Heritable artificial sex chromosomes in the medaka, Oryzias latipes

Chromosomal sex determination is widely used by vertebrates, however, only two genes have been identified as master sex-determining genes: SRY/Sry in mammals and DMY in the teleost medaka. Transfer of both genes into genetically female (XX) individuals can induce male development. However, transgenic strains have not been established in both cases because of infertility of the transgenic founders in mammals and low germline transmission rates in medaka. In this study, we used a BAC clone containing DMY in a 117 kb genomic region and two types of fluorescent marker to establish two DMY-transgenic medaka strains. In these strains, exogenous DMY is completely linked to a male phenotype and early gonadal development is not different from that of the wild-type strain. Sex-linkage analysis showed that the exogenous DMY was located on linkage group (LG) 23 in one strain and on LG 5 in the other strain, whereas the sex chromosome in medaka is on LG 1. Real-time PCR analysis indicated that these strains have multiple copies of DMY and higher DMY expression levels than the wild-type strain. These results showed that LGs 23 and 5 function as sex chromosomes in the two strains, respectively. This is not only the first example of the artificial generation of heritable sex chromosomes in vertebrates but also the first evidence showing plasticity of homomorphic sex chromosomes. This plasticity appears to be a characteristic of lower vertebrates and the underlying cause of frequent sex chromosome switching, recently reported in several fish and frog species.