Male-specific restriction of recombination frequency in the sex chromosomes of the medaka, Oryzias latipes

In the medaka, Oryzias latipes, the mechanism of sex determination (XX/XY) can be revealed by genetic crosses using a body-colour gene, though it does not have cytologically recognizable sex chromosomes. The recombination res triction of sex chromosomes in heterogametic (XY) males has been demonstrat ed. To elucidate whether the recombination is prevented by the heterogamety of the sex chromosomes or by maleness, we examined the recombination frequ encies among three loci located on the sex chromosomes (r, SL1 and SL2) in heterogametic males (XY), homogametic males (XX and YY), homogametic female s (XX) and heterogametic females (XY). The recombination frequencies betwee n r-SL1 and SL1-SL2 were as follows: 0, 0 (XY males); 0, 1.5 (XX males); 1. 6% (YY males; 1.2%, 14.4% (XY females); 0.8%, 21.8% (XX females). These res ults indicate that the recombination restriction of the sex chromosomes in heterogametic males does not result from heterogametic sex chromosomes, but from maleness. Such sex-chromosome-specific recombination restriction in h eterogametic sex may have triggered the differentiation of sex chromosomes in vertebrates.