THE MUTATION MASCULINIZER (MAN) DEFINES A SEX-DETERMINING GENE WITH MATERNAL AND ZYGOTIC FUNCTIONS IN MUSCA-DOMESTICA L

In Musca domestica, the primary signal for sex determination is the do minant factor M, which is assumed to regulate a postulated female-dete rmining gene F. Presence of M prevents expression of F so that male de velopment ensues. In the absence of M, F can become active, which dict ates the female pathway. The existence of F is inferred from F-D, a do minant factor that is epistatic to M. We describe a new mutation mascu linizer, which has all the properties expected for a null or strongly hypomorphic allele of F: (1) it maps to the same chromosomal location as F-D, (2) homozygous man/man animals develop as males, (3) homozygou s man/man clones generated in man/+ female larvae differentiate male s tructures, (4) man has a sec-determining maternal effect. About a thir d of the morphological males synthesize yolk proteins, which indicates that they are intersexual in internal structures. The maternal effect of man is complete in offspring that derive from homozygous man/man p ole cells transplanted into female hosts. In this case, all man/+ prog eny become fertile males that do not produce yolk proteins. A sex-dete rmining maternal effect has previously been demonstrated for F-D. Like F, maternal man(+) is needed for zygotic man(+) to become active, pro viding further evidence that man is a loss-of-function allele of F.