DESCRIPTION TITLE OF INVENTION METHOD FOR ENGRAFTING GERM CELLS Technical Field [0001] The present invention relates to a method for improving the ability of transplanted isolated germ cells to colonize in the gonad of host fish and thereby enhancing transplantation efficiency in a method for inducing differentiation into a germ line by the transplantation of isolated germ cells (for, e.g., surrogate parent fish farming which involves transplanting isolated germ cells to host fish to thereby induce their differentiation into the germ line, the isolated germ cells being derived from fish differing in strain or species from the host fish). Background Art [0002] Many researchers have attempted to establish ES cell lines of fish so far. Fish cell lines similar in morphological and biochemical features to mouse-derived ES cells have been established from blastula cells of Oryzias latipes (Protein, Nucleic acid and Enzyme, 40, 2249-2256, 1995; and Fish Phys. Biochem. 22, 165-170, 2000), zebrafish (Methods Cell Biol. 59: 29-37, 1999), and Sparus aurata (Biomolecular Engineering, 15, 125-129, 1999). These cells have already been confirmed to differentiate 1 into various somatic cells after being transplanted to a host embryo around the blastula stage. [0003] In spite of these attempts by many researchers to establish ES cell lines of fish, as described above, none of them have published a paper showing that fish ES-like cells differentiate into a germ line and thereby contribute to the production of a next generation. In fact, cells cultured only for a few days in vitro differentiate into a germ line (Proc. Natl. Acad. Sci. USA, 98, 2261-2266, 2001), but their ability to differentiate into germ cells rapidly disappears as the culture period is prolonged. [0004) Meanwhile, an attempt has also been reported to produce fish individuals using isolated cells transplanted to a host embryo or the like and thereby incorporated in the germ line. For example, it has been reported as to the production of germ-line chimeras in Oncorhynchus mykiss by blastomere transplantation that Oncorhynchus mykiss egg-derived blastodermal cells labeled with FITC dextran were dispersed, then collected, and transplanted to a host embryo, resulting in the incorporation of the transplanted labeled cells into the germ line to produce offspring (Mol. Rep. Dev., Vol. 59, p. 380-389, 2001). The method of this report has the technical disadvantage that these chimeras are prepared only from limited related species. There is also a report on the preparation of transgenic Oryzias latipes by incorporating gene constructs containing the GFP gene linked to the vasa gene 2 promoter of Oryzias latipes to expression vectors, which are in turn introduced to eggs. This report discloses that GFP is strongly expressed in primordial germ cells (PGCs) located in the dorsal region of the intestinal membrane of the transgenic Oryzias latipes (Proc. Natl. Acad. Sci. USA, Vol. 98, No. 5, p. 2544-2549, 2001). [00051 It has further been reported as to a method for preparing germ-line chimeras by germ layer transplantation between goldfish and crucian carp that crucian carp derived primordial germ cells (PGCs), after being transplanted to goldfish embryos or mesoderms, migrate to the genital ridges and further differentiate into various cells including germ cells (Genetica, Vol. 111, No. 1-3, p. 227-236, 2001). This report relates to a method for transplanting cells isolated from fish to other fish, but not to the transplantation of isolated cells of fish involving transplanting the germ cells isolated from fish to host individuals of other fish so that the differentiation of the isolated germ cells into the germ line is induced in the gonads. [0006] A mechanism by which germ cells are discriminated from somatic cells during the course of differentiation in fish remains to be elucidated. In recent years, however, data has been shown, suggesting the possibility that maternal factors, such as RNAs or proteins, accumulated in eggs during maturation in the parent ovary play an important role in the determination of the germ line (KAIYO Monthly, 31-5, 266-271, 1999). Only some 3 blastomeres receive these maternal factors by cell division due to the uneven distribution of these factors in fertilized eggs. As a result, only these cells that have received the maternal factors probably differentiate into a future germ line. By contrast, an undifferentiated population of mouse ES cells, which are known to differentiate into a germ line, is considered to differentiate into germ cells by stimulation from their neighboring cells (Protein, Nucleic acid and Enzyme, 43, 405-411, 1998). [0007] Heretofore, attempts have been made to modify or clone animal individuals for the preparation of transgenic or cloned animals in vertebrates such as fish. Unfortunately, a technique of transplanting such genetically modified cells or cells intended for cloning to host individuals and inducing their differentiation for transformation as new individuals has not yet been established for vertebrates such as fish. Thus, for genetically modifying or cloning individuals of vertebrates such as fish and breeding them or preparing cloned animals, an important challenge is to establish the technique of transplanting modified or isolated cells to host individuals and inducing their differentiation for transformation as new individuals. [0008] Thus, the present inventors have previously conducted diligent studies on a method for transplanting genetically modified or isolated cells to host individuals to thereby induce their differentiation into the germ line, 4 particularly in the case of cold-blooded vertebrates such as fish and a method for propagating or breeding vertebrates such as fish using the differentiation induction method. Consequently, the present inventors have found that: (1) in the case of vertebrates such as fish, maternal factors, such as RNAs or proteins, accumulated in eggs during maturation in the parent ovary play an important role in the determination of the germ line, while only some blastomeres receive these factors by cell division due to the uneven distribution of these maternal factors in fertilized eggs and as a result, only these cells that have received the maternal factors probably differentiate into a future germ line; (2) in consideration of such a germ-line determination mechanism, vertebrates such as fish "involves the maternal factors that determine differentiation into germ cells"; and (3) in consideration of these facts, in the case of vertebrates such as fish, cells to be used for transplanting genetically modified or isolated cells to host individuals to thereby induce their differentiation into the germ line (i.e., cells capable of differentiating into eggs or sperms, after being transplanted to host individuals, to achieve transgenic individuals in the next generation) are not undifferentiated embryonic cells, but are germ cell stem cells determined to differentiate into future germ cells, i.e., primordial germ cells (germ cells). 10009] The present inventors have further found that the germ cells can be transplanted to perihatching fish 5 individuals of vertebrates such as fish to thereby induce the differentiation of the germ cells into the germ line, i.e., isolated germ cells derived from vertebrates such as fish can be transplanted to perihatching fish individuals of host vertebrates, particularly, to the dorsal regions of the intraperitoneal intestinal membranes of fish individuals in the perihatching developmental stages, to thereby induce the differentiation of the germ cells into the germ line, and successfully established a method for inducing the differentiation of isolated germ cells (isolated primordial cells) of fish into a germ line (Japanese Patent No. 4300287). [0010] It has been found that in fish, even testicular cells presumed to be spermatogonia can be transplanted to perihatching fish individuals to thereby induce their differentiation into the germ line, as in the primordial germ cells (Proc. Natl. Acad. Sci. USA, 103, 2725-2729, 2006). In the course of this process, the transplanted spermatogonia differentiate into egg cells, after being transplanted to perihatching female fish individuals, demonstrating that these cells possess sexual plasticity. As a result, the method has established itself as the general transplantation technique of germ cells. [0011] In addition, such transplantation succeeds using fish of the same species as host fish, as a matter of course, and even using fish differing in strain or species from host fish. This allows production of germ cell derived fish differing.in strain or species from host fish 6 by the transplantation of the germ cells to the host fish of different species, i.e., fish production using a surrogate. Unfortunately, this method often ends in the unsuccessful transplantation of the fish differing in strain or species from host fish. Conditions for successful transplantation have been totally unknown. Particularly, fish of the genus Thunnus is difficult to transplant. In the case of using germ cells of Thunnus orientalis, which is a fish species important as cultured fish, the transplanted germ cells of Thunnus orientalis have failed to colonize in Nibea mitsukurii, Scomber japonicus, Scomber australasicus, Trachurus japonicus, Seriola quinqueradiata, Pagrus major, or the like. Thus, successful transplantation of fish has not yet been reported. [0012] Such a method for inducing the differentiation of isolated germ cells of fish into a germ line requires improving the colonization rate of the germ cells introduced in the host fish. Some methods for promoting cell colonization have been disclosed so far. For example, Japanese unexamined Patent Application Publication (Translation of PCT Application) No. 2000-500327 discloses a method for increasing the fertilization potential of sperms during sperm collection, comprising contacting a sample comprising sperms with a solution comprising a polysaccharide containing arabinose, galactose, and/or hexuronic acid. Also, Japanese unexamined Patent Application Publication No. 8-27011 discloses a method -7 using a fetus fixation enhancer for pregnant animals comprising bacterial cells of the genus Bifidobacterium having IgA production-promoting effect as an active ingredient, the method comprising preventing the abnormal development or abortion of a fetus to thereby stabilize the fixation of the fetus. Furthermore, Japanese unexamined Patent Application Publication (Translation of PCT Application) No. 2009-517078 discloses a method for enhancing the ability of cells to colonize for bone marrow transplantation (BMT) or the like, comprising treating a cell population with a predetermined amount of nicotinamide. None of these methods, however, are applicable to improvement in the ability of transplanted germ cells to colonize in the host gonad for the above described method for inducing the differentiation of isolated germ cells of fish into a germ line. Prior Art Documents Patent Documents [0013] Patent Document 1: Japanese unexamined Patent Application Publication No. 8-27011 Patent Document 2: Japanese unexamined Patent Application Publication (Translation of PCT Application) No. 2000 500327 Patent Document 3: Japanese unexamined Patent Application Publication (Translation of PCT Application) No. 2009 517078 Patent Document 4: Japanese Patent No. 4300287 Non-Patent Documents 8 [0014] Non-patent Document 1: Protein, Nucleic acid and Enzyme, 40, 2249 2256, 1995 Non-patent Document 2: Fish Phys. Biochem. 22, 165-170, 2000 Non-patent Document 3: Methods Cell Biol. 59: 29-37, 1999 Non-patent Document 4: Biomolecular Engineering, 15, 125-129, 1999 Non-patent Document 5: Proc. Natl. Acad. Sci. USA, 98, 2261-2266, 2001 Non-patent Document 6: Mol. Rep. Dev., Vol. 59, p. 380-389, 2001 Non-patent Document 7: Proc. Natl. Acad. Sci. USA, Vol. 98, No. 5, p. 2544-2549, 2001 Non-patent Document 8: Genetica, Vol. 111, No. 1-3, p. 227-236, 2001 Non-patent Document 9: KAIYO Monthly, 31-5, 266-271, 1999 Non-patent Document 10: Protein, Nucleic acid and Enzyme, 43, 405 411, 1998 Non-patent Document 11: Proc. Natl. Acad. Sci. USA, 103, 2725-2729, 2006 Summary of the Invention [0015] An aim of the present invention is: to improve the ability of transplanted isolated germ cells to colonize in the gonad of host fish and to enhance transplantation efficiency for induction of the differentiation of the transplanted isolated germ cells into the germ line for, for example, surrogate parent fish farming which involves transplanting isolated germ cells to host fish to thereby induce their differentiation into the germ line, the isolated germ cells being derived from fish differing in strain or species from the host fish. [0016] The present inventors have conducted diligent studies on a method for improving the ability of transplanted isolated germ cells to colonize in the host gonad for, for example, surrogate parent fish farming which involves transplanting isolated germ cells to host fish to thereby induce their differentiation into the germ line, the isolated germ cells being derived from fish differing in strain or species from the host fish. Consequently, the present inventors have found that: a temperature at which the resultant host fish individual is raised influences the activity of protein factors involved in the migration or colonization of the transplanted isolated germ cells into the host, and influences the recovery of the host fish itself from damage caused by the transplantation or the ability of the transplanted germ cells to migrate or maintain division activity; and these factors have a great impact on the ability of the transplanted isolated germ cells to colonize in the host gonad. [0017] Then, the present inventors have transplanted the germ cells into an abdominal cavity of a perihatching host fish and then raised the resultant host fish within a temperature range applicable to the fish being the origin of the germ cells, particularly, a temperature range near the temperature applicable to growth from the spawning to larval and juvenile stages thereof. Consequently, the present inventors have found that: this approach preserves the activity of protein factors involved in the migration or colonization of the transplanted isolated germ cells into the host; and the maintenance of this host fish raising temperature preserves an environment involved in colonization, such as the recovery of the host fish itself from damage caused by the transplantation or the ability of the transplanted germ cells to migrate or their division activity, and can drastically increase the colonization rate of the isolated germ cells in the host fish gonad. The present invention has been completed on the basis of these findings. [0018] Specifically, the present invention provides a method for inducing the differentiation of isolated germ cells into a germ line with the improved ability of the isolated germ cells to colonize in the gonad of host fish, the differentiation induction consisting of transplanting the isolated germ cells into an abdominal cavity of a perihatching host fish individual, the isolated germ cells being derived from fish differing in strain or species from the host fish, the method comprising raising the resultant host fish individual within a temperature range that is applicable to growth from the spawning to larval and juvenile stages of the fish being the origin of the 11 isolated germ cells and allows raising of the host fish. In the present invention, examples of the isolated germ cells can include primordial germ cells, spermatogonia, and oogcnia of the fish being the origin of the isolated germ cells. [0019] For successful surrogate technology based on germ cell transplantation, it is inevitably required that intraperitoneally transplanted donor-derived germ cells should spontaneously migrate to the host gonad and colonize therein. According to the present invention, the germ cells are transplanted into an abdominal cavity of a perihatching host fish, which is then raised within a temperature range near the temperature applicable to the fish being the origin of the germ cells. This approach has been found to be able to reduce the influence of damage on the cells, enhance the efficiency of cell migration, maintain cell division activity, and drastically increase the colonization rate of the isolated germ cells in the host fish gonad. [0020] Transplantation operation physically forms voids in, for example, the peritoneal cavities of host fish individuals and injects thereinto donor cells (isolated germ cells to be transplanted). The sites that have received the transplantation in, for example, the peritoneal cavities of the host fish individuals have large damage immediately after the transplantation. This damage may also damage the transplanted isolated germ cells through induction of many proteolytic enzymes, etc. 12 In addition, the transplanted isolated germ cells can preserve the original activity of protein factors (including host fish chemokines) involved in germ cell migration or colonization, within the original temperature range of the germ cells in the nature. Thus, the transplanted isolated germ cells within this temperature range exhibit the highest repair efficiency for recovering from the damage and the highest ability to migrate or maintain division activity. The germ cells, even after being transplanted, is placed within this temperature range to thereby be able to efficiently recover from the damage, migrate, maintain division activity, and enhance the ability to colonize. [00211 A requirement of the present invention is to raise the resultant host fish individual within the temperature range applicable to the fish being the origin of the isolated germ cells. Particularly, a temperature range applicable to growth from the spawning to larval and juvenile stages of the fish being the origin of the isolated germ cells is adopted as this raising temperature range. The adoption of this growth temperature range can give the transplanted isolated germ cells an environment applicable to the spawning to larval and juvenile stages of the fish being the origin of the isolated germ cells, in the resultant host fish individual. The temperature range that allows raising of the host fish is preferably the range of ±3 0 C of the optimum temperature applicable to growth from the spawning to larval and juvenile stages of the fish being the origin of the isolated germ cells. 13 Particularly preferably, the range of ±1C of the optimum temperature applicable to growth in larval and juvenile stages is adopted. [0022] In the present invention, the adopted temperature at which the resultant host fish individual is raised is a temperature range that is applicable to the growth of the fish being the origin of the isolated germ cells and allows raising of the host fish. The adoption of this temperature range that allows raising of the resultant host fish individual preserves an environment involved in the colonization of the transplanted germ cells, such as the recovery of the host fish itself from damage caused by the transplantation, or the ability of the germ cells to migrate or their division activity. These raising temperature conditions (i.e., the temperature range that is applicable to the growth of the fish being the origin of the isolated germ cells and allows raising of the host fish) can be satisfied by selecting host fish whose raising temperature range can fall within the growth temperature range of fish serving as an origin of the isolated germ cells to be transplanted, and raising the resultant host fish within this growth temperature range. [0023] Specifically, one aspect of the present invention provide a method for inducing the differentiation of isolated germ cells into a germ line using isolated germ cells derived from fish of the genus Thunnus (e.g., Thunnus orientalis) as the isolated germ cells and fish of the genus Sarda or Euthynnus (e.g., Sarda orientalis or 14 Euthynnus affinis) selected as host fish, the method comprising transplanting the isolated germ cells derived from fish of the genus Thunnus into an abdominal cavity of a perihatching host fish individual of the genus Sarda or Euthynnus, and raising the resultant host fish individual at ±3'C of the optimum temperature 26'C for growth from the spawning to larval and juvenile stages of the fish of the genus Thunnus as the origin of the isolated germ cells. [0024] The present invention also encompasses a method for improving the ability of isolated germ cells to colonize in the gonad of host fish for a method for inducing the differentiation of the isolated germ cells into a germ line, the differentiation induction consisting of transplanting the isolated germ cells into an abdominal cavity of a perihatching host fish individual, wherein the isolated germ cells being derived from fish differing in strain or species from the host fish, and the improvement method comprising raising the resultant host fish individual within a temperature range that is applicable to growth from the spawning to larval and juvenile stages of the fish being the origin of the isolated germ cells and allows raising of the host fish. [0025] Specifically, the present invention provides (1) a method for inducing differentiation of an isolated germ cell into a germ line with improved ability of the isolated germ cell to colonize in a gonad of a host fish, consisting of transplanting the isolated germ cell derived from fish differing in strain or species from the host 15 fish to a host fish by a transplant into an abdominal cavity of a perihatching host fish, wherein the method comprises raising the implanted host fish within a temperature range applicable to growth from spawning to larval and juvenile stages of the fish being the origin of the isolated germ cell and which temperature range allows raising of the host fish, and (2) the method for inducing differentiation of an isolated germ cell into a germ line with improved ability of the isolated germ cell to colonize in a gonad of a host fish according to (1), wherein the temperature range that allows raising of the host fish is within a range of ±30C of an optimum temperature applicable to growth from spawning to larval and juvenile stages of the fish being the origin of the isolated germ cell. [00261 The present invention also provides (3) the method for inducing differentiation of an isolated germ cell into a germ line with improved ability of the isolated germ cell to colonize in a gonad of a host fish according to (1) or (2), wherein the isolated germ cell is a primordial germ cell, spermatogonia, or oogonia of the fish being the origin of the isolated germ cells, and (4) the method for inducing differentiation of an isolated germ cell into a germ line with improved ability of the isolated germ cell to colonize in a gonad of a host fish according to (1) or (2), comprising using an isolated germ cell derived from fish of the genus Thunnus as the isolated germ cell and fish of the genus Sarda or Euthynnus as the host fish, and comprises transplanting the isolated germ cell derived 16 from fish of the genus Thunnus to a host fish of the genus Sarda or Euthynnus by a transplant into an abdominal cavity of the perihatching host fish, and raising the implanted host fish at ±3'C of the optimum temperature of 26*C for growth from spawning to larval and juvenile stages of the fish being the origin of the isolated germ cell. [0027] The present invention further provides (5) the method for inducing differentiation of an isolated germ cell into a germ line with improved ability of the isolated germ cell to colonize in a gonad of a host fish according to (4), wherein the isolated germ cell derived from fish of the genus Thunnus is an isolated germ cell derived from Thunnus orientalis, and the host fish of the genus Sarda or Euthynnus is Sarda orientalis or Euthynnus affinis. [00281 The present invention further provides (6) a method for improving ability of an isolated germ cell to colonize in a gonad of a host fish in a method for inducing differentiation of the isolated germ cell into a germ line, consisting of transplanting the isolated germ cell derived from fish differing in strain or species from the host fish to a host fish by a transplant into an abdominal cavity of a perihatching host fish, wherein the improvement method comprises raising the implanted host fish within a temperature range applicable to growth from spawning to larval and juvenile stages of the fish being 17 the origin of the isolated germ cell and which temperature range allows raising of the host fish. Effect of the Invention [0029] The present invention can improve the ability of transplanted isolated germ cells to colonize in the gonad of host fish and thereby can enhance transplantation efficiency in a method for inducing differentiation into a germ line by the transplantation of isolated germ cells for, for example, surrogate parent fish farming which involves transplanting isolated germ cells to host fish to thereby induce their differentiation into the germ line, the isolated germ cells being derived from fish differing in strain or species from the host fish. Thus, for example, isolated germ cells derived from fish of the genus Thunnus can be transplanted to host fish (including fish of the genus Sarda) to thereby induce their differentiation into the germ line. Brief Description of Drawings [00301 [Figure 1] Figure 1 is a diagram showing the monitoring of donor cells in the peritoneal cavity of a host. [Figure 2] Figure 2 is a diagram showing the detailed observation of the donor cells under a confocal microscope. [Figure 3] Figure 3 is a diagram showing in situ hybridization using Thunnus orientalis vasa probes. Mode of Carrying Out the Invention 18 [0031] The present invention provides a method for inducing the differentiation of isolated germ cells into a germ line with the improved ability of the isolated germ cells to colonize in the gonad of host fish, the differentiation induction consisting of transplanting the isolated germ cells into an abdominal cavity of a perihatching host fish individual, the isolated germ cells being derived from fish differing in strain or species from the host fish, the method comprising raising the resultant host fish individual within a temperature range that is applicable to growth from the spawning to larval and juvenile stages of the fish being the origin of the isolated germ cells and allows raising of the host fish. [00321 In the present invention, any cell that has the ability to differentiate into a germ line after being transplanted can be used as the isolated germ cells of fish used in transplantation. Examples thereof include primordial germ cells, spermatogonia, and oogonia. Primordial germ cells are particularly preferable in terms of high differentiation activity. Also, spermatogonia are preferable because they are easily obtained and a large number thereof can be prepared. [0033] Any fish can be selected as the origin of the isolated germ cells. Examples of particularly useful fish can include fish of the genus Thunnus. The fish of the genus Thunnus, also called tuna, belongs to the family Scombridae within the suborder Scombroidei of the order 19 Perciformes. Specific examples thereof can include Thunnus orientalis, Thunnus obesus, Thunnus maccoyii, Thunnus albacares, Thunnus alalunga, Thunnus atlanticus, and Thunnus tonggol. Among them, preferable examples can include Thunnus orientalis. [100341 Any hatched larval fish that can be raised while surviving donor cell transplantation may be used as the host fish in the present invention. Fish that can be raised within the growth temperature range of the fish being the origin of the isolated germ cells can be selected as the host for improving the ability of the transplanted isolated germ cells to colonize in the host gonad. Specifically, the host fish is preferably fish that has been spawned at a temperature included in or around the growth temperature range of the fish being the origin of the isolated germ cells and can undergo larval and juvenile stages in substantially the same sea area thereas. [0035] Fish of the genus Sarda or Euthynnus can be selected as the host fish for transplanting isolated germ cells of, for example, fish of the genus Thunnus such as Thunnus orientalis. The fish of the genus Sarda belongs to the family Scombridae within the suborder Scombroidei of the order Perciformes. Specific examples thereof include Sarda orientalis and Maldives fish. Among them, preferable examples can include Sarda orientalis. Also, the fish of the genus Euthynnus belongs to the family Scombridae within the suborder Scombroidei of the order 20 Perciformes. Among others, preferable examples thereof can include Euthynnus affinis. [0036] The transplantation of isolated germ cells of fish of the genus Thunnus such as Thunnus orientalis will be described below. For improving the ability of Thunnus orientalis-derived isolated germ cells to colonize in the host gonad, it is preferred to use fish that has been spawned in a high-water temperature region as in Thunnus orientalis and undergoes larval and juvenile stages in the substantially same sea area thereas. Fish that meets such conditions is for example, Sarda orientalis belonging to the genus Sarda of the family Scombridae or Euthynnus affinis belonging to the genus Euthynnus of the family Scombridae, which is preferably selected as the host. Thunnus orientalis larvae or juveniles usually grow at 260C, while those of Sarda orientalis or Euthynnus affinis can be raised at 250C. By contrast, the larvae or juveniles of Nibea mitsukurii, Scomber japonicus, and Scomber australasicus, which have previously produced unsuccessful results in such transplantation, are usually raised at 20*C to 220C and are difficult to be raised at 250C to 260C. [0037] In the present invention, the resultant host fish individual needs to be raised within a temperature range including or around the growth temperature of the fish being the origin of the isolated germ cells. Fish is sensitive to water temperature and may become significantly difficult to be raised even due to 21 temperature variation by a few degrees of centigrade. Particularly, the isolated germ cells within the temperature range have high repair efficiency and the high ability to migrate or maintain division activity. The germ cells, even after being transplanted, can be placed within this temperature range to thereby efficiently recover from damage, migrate, and maintain division activity. For enhancing their ability to colonize in the host fish individual, it is preferred that the resultant host fish individual should be raised at or near the temperature of differentiation of the isolated germ cells into a germ line in their original developmental stages, i.e., the temperature applicable to the spawning to larval and juvenile stages of the donor fish. The temperature variation thereof is preferably within ±5*C, more preferably within ±3*C, more preferably within ±1C. [0038] Hereinafter, the present invention will be described more specifically with reference to Example. However, the technical scope of the present invention is not intended to be limited to these examples. Example 1 [0039] In this study, fertilized eggs of Sarda orientalis kindly provided by Tokyo Sea Life Park were raised at 25*C and used in subsequent experiments. First, in order to estimate the optimum time of transplantation, the gonadal development of Sarda orientalis larvae of 4.0, 5.2, and 6.8 mm long (3, 5, and 7 days old, respectively) was 22 histologically examined. Next, the Thunnus orientalis testicle was enzymatically dispersed, and the obtained donor testicular cells were fluorescently labeled with PKH26 and then intraperitoneally transplanted to the respective larvae of Sarda orientalis, Nibea mitsukurii, Scomber japonicus, and Scomber australasicus. After the transplantation, these larvae of Sarda orientalis (250C), Euthynnus affinis (250C), Nibea mitsukurii (22*C), Scomber japonicus (22*C), and Scomber australasicus (22 0 C) were separately raised. Ten days after the transplantation, the survival rate of the recipient individuals and the ratio of individuals in which the donor cells successfully colonized in the host gonad were examined. The results are shown in Table 1. [0040] [Table 1] Colonization rate of transplanted Thunnus orientalis testicular cells Fish species The number The number The number of Colonization of of observed individuals rate (%) recipient individuals in which individuals donor cells success fully colonized Nibea 1?54 504 0 0 mitsukurii Scomber 2?5 62 0 0 australasicus Scomber 617 128 0 0 japonicus Sarda 140 6 2 33.3 orientals Euthynnus 1870 104 20 19.2 affinis [0041] 23 In order to examine whether the donor cells that successfully colonized were germ cells, the cells were morphologically observed. The results are shown in Figures 1 and 2. In Figure 1, the middle left box shows a differential interference microscope image, and the middle right box shows a fluorescence microscope image. The dotted lines in the middle boxes denote the gonads. The triangle arrowhead in the middle right box denotes PKH+ cells stained with PKH26. The triangle arrowheads in the lower boxes denote enlarged views thereof. The lower left box shows a differential interference microscope image, and the lower right box shows a fluorescence microscope image. In Figure 2, the upper left box shows a differential interference microscope image, the upper right box shows a fluorescence microscope image for PKH26 observation, the lower left box shows a fluorescence microscope image for DAPI observation, and the lower right box shows the superposition of the observed PKH image and the observed DAPI image. In addition, the donor germ cells were detected by in situ hybridization using a Thunnus orientalis vasa probe. The results are shown in Figure 3. In Figure 3, the right boxes show the observation of one of the PKH+ individuals, and the left boxes show the observation of the other PKH+ individual. The upper boxes show differential interference microscope images, the middle boxes show fluorescence microscope images for PKH26 observation, and the lower boxes show images observed by in situ hybridization using the Thunnus orientalis vasa probe. [0042] 24 The Sarda orientalis larvae of 5.2 mm long (5 days old) were in a stage just before primordial germ cells that completely migrated were incorporated into the genital ridges. Thus, this period was presumed to be the optimum time of transplantation. Thunnus orientalis testicular cells were intraperitoneally transplanted to 140 Sarda orientalis larval individuals at the optimum time of transplantation. As a result, 6 individuals (4.2%) survived up to 10 days after the transplantation, and the PKH26-positive cells were confirmed to colonize in the host gonads of two (33.3%) out of these 6 individuals. The donor cells that successfully colonized had a large round nucleus characteristic of germ cells and exhibit Thunnus orientalis vasa-positive, demonstrating that they were donor-derived Thunnus orientalis germ cells. [00431 Likewise, 104 Euthynnus affinis individuals survived up to ten days after the transplantation, and PKH26 positive cells were confirmed to colonize in the host gonads of 20 (19.2%) out of these 104 individuals. Thus, use of Sarda orientalis or Euthynnus affinis larvae, which can be raised at high water temperature, as a host allows colonization of Thunnus orientalis spermatogonia in the host gonads. These results suggest that Sarda orientalis or Euthynnus affinis is a promising candidate of a surrogate for Thunnus orientalis production. Industrial Applicability [0044] 25 The present invention allows surrogate propagation or breeding of, for example, fish of the genus Thunnus, which has previously failed to be transplanted in host fish differing therefrom in strain or species. As a result, germ cells of giant fish species such as tuna can be transplanted to small fish to thereby produce seeds of the giant fish such as tuna in a small water tank. [0045] Throughout this specification the word "comprise", or variations such as "comprises" or "comprising", will be understood to imply the inclusion of a stated element, integer or step, or group of elements, integers or steps, but not the exclusion of any other element, integer or step, or group of elements, integers or steps. [0046] All publications mentioned in this specification are herein incorporated by reference. Any discussion of documents, acts, materials, devices, articles or the like which has been included in the present specification is solely for the purpose of providing a context for the present invention. It is not to be taken as an admission that any or all of these matters form part of the prior art base or were common general knowledge in the field relevant to the present invention as it existed in Australia or elsewhere before the priority date of each claim of this application. [0047] It will be appreciated by persons skilled in the art that numerous variations and/or modifications may be made to the invention as shown in the specific embodiments without departing from the spirit or scope of the invention as broadly described. The present embodiments are, therefore, to be considered in all respects as illustrative and not restrictive.