Wild-derived XY sex-reversal mutants in the medaka, Oryzias latipes

The medaka, Oryzias latipes, has an XX/XY sex-determination mechanism. A Winked DM domain gene, DMY has been isolated by positional cloning as a sex-determining gene in this species. Previously, we found 23 XY sex-reversed females from 11 localities by examining the genotypic sex of wild-caught medaka. Genetic analyses revealed that all these females had Y-linked gene mutations. Here, we aimed to clarify the cause of this sex reversal. To achieve this, we screened for mutations in the amino acid coding sequence of DMY and examined DMY expression at 0 days after hatching (dah) using densitometric semiquantitative RT-PCR. We found that the mutants could be classified into two groups. One contained mutations in the amino acid coding sequence of DMY while the other had reduced DMY expression at 0 dab although the DMY coding sequence was normal. For the latter, histological analyses indicated that (YYwOur)-Y-wOur (Y-wOur, Y chromosome derived from an Oura XY female) individuals with the lowest DMY expression among the tested mutants were expected to develop into females at 0 dah. These results suggest that early testis development requires DMY expression above a threshold level. Mutants with reduced DMY expression may prove valuable for identifying DMY regulatory elements.