Frequent Turnover of Sex Chromosomes in the Medaka Fishes
The mechanisms that govern the decision to become male or female are diverse, ranging from environmental to strict sex chromosomal systems. In vertebrates, only two sex-determining genes have been isolated: SRY/Sry from mammals and DMY from the medaka, Oryzias latipes. In contrast to the widespread distribution of SRY/Sry in mammals, DMY is absent in most Oryzias species, suggesting that closely related species have different sex-determining genes. Recent studies have demonstrated that Oryzias species have different sex determination systems (XX/XY and ZZ/ZW). Furthermore, the sex chromosomes differ in their origin and degree of differentiation. These findings suggest the repeated creation of new sex chromosomes from autosomes during evolution of Oryzias fishes, possibly in association with the formation of new sex-determining genes.
KeywordsNonmammalian Vertebrate Oryzias Species
The author is grateful to Dr. Mitsuru Sakaizumi and Dr. Masaru Matsuda for valuable comments on this manuscript.