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サケ科魚類の人為雑種において，性比が一方に偏る例が報告されてきたが，これらは生殖腺の表現型に基づく。本研究では，カラフトマスとサクラマス間の正逆雑種について，雄特異的な性決定遺伝子sdYの有無による遺伝的性を検討した。その結果，カラフトマス雌×サクラマス雄，サクラマス雌×カラフトマス雄の8か月齢魚の遺伝的性は雌に，サクラマス雌×カラフトマス雄の12か月齢魚は雄に偏った。初期胚と孵化仔魚の遺伝的性比は1:1であったことから雑種の性比の偏りは成長過程における生残率の雌雄差により生じたと考えられた。, Skewed sex ratio has often been observed in hybrids, but the sex was always determined by gonadal phenotypes. Here, we artificially induced reciprocal hybrids between pink salmon and masu salmon and then examined the genetic sex ratio in the hybrids based on the presence or absence of the sex-determining gene sdY. As a result, the sex of 8-month-old hybrids between pink salmon female x masu salmon male and masu salmon female x pink salmon male skewed to female, while 12-month-old hybrids between masu salmon female x pink salmon male were exclusively male. Since the sex ratio between the early embryos and hatching alevin was 1:1, it was considered that the skewed sex ratio was due to the difference of survival rate between genetic males and females.

In dojo loach (Misgurnus anguillicaudatus), although most wild types are gonochoristic diploids that are genetically differentiated into 2 groups, A and B, clonal lineages appear in certain localities. Clonal loaches have been considered to have hybrid origins between the 2 groups by a series of genetic studies. In this study, using FISH with a newly developed probe (ManDra-A), we identified 26 (1 pair of metacentric and 12 pairs of telocentric chromosomes) of 50 diploid chromosomes in contemporary wild-type group A loach. In contrast, ManDra-A signals were not detected on metacentric chromosomes derived from the ancestral group A of clonal loach. The FISH results clearly showed the presence of certain differentiations in metacentric chromosomes between ancestral and contemporary group A loach. Two-color FISH with ManDra-A and group B-specific ManDra (renamed ManDra-B) probes reconfirmed the hybrid origin of clones by identifying chromosomes from both groups A and B in metaphases. Our results showed the hybrid origin of clonally reproducing fish and the possibility that chromosomal differentiation between ancestral and contemporary fish can affect gametogenesis. In meiotic spermatocytes of sex-reversed clones, ManDra-A, and not ManDra-B, signals were detected in 12 out of 50 bivalents. Thus, the results further support the previous conclusion that clonal gametogenesis was assured by pairing between sister chromosomes duplicated from each ancestral chromosome from group A or B. Our study deepens the knowledge about the association between clonality and hybridity in unisexual vertebrates.

Production of sterile individuals is the key technique for surrogate propagation in teleosts. Sterile hybrids may be the ideal surrogate host when they do not generate their own germ cells in their gonads. Here, we attempted hybridization experiments between zebrafish (Danio rerio) and six closely related species (Danio albolineatus, Aphycypris chinensis, Hemigrammocypris rasborella, Opsariichthys platypus, Nipponocypris temminckii, N. sieboldii) and one remotely related species (Tanichthys albonubes). Intersubfamilial hybridizations in the family Cyprinidae resulted in the occurrence of inviable abnormal larvae with the two parental genomes, except for the T. albonubes x A. chinensis hybridization, in which normal larvae survived. Allotetraploidy and spontaneous gynogenetic diploidy were infrequently detected in T. albonubes x A. chinensis and D. albolineatus x A. chinensis, respectively

Tetraploid strains are important sources of diploid gametes available for further expansion of ploidy manipulation. Although artificial induction of tetraploid strains has been attempted by inhibition of a mitotic cell division of zygotes using hydrostatic pressure or temperature treatments at the early developmental stage, successful examples of viable and fertile tetraploid fish were very rare. Because most resultant tetraploid progeny exhibited extremely high mortality. Natural tetraploid variants are considered another source of diploid gametes. In the present study, we tried to induce new strains of tetraploid, triploid and androgenetic diploid using diploid sperm of a tetraploid ginbuna (Japanese silver crucian carp Carassius auratus langsdorfii), which caught at the Jounuma lake, Gunma Prefecture, Japan. New tetraploids, i.e., neo-tetraploids were induced by fertilizing eggs of a diploid goldfish Carassius auratus auratus with diploid sperm of the tetraploid ginbuna, followed by heatshock (40°C for 45, 60 and 75 s) to inhibit the second polar body release at 5 min after fertilization. Although a small number of neo-tetraploid fish survived, we could not obtain any fertile gametes from them in the present study. Triploids were successfully induced by fertilizing eggs of a diploid goldfish with diploid sperm of the tetraploid ginbuna. Some resultant triploid males produced aneuploid sperm at the age of maturation. Androgenetic diploids were induced by fertilizing UV-irradiated eggs of diploid goldfish with diploid sperm of the tetraploid ginbuna. A mature androgenetic diploid produced fertile haploid sperm.

Gonochoristic wild-type dojo loaches (Misgurnus anguillicaudatus) are diploid (2n = 50) and reproduce bisexually. However, sympatric clonal diploids generate unreduced diploid isogenic eggs that develop gynogenetically. Clone-origin triploidy arises following the incorporation of a haploid wild-type sperm nucleus into the diploid egg. Triploid females produce fertile haploid eggs by meiotic hybridogenesis, while triploid males are sterile. Clonal loaches arose from past hybridization event(s) between genetically diverse groups, A and B. Artificial hybrid females between the 2 groups produce unreduced and/or aneuploid eggs, but the hybrid males are sterile. In this study using FISH, we analyzed chromosome pairing in meiotic cells of clone-origin triploid and inter-group hybrid males to clarify the cytogenetic mechanisms underlying the male-specific sterility. We used a repetitive sequence probe to identify group B-derived chromosomes and a 5.8S + 28S rDNA probe to identify pairs of homologous chromosomes. We found that asynapsis and irregular synapsis occur in triploid and hybrid males containing 2 different genomes and that this may cause the formation of sterile germ cells. These results will help us to understand hybrid sterility from the viewpoint of synapsis behavior.

Skewed sex ratio has often been observed in hybrids, but the sex was always determined by gonadal phenotypes. Here, we artificially induced reciprocal hybrids between pink salmon and masu salmon and then examined the genetic sex ratio in the hybrids based on the presence or absence of the sex-determining gene sdY. As a result, the sex of 8-month-old hybrids between pink salmon female x masu salmon male and masu salmon female x pink salmon male skewed to female, while 12-month-old hybrids between masu salmon female x pink salmon male were exclusively male. Since the sex ratio between the early embryos and hatching alevin was 1:1, it was considered that the skewed sex ratio was due to the difference of survival rate between genetic males and females.