Genetics in natural polyploidy in loach

다국어 초록 (Multilingual Abstract)

Loaches widely distribute in the Eurasian Continent and are important for food both in Japan and Korea. Chromosome manipulation including induction of gynogenesis, androgenesis, and polyploidy is powerful tool for potential applications in aquaculture as well as for basic genetic research.
The loach Misgrunus anguillicaudatus in Japan has a diploid karyotype(2n=50), but a small number of natural polyploidy have been found. In diploid vertebrates, artificially induced gynogenesis and androgenesis normally give rise to inviable haploid embryos, In contrast, gynogenetic and androgenetic progeny generated by using gametes of the natural tetraploid loach with 100 chromosomes were viable without any treatments for chromosome duplication. The results conclusively demonstrate that the loach with 100 chromosomes is not an evolved-back diploidy, but a true tetraploidy with four sets of homologous chromosomes, The same conclusion was obtained in the tetraploid form of spinous loach Cobitis biwae.
Triploid loach produced by hybridizations between normal diploids and natural tetraploids showed male - sterility. While, triploid females laid two sizes of eggs, large and small, and gave viable progeny after backcrossing to normal diploid loach and chromosome manipulation(induced gynogenesis by UV - irradiated spermatozoa and inhibition of the second polar body release by pressure shock). Cytogenetic and flowcytometric analyses on these progeny demonstrate that triploid loach produce 3n large and 1n small eggs. Multilocus - DNA - fingerprintind revealed clonal nature of gynogenetic triploids which were produced by fertilizing the large eggs with UV - irradiated spematozoa. This suggests involvement of unreduced oogenesis in the formation of the large eggs. Cytology and biochemical genetics are now underway to disclose mechanisms responsible for the small egg formation.
Pentaploid females produced by inhibiting the polar body release after 4n × 2n hybridizations laid only 2n eggs with medium size. Pentaploid males produced very small number of aneuploid spermatozoa. Hexaploid loach were produced by inhibiting the polar body release after 4n × 4n crosses. Flowcytometry of the testes of hexaploid males showed formation of functuonal 3n spermatozoa.
The presence of polyploid in the loach species suggests specially divierged sex determination mechanisms. Deviation of sex from the theoretical ratio was observed when the loach were reared in unusually high temperature.