THE Y-CHROMOSOMAL AND AUTOSOMAL MALE-DETERMINING M-FACTORS OF MUSCA-DOMESTICA ARE EQUIVALENT

In Musca domestica, male sex is determined by a dominant factor, M, lo cated either on the Y, the X or on an autosome. M prevents the activit y of the female-determining gene F. In the absence of M, F becomes act ive and dictates female development. The various M factors may represe nt translocated copies of an ancestral Y-chromosomal M. Double mutants and germ line chimeras show that M-Y, M-I, M-II M-III and M-V perform equivalent functions. When brought into the female germ line, they pr edetermine male development of the offspring. This maternal effect is overruled by the dominant female-determining factor F-D. M-I and M-II are weak M factors, as demonstrated by the presence of yolk proteins i n M-I/+ males and by the occurrence of some intersexes among the offsp ring that developed from transplanted M-I and M-II/+ pole cells. The a rrhenogenic mutation Ag has its focus in the female germ line and its temperature-sensitive period during oogenesis. We propose that M-I and Ag represent allelic M factors that are affected in their expression. Analysis of mosaic gonads showed that in M. domestica the sex of the germ line is determined by inductive signals from the surrounding soma . We present a model to account for the observed phenomena.