Abstract
Geographic variation in sex-determining factors was studied in 19 localities in Japan between 1981 and 1986. The association of the male-determining factor, M, was demonstrated with all linkage groups, and the most predominant carrier of the M in each locality was either the IIIM or the Y chromosome. The fraction of Y in M-carrying chromosomes was higher in Hokkaido, and declined significantly on moving to the south. The frequency of the females carrying the female-determining factor, F, which is epistatic to theMfactor, was sporadically distributed in the populations surveyed. However, male heterogamety was largely retained in the populations in Hokkaido. We present a hypothesis that broadly ranging geographic variation in sex-determining factors reflects a transient state caused by recent invasions of autosomal sex-determining factors. No significant difference in the index combined with segregation rate of theM-carrying gamete and viability was indicated between the males ofX /Yand IIIM/III.
