Autosomal Sex-Determining System of the Housefly : Discovery of the First-Chromosomal Male Factor in Kitakyushu, Japan

Besides the usual sex-determining system by sex-chromosomes (X and Y), several autosomal sex-determining factors are known in the housefly. Among them the 2nd and 3rd chromosomal male factors are most frequent in various areas of the world. By using multichromosomal mutant marker strains, the autosomal sex-determining system has been analyzed in field-collected colonies of the housefly from Kitakyushu, Japan. In addition to the majority of ⅡM and ⅢM male factors, one ⅠM and two ⅤM cases have been discovered from these flies. No published data had been available on the presence of ⅠM type flies until evidence was recently reported from Fiji populations. Thus the present paper is not only to report the first discovery of the ⅠM case in Japan but also the second record on such a rare type of the sex-determination in the world. The presence of neither feminizing factor F nor the 4th chromosomal male factor ⅣM was detected from these populations examined so far. Cytological examinations also showed that almost all flies possessed XX karyotype with a few XO or XXX types instead of the normal XX-XY types. A new visible mutant allele bwb79K (brown-body) was isolated from one of the colonies.