 4 chromosomes are entirely complementary), Or in the near- complete reconsti tition o: the original hybrid plant (when one or two ol the Sour chromosomes are not entirely complementary). The situation that more than two chromosomes would not be complementary does not exist, because the s originated from the same meiotic even' In addition, it is clear from the drawing that a large par O__ the “non— complementary” somes is in fact complementary, and that crossing wil:_ lead to a highly heterozygous progeny; the only non—comp:_ementary chromosome parts are due to the cross-over events at the chromosomes’ telomeres, which cross-over regions would become homozygous in the resulting progeny. When the numbers below the chromosome pairs are added up to group the events that have 0, l or 2 non— complementary somes in comparison to the chromosome set on the _CaT‘ 1e.:t, this leads to the 2 — 8 — 6 distribution for n = 4, which can also be found in the ‘ourth I] row 0' the triangle 0 "igure 1 (where they are displayed as — 4 _6_4_ l).

On the far right " 0 ohis ”figure a second possible pair 0: gametes is depicted, that can be derived from the lej,—most diploid cell during meiosis The numbers listed at the bootom or the jigure represen, ,he number 0; 2O complementary chromosomes when one or ,he gametes from this pair is combined with any 0: the 16 s derived from the right—most diploid cell. Again the same outcome can be observed: 0: the 16 s 2 are fully complementary (i.e. 4 complementary chromosomes), 8 have one non-complementary chromosome and 6 have two non-complementary chromosomes. 3y g the four ts derived from a single meiotic event cogether, and by providing a means to obtain progeny plants thao are cally identical to these four meiotic prodJcts, ,he present invention maximizes the chance or iden,i Lying pairs 0: progeny plants that upon crossing can give rise to the origina' hybrid plant, or to a hybrid that is genetically essential:_y the same as the original hybrid.

Figure 4: simplified overview 0: a pedigree according to the present invention, assuming tha'I DO recombination occurs. A hybrid plant, resulting from the CIOSS Ol ,wo (homozygous) parental lines (with genotypes AA and %%, respectively) produces pollen grains in ,he "orm 0' tetrads. When a single tetrad is used to a:e a haploid inducer mOther plant (with random genOtype) the progeny resulting from this cross will consist o: a set 0 "our haploid seeds. Genetically these seeds are A, A, 3 and 3 (in the absence 0: recombination), and there is no genetic contribution from the mOther plant. Hence these four haploid seeds only have two grandparen':s, namely the parental lines of the hybrid "ather p'ant tha': produced the pollen tetrad.

Aj,er genome doubling four doubled haploid plants can be obtained, which — in the e 0: ination - will always be pairwise genetically “ul 'y complementary. Crossing the two genetically mentary plants from any of the pairs leads to the reconstitution o: the original hybrid plant. The situation is more ex when recombination does 2O occur, as illustrated by ‘igures ’, 7 and 3. When recombination occurs the chance 0 ”inding a pair 0 Fully complementary plants among the progeny o: a pollen ,eorad decreases in on o: the haploid chromosome number, but in the absence 0: recombination the chromosome number has no e "ect on the outcome.

EXAMPLES EXAMPLE 1 Tdentification of guartet—pollen—shedding Brassica plants An essential resource ‘or ng out this invention is a plant or the species ol interest that sheds its pollen grains in a tetrad form, whereby the four products from a meiotic division remain physically attached to each other. Such a plant can either be obtained in a mutant screen or through a transgenic approach. This example illusurates the jirst option, while the second option will be explained in subsequent examples. "n order to iderti‘y a plant exhibiting the quartet pollen phenotype, Brassica oleracea EMS-mutant population was ed phenotypically, to detect the occurrence 0; pollen grains with the quartet phenotype (i.e. pollen tetrads) in the anthers o " individual . In a bulk— approach, pollen from multiple plants was pooled together and diluted in solution so that individual po'len grains could be clearly discerned. These pollen pools were then screened by eye under a binocular, bat atively this screen is also possible with ow cytometry or by "ilvra,ion (with a "i'ter that has a pore size that is larger than the diameter 0 an individual pol'en grain, but smaller than the diameter ol a pollen tetrad). Upon detecting the desired quartet phenotype in a pollen pool, all plants that contributed pollen to that pool were screened individually, 2O until a t mutant plant was positively identi‘ied. "n the next generation the transmissibility O" the quartet phenotype was confirmed.

EXAMPLE 2 Creating a Brassica plant that eliminates the al genome from its Pl-progeny A publication by Maruthachalam & Chan (Nature 464, 615-619; 2010) teaches that the ,rans"ormation 0' Arabidopsis thaliana cenh3 mutant plants with an overexpression construct for a NH3—tailswap protein results in an aberrant mitotic division in the zygote, ing ization. During this aberrant mitosis the al chromosom s ar s l ctiv ly liminated jrom the dividing zygote. As the CENHB protein is universal in eukaryotes and its function is very wel' conserved this strategy "rom Arabidopsis is widely app:_icable in Other plant species.

In order to create a Brassica ea plant in whose Fl progeny the maternal genome is selectively ated, a ca plant was created that lacks a 'Jnctional version of the protein that is orthologous to C?NH3 from opsis. This was ed by means 0; an QWAi approach. This plant was subsequently genetically transformed by means 0: Agrobacterium in:fection, with the GFP-CENHB-tailswap construct described by Maruthachalam & Chan (Nature 464, 615—619; 20l 0). Due to the lethality o: the homozygous cenh3 mutant plant (as is the case in Arabidopsis), it was necessary to transform a heterozygous silenced plant with this ht. Transformant plants were subsequently selected based on the construct’s selection marker, and the presence and correct expression of the GFP— CENHB—tailswap fusion protein could be detected with 2O fluorescence microscopy during mitotic division.

EXAMPLE 3 Combining the quartet phenotype with maternal genome e7imination in Arabidopsis This example illustrates how the genotype I) 0; a hybrid plant can be e ”iciently reconstituted.

A transgenic Arabidopsis thaliana plant 0: the Ler ion was created, harbouring a single copy (in gous state) ol an RNAi construct that targets the QRTl gene (At5g55590), driven by the CaMV 358 constitutive er. Alternatively other techniques such as artificial micro—RNAs (amiRNAs) can be used for this purpose. This plant was then crossed to a wild-type Arabidopsis thaliana plan, ol the Ws accession.

The resulting F1 generation consisted 0: hybrid plants with a mixed Ler/Ws background, which were hemizygous for the QNAi construct. Because the RNAi construct aCts sporophytically and in a dominant manner, al' F’ plants exhibited the quartet pollen phenotype. An F’ p'ant — exhibiting the quartet pollen phenotype — was then d as a father with an Arabidopsis thaliana cenh3 mutant plant o: the Col-O accession, which had been genetically transformed with the GFP—CENH3-tailswap construct as reported by Maruthachalam & Chan (Nature 464, 9; 2010). This cross resu'ted in the elimination o: the al chromosomes during the first mitotic division in the zngte, leading to the "ormation O" haploid seeds. "n preparation for the crossing, nearly dehiscent anthers of the father plant were opened under a lar cope to allow the colleCtion 0" individual, ripe pollen tetrads. Each pollen tetrad was carefully ted 2O onto the pistil o: a mother plant, using an eyelash or a fine brush hair, and the four pollen grains were allowed to "ertilixe "our . The four seeds resul ling from this limited pollination with a single pollen te':rad were allowed to mature, and were subsequently harvested and allowed to germinate. Alternatively, more than one tetrad can be deposited onto the pistil o: the mother plant, but care should be taken that the number 0: pollen grains does not xc d th av rag number 0: ovules in the female reproductive par, ol the species. The use 0: more than one tetrad "or pollination will namely reduce the e "iciency with which genetically complementary progeny plants can be identified.

The ploidy o: the seedlings resulting from the d po" ination o _ the transgenic Col-O mother plant was tested by _ ow cytome'try. Their ploidy was n, except in some cases in which spontaneous genome doubling to 2n had meanwhile occurred. For haploid individuals genome doubling was subsequently accomplished by standard methods known to the skilled person (e.g. colchicine treatment). Once the four ngs resu' ting from this cross were 2n, their genomic DNA was isolated and genetically analysed for genetic markers covering the entire opsis genome. ally markers polymorphic between Col-O and Ler and between Col-O and Ws were tested , to distinguish the contribution 0: both paren':al genomes to the four progeny plants. Due to the ation o : the maternal genome from the rygo ,es ,he four progeny plants only contained recombined chromosomes from the hybrid father plant, and h nc th y t st d n gative for all Col—O—specific markers.

Figure 2 shows that the occurrence rate 0: progeny plants with a certain number 0: complementary chromosomes 2O within a pollen tetrad is dependent on the haploid some number (n). opsis has five chromosomes, and because of ,he quartet pollen phenotype in the al father plan ,he chromosome constellation of the four seedlings relets from a single meiotic division. This implies ,hat ,here is a theoretical chance of l in 16 (6.3%) for two "u' ly complementary genomes to be present in a single pol:_en tetrad. The chance is 3l.3% for having two genomes that have one non-complementary chromosome, and 62.5% for having two genomes that have two non-complementary chromosomes. In all cases the individual pollen grains in a tetrad are there:fore at least 50% complementary to each other. "mportant' le _ ination results in e.g. 40% heterozygosity, the overall complementarity between the 2012/069191 genome o: the individual pollen grains will always be higher than 50%, because even the “non-complementary” somes would then still be 60% complementary to each other.

Therefore, in theory only l6 individual crosses with a pollen tetrad are required on average to identify two Arabidopsis seedlings that have essentially complementary sets 0: chromosomes. In practice, however, more are needed because the e ”iciency o: the pollination, seed formation, and plant germination and al is generally below 100 percent. As a rule 0: thumb 10 times more crosses were done with single tetrads, i.e. 160 in this case, to maximize the chance 0: success.

After fication, two genetically essentially complementary plants were grown to anthesis and then crossed. The genetic constitution of the Fl y resu'ting "rom this cross was experimentally shown to be essentially identical to the genetic tution 0: its paternal grand ather, i.e. the grtl mutant plant with a Ler/ Ws hybrid background.

EXAMPLE 4 Combination of the quartet phenotype with maternal genome elimination in Arabidopsis, with non—transgenic progeny In Example 3 the F1 progeny remained transgenic, because it ed the RNAi construct targeting the QRTl gene, and hence also the quartet pollen phenotype. r, when a GFP reporter te that specifically expresses the Green Fluorescent Protein in mature pol'en grains is integrated into the RNAi construct used in Example 3, this allows another approach, in which the progeny plants are not transgenic. The T—DNA construct then also contains a GFP protein with a nuclear localization signal under a late- pollen—specific promoter (the LAT52 promoter; Twell et al, 1990, Development L09: 705-713), which allows the easy visual ion 0: this construct in mature pollen grains.

In anther es o: the Ler/Ws hybrid plant mentioned in Example 3 ygous for the RNAi construct targeting QRTl), pollen tetrads were selected in which two of the "our pollen grains did not express GFP in their nucleus, either visually (using a fluorescence binocular or microscope) or by FACS (fluorescence—activated cell sorting). Only two of the "our pol'en grains thus contained the RNAi construct targeting the QRTl gene. Pollination o; the above-mentioned Col-O mother plant - which induced the elimination of the al chromosomes during the first mitotic division in the zygote - with s JCh a pollen tetrad gave rise to two transgenic haploid progeny plants (harbouring the RNAi construct) and to :wo non-transgenic haploid progeny plants (not harbouring the RNAi construct).

The non—transgenic progeny plants per definition lacked the Ler chromosome fragment harbouring the RNAi— construct for QRTl, and the crossing of these two plants can 2O thus never lead ,0 the exact reconstitution o: the original Fl , e the reconstituted plants will be homozygous for chromosome ] a, least one Ws region, namely :or the chromosome region which corresponds :o the chromosome region that contains the RNAi-construct for QRTl in the Ler parent plant.

EXAMPLE 5 Combining the quartet pollen phenotype with second division restitution in sweet pepper (Capsicum annuum) When second division restitut ion (SDR) occurs, the second meiotic division does not take place, and the result o: meiosis will be two diploid po'len grains, instead 0; four haploid pollen grains. Th r for wh n , th m iotic 2012/069191 products remain physically attached to each other — as is the case with the quartet pollen phenotype — a plant will produce pollen dyads when SDR occurs. "n this preferred embodiment o: the current invention, the two diploid meiotic products remain physically attached to each other, and because their chromosomes have identical recombination break points the two pollen grains are 100% genetically complementary to each other.

A sweet pepper plant (Capsicum annuum) exhibiting the quartet pollen phenotype was obtained through an RNAi approach, similarly as described in Example 3. A progeny plant thereo: — homozygous for the quartet phenotype — was subjeCted to cold stress, in order to se the frequency 0: unreduced microspore (gamete) formation, as described by Zhang et al. (2002) 7 of Horticu7tura7 Science & Biotechnology 78: 84-88 and in examp'e 2 of ).

Up to 25% of the microspores and pollen ed in the anthers of ,he cold-ureaued plant had ,he "orm O" a dyad.

Isolated microspore fracuions were fur,her enriched for 2O dyads by microscopic analysis (alternative'y flow cytometry can be used). Using this approach, the app'ication referred to as Near-Reverse Breeding () could be enabled in a preferred embodiment.

For this purpose, a second sweet pepper plant was created which eliminates the maternal genome from its zngtes during the first mitotic division, “o'lowing the experimental approach outlined in e 2. This transgenic sweet pepper plant was ated with a single pollen dyad derived from the mentioned cold-tr at d sw t p pp r plant, and the resulting two diploid seeds were harvested and germinated. The pollen dyad was ed manually in an anther squash from among the pollen quartets that resulted from non—SDR c events.

The plants grown from these two diploid seeds were subsequently crossed, and — using genetic mar<ers — the genetic composition 0: the progeny plants or ,his cross could be confirmed as being essentially identical to the c composition 0: the sweet pepper plant that produced the po"en dyad we used for pollination. However, due to cross-over events that had occurred during the ‘ormation 0' the po"en dyads some telomeric variation had been introduced, which provided onal genetic ion in the selected hybrid ound.

Thus, the sweet pepper plant created in this example by pollinating a haploid inducer mother plant with a pollen dyad produced by another sweet pepper plant was genetically only “essentia"y identical” to the sweet pepper plant that ed the po"en dyad for pollination, because additional genetic ion had been introduced at the telomeres, as a resu't O" cross—over events that had occurred during the ‘ormation o: the pollen dyads. All genetic material 0: ,he Lather plant had been maintained, 2O but some paros or it had been rearranged through cross-over events, and this rearrangement may cause additional phenotypic e ecos.

This e thus allows jor ,he introduction I) additional genetic variation in a seleCted ) hybrid plant. This additional variation may have positive or negative additional phenotypic e "ecLs, when compared to the original hybrid phenOtype, and it thus provides an interesting opportunioy to jurther improve (and/or fine— tune) a hybrid ype, without losing the combination I) selected traits comprised in the original hybrid.

Alternatively, the sweet pepper plant exhibiting the quartet phenotype can be crossed with a sweet pepper plant that naturally exhibits an above-average degree or SDR. Their progeny will then naturally produce an above- average percentage 0“ pol 'en dyads. Another strategy is to mutagenize a tion 0: sweet pepper plants that naturally exhibit an above-average degree o: 83?, and to screen for plants displaying the t pollen phenotype in this mucano population, in a ‘orward genetics approach.

In each and every dyad the chance is 100% that the two pollen grains are genetically essentially complementary to each other (with identical somal break-points). The two microspores comprised in a dyads are per definition genetically essentially complementary, and crossing the two plants that can be derived from any one o: these dyads will always result in a hybrid plant that is essentially genetically identical to the original hybrid p— ant that produced the dyads. This embodiment thus greatly improves the e ”iciency o: th N ar R v rs 3r ding technology.

Generally, in this embodiment o: the invention a plant carrying a genetic jeature that causes the elimination O: the al chromosomes from its geny is 2O pollinated through limited pollination. The pollen used in this cross is a single pollen dyad obtained from a hybrid plant 0: the same species, which exhibits the quartet pollen phenotype in combination with SDR. This pollen dyad is obtained by visually screening squashed anthers and by subsequently selecting a dyad cons ,ellation jrom among the tetrad constellations (which resul ,ed jrom meiotic divisions during which SDR had not occurred), or by ing a pollen "raction "or dyads — in a more high-throughput g — by means 0 "low cytometry or cell sorting.

A‘ter pol" ination E 0' the mother plant ng a genetic jeature tha causes the elimination o: the maternal chromosomes from its Fl—progeny, each 0. the ,wo diploid pollen grains sed in the dyad jer,ilise an ovule, and seed is allowed ,0 form and mature. Upon ripeness the two seeds resu'ting rom ,his cross are harvested and germinated. The ing seedlings are then tested for their ploidy level by means 0 "low try, to confirm that they are indeed 2n, as would be expeCted.

Subsequently, genomic DNA is isolated from the two ngs, and genetic markers covering the entire genome are tested in both individuals. Due to the elimination o; the maternal chromosomes from the zygote, both seedlings are expeCted only to possess paternal chromosomes. Due to the fact ,hat the chromosomes from both seedlings originated in a single meiotic division, all chromosome break points are identical, and their genomes are 100% complementary. This is confirmed by the genome-wide marker analysis.

The seedlings are subsequently grown to maturity and d with each other. Their Fl progeny is genetically essentially identical to the original starting hybrid plant that was used as a laoher in the first cross, except for cross-over events that have occurred during the "orma,ion I] 2O the SDR gametes. These cross-over events introduce some telomeric ion, which provides additional genetic variation in the selected hybrid ound.

The xact reconstruction of the (hybrid) genotype 0: the al grandfather organism was achieved in only two generations’ time and without the need :or intermediate regeneration or genome doubling steps or tissue culture.

EXAMPLE 6 Combining the quartet pollen phenotype with suppression of some recombination and maternal genome elimination in Arabidopsis From a cross between an Arabidopsis plant that exhibits the quartet pollen phenotype and anOther Arabidopsis plant in which chromosome recombination is partially or completely ssed (either by enic, mutational or al means), an F2 progeny plant can be selected in which both properties are combined: the four po'len grains resulting "rom a single meiotic division remain physically attached to each other to the time I) up o; anthesis and pollen shedding, and during s the recombination o: homologous chromosomes is suppressed.

This progeny plant allows the application 0; Reverse Breeding (WO 03/017753; Dirks et al 2009) in a more e "icient way. Because the meiotic products remain physically attached to each other, the chance 0" identi‘ying two pollen grains with essentially complementary sets ol 2O chromosomes is greatly increased. The number 0" di""erent tetrads is a on 0‘ the chromosome number ol the species, but within a single tetrad, however, the chance that two pollen grains have fully complementary sets 0; chromosomes is always 50%, as the tour pollen grains are pairwise complementary. Within each and every tetrad the chance 0 g two pairs 0: complementary pollen grains is thus lOO%.

As described in Example 3, an Arabidopsis thaliana Ler plant was created, harbouring a single copy (in homozygous state) 0: an RNAi construct that targets the QRTl gene, driven by the CaMV 358 constitutive promoter. This plant was then crossed with an opsis thaliana plant I) the Ws accession which harboured I) a homozygous single copy 0; an RNAi construct that targets the DMCl gene, also driven by the CaMV 358 constitutive er.

The resulting F1 generation consisted of hybrid plants with a mixed Ler/Ws background, which were all hemizygoas for both BNAi construCts. Because the RNAi constructs both junction sporophy':ically and in a dominant , the Fl plants exhibi':ed the quartet pollen phenotype and suppression o: chromosome recombination during meiosis.

As an undesired side-e ect the suppression o: chromosome recombination also ed in the ence 0: unbalanced pollen tetrads (alongside balanced tetrads), because in the absence 0 su "icient UHC ,ional DMCl protein each chromosome is randomly dis:ributed over the daughter cells 0: a division. However, balanced tetrads could be selected from s experimentally (through visual inspection and/or flow try).

Balanced pollen tetrads derived from an Fl plant — exhibiting the quartet pollen pheno type in combination with suppression o: chromosome recombina :ion, being hemizygous for both transgenic cons:ructs — were subsequently used to pollinate Arabidopsis thaliana cenh3 mutant plants ol the Col-0 accession (as crea:ed and ed by Maruthachalam & Chan, 2010), which had been genetically trans formed with the GFP—CENH3-tailswap construct reported by Maruthachalam & Chan (2010), which results in the ation of the maternal chromosomes during the ll”S mitotic division in the zygote.

When 1 a single balanced pol: _en tetrad was used :or pollination O" a pistil O the mother plant, this cross ideally resulted in tour haploid progeny seeds. The four y seeds were allowed to germinate, and the seedlings were genetically tested with s. Because no chromosome recombination had occurred during meiosis (due to the RNAi construct targeting DMCl all chromosomes were passed onto the next generation in their entirety), only very few markers needed to be tested for each chromosome, to determine r it was inherioed from the Ler grandparent or from the Ws grandparent (in fac, a single rphic mar<er per chromosome would be su "icient; in our experimental approach we used one marker per chromosome arm, i.e. two markers per chromosome).

To ensure that the mother plant had not contributed genetically to the progeny also Col-O specific markers were , but no Col-O c material could be identified, as was also the case in Example 3.

The four progeny plants were found to be pairwise perfectly genetically complementary, and based on the genetic marker ana'ysis these pairs could be e ”iciently idenoified. Crossing such a pair 0: genetically complementary progeny plants to each other resulted in the e "icient genetic reconstitution o: the original hybrid plant that produced the pollen tetrad that was used :OI pollination, i.e. the Ler/Ws hybrid plant gous :OI both RNAi construC' Figure 4 illustrates the pedigree I] :s. o; the plants used in this experiment.

Claims (33)

1. Method for the production of a set of seeds which are genetically cal to the male s from which they arise, comprising: a) placing a limited number of paternal gametes that have the form of tetrads or dyads on the stigma of a flower to fertilize maternal egg cells to obtain a number of s, wherein the number of al gametes is limited to be equal or lower than the number of egg cells contained in the female reproductive organ carrying the stigma; b) inducing the loss of maternal chromosomes from the zygotes to obtain a seed set containing a limited number of seeds in which the maternal chromosomes are absent.

2. Method as claimed in claim 1, wherein the limited number of paternal gametes is equal to or lower than the number of egg cells contained in the female reproductive organ carrying the stigma.

3. Method as claimed in claim 1 or 2, wherein the limited number of paternal gametes is two or four.

4. Method as claimed in any one of the claims 1-3, wherein the paternal gametes that have the form of tetrads or dyads are the result of erence with microspore tetrad separation.

5. Method as claimed in claim 4, wherein interference with microspore tetrad separation comprises interference with one or more target genes involved in the break-down of the pectin layer between the microspores resulting from a single meiotic division.

6. Method as claimed in claim 5, wherein the one or more target genes are selected from the group consisting of QRT1 , QRT2 , QRT3 , or their functional homologues.

7. Method as claimed in claim 4, wherein interference with microspore tetrad separation is achieved by chemical means.

8. Method as claimed in any one of the claims 1-7, n the father plant exhibits suppression of some recombination.

9. Method as claimed in any one of the claims 1-7, wherein the father plant ts second division restitution (SDR) during s.

10. Method as claimed in claim 8, wherein suppression of chromosome recombination is ed by interfering with one or more target genes involved in recombination.

11. Method as claimed in claim 10, wherein the one or more target genes are involved in double strand breaks, such as SPO11 , MER1 , MER2 , MRE2 , MEI4 , REC102 , REC104 , REC114 , MEK1 /MRE4 , RED1 , HOP1 , RAD50 , MRE11 , XRS2 , or their functional homologues, or wherein the one or more target genes are involved in chromosome pairing and/or strand exchange, such as RHD54 /TID1 , DMC1 , SAE3 , RED1 , HOP1 , HOP2 , REC8 , MER1 , MRE2 , ZIP1 , ZIP2 , MEI5 , RAD51 , RAD52 , RAD54 , RAD55 , RAD57 , RPA , SMC3 , SCC1 , MSH2 , MSH3 , MSH6 , PMS1 , SOLODANCERS , HIM6 , CHK2 , or their functional homologues, or wherein the one or more target genes are involved in the meiotic ination process, or their functional homologues, or wherein the one or more target genes are selected from the group consisting of PRD1 , PRD2 , PRD3 , PHS1 , NBS1 , COM1 , MND1 , MER3/RCK , ZIP3 , ZIP4, PTD , SHOC1 , ZYP1 , MLH1 , MLH3 , or their functional homologues.

12. Method as claimed in claim 11, wherein the one or more target genes involved in the meiotic recombination process are SGS1, MSH4, MSH5, ZIP1 and/or ZIP2.

13. Method as claimed in claim 5 or 10, wherein the interfering with the one or more target genes consists of ting transcription thereof.

14. Method as claimed in claim 13, wherein ription is prevented by means of RNA oligonucleotides, DNA oligonucleotides or RNAi molecules directed against the target gene promoter, or wherein transcription is prevented by means of the expression of a negatively acting transcription factor acting on the target gene promoter.

15. Method as claimed in claim 5 or 10, wherein the interfering with the one or more target genes ts of destabilizing the target gene mRNA or transcript, or wherein the interfering with the one or more target genes consists of inhibiting the target gene expression product.

16. Method of claim 15, wherein the target gene mRNA or transcript is destabilized by means of nucleic acid molecules that are complementary to the target gene mRNA or transcript, selected from the group consisting of nse RNA, RNAi molecules, Virus-Induced Gene Silencing (VIGS) molecules, pressor molecules, RNA oligonucleotides or DNA oligonucleotides.

17. Method of claim 15, wherein inhibiting the target gene expression product is by means of the expression product(s) of one or more dominant negative nucleic acid constructs.

18. Method of claim 15, wherein inhibiting the target gene expression product is by means of one or more chemical compounds.

19. Method as claimed in claim 5 or 10, wherein the ering with the one or more target genes consists of the introduction of one or more mutations into the target gene, leading to perturbation of its biological function.

20. Method as claimed in claim 19, n the one or more mutations are introduced randomly by means of one or more chemical compounds, and/or by physical means, and/or by insertion of genetic elements and/or wherein the one or more mutations are introduced ically by means of homologous recombination or oligonucleotide-based mutation induction.

21. Method as claimed in claim 20, n the one or more chemical compounds selected from methanesulphonate, nitrosomethylurea, hydroxylamine, proflavine, N-methyl-N- nitrosoguanidine, N-ethyl-N-nitrosourea, N-methyl-N-nitronitrosoguanidine , diethyl sulphate, ethylene imine, sodium azide, formaline, urethane, phenol and ethylene oxide.

22. Method as claimed in claim 20, wherein the al means comprise UV-irradiation, fast-neutron exposure, X-rays, gamma irradiation.

23. Method as claimed in claim 20, wherein the insertion of genetic elements includes transposons, T-DNA, iral elements.

24. Method as claimed in claim 9, wherein second division restitution occurs spontaneously, in particular without interference with the starting organism.

25. Method as d in claim 9, wherein second division restitution is induced by means of genetic modification, wherein the genetic modification is transient, or n the genetic modification is achieved by stable oration into the genome of a genetic t increasing the number of second division restitution events in the organism, or wherein second division restitution is achieved by subjecting the father plant to environmental stress, such as ature stress, NO2, nitrous oxide (N2O), or combinations thereof.

26. Method as d in any one of the claims 1- 17, wherein the loss of maternal chromosomes from the zygote is induced by using a haploid inducer line as the female.

27. Method as claimed in claim 26, wherein the female is a plant of a ent species.

28. Method as claimed in any one of the claims 1- 26, wherein the female plant is a transgenic plant that comprises a heterologous ene expression cassette, the expression te comprising a promoter operably linked to a polynucleotide encoding a recombinantly altered CENH3, CENPC, MIS12, NDC80 or NUF2 polypeptide, and having a corresponding inactivated endogenous CENH3 , CENPC , MIS12 , NDC80 or NUF2 gene.

29. Set of seeds containing a limited number of seeds in which the maternal chromosomes are absent, which set is composed of pairs of genetically complementary seeds which when plants grown from the seeds are crossed result in essentially the same hybrid, and which seed set is obtainable by a method as claimed in any one of the claims 1-28.

30. Method for providing a set of parent plants for the production of a plant of which the genetic constitution is essentially identical to the c constitution of its male grandparent, comprising growing plants from seeds of the set of seed as claimed in claim 29, after or prior to doubling the chromosome number of the seeds, and identifying two genetically complementary plants as the parent plants.

31. Method as claimed in claim 30, n the set of seeds or the plants grown thereof are screened for their genetic constitution, to identify a plant of which the genetic constitution is essentially identical to the genetic constitution of its paternal ather, and to identify another plant of which the genetic constitution is essentially identical to the genetic constitution of its paternal grandmother.

32. Method as d in any one of the claims 29- 31, wherein a plant of which the genetic constitution is ially identical to the genetic constitution of its paternal grandfather, is crossed to another plant of which the genetic tution is essentially identical to the genetic constitution of its paternal grandmother, in order to obtain progeny plants of which the genetic constitution is essentially identical to the c constitution of their own grandfather.

33. A method according to claim 1, substantially as herein described with reference to any one of the accompanying examples and/or figures thereof.