DEVELOPMENTAL ANALYSIS OF 2 SEX-DETERMINING GENE-M AND GENE-F, IN THEHOUSEFLY, MUSCA-DOMESTICA

In the housefly, Musca domestica, a single dominant factor, M, determi nes maleness. Animals hemi- or heterozygous for M are males, whereas t hose without M develop as females. In certain strains, however, both s exes are homozygous for M, and an epistatic dominant factor, F(D), dic tates female development. The requirement for these factors was analyz ed by producing, with mitotic recombination, mosaic animals consisting of genetically male and female cells. Removal of F(D) from an M/M;F(D )/+ cell at any time of larval development, even in the last larval in star, resulted in sex-reversal, i.e., in the development of a male clo ne in an otherwise female fly. In contrast, when M was removed from M/ + cells, the resulting clones remained male despite their female genot ype, even when the removal of M happened at embryonic stages. The occu rrence of spontaneous gynandromorphs, however, shows that the loss of M in individual nuclei prior to blastoderm formation causes the affect ed cells to adopt the female pathway. These results are consistent wit h the hypothesis that M is the primary sex-determining signal which se ts the state of activity of the kev gene F at around the blastoderm st age. Parallels and differences to the sex-determining system of Drosop hila are discussed.