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 restriction of sex chromosomes in heterogametic (XY) males has
been demonstrated. To elucidate whether the recombination is prevented by the heterogamety of
the sex chromosomes or by maleness, we examined the recombination frequencies among three loci
located on the sex chromosomes (r, SL1 and SL2) in heterogametic males (XY), homogametic
males (XX and YY), homogametic females (XX) and heterogametic females (XY). The
recombination frequencies between 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
results 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 heterogametic sex may have triggered the differentiation of sex
chromosomes in vertebrates.