THE MALE-DETERMINING ACTIVITY ON THE Y-CHROMOSOME OF THE HOUSEFLY (MUSCA-DOMESTICA L.) CONSISTS OF SEPARABLE ELEMENTS

In the common housefly. the presence or absence of a male-determining factor, M is responsible for sex determination. In different strains, M has been found on the I: on the X or on any of the five autosomes. B y analyzing a Y-autosomal translocation and a ring-shaped, truncated Y chromosome, we could show that M on the I:consists of at least two re gions with M activity: One of them can be assigned to the short arm of the Y chromosome (M-YS), which is largely C-banding negative, the oth er region lies on the G-banding positive long arm of the Y, including the centromeric part (M-YL). Each region alone behaves as a hypomorphi c M factor, causing many carriers to develop as intersexes of the mosa ic type instead of as males. When introduced into the female germ line by transplantation of progenitor germ cells (pole cells), the M-YS sh ows an almost complete maternal effect that predetermines 96% of the g enotypic female (Non I) animals to develop as males. In contrast, the M-YL has largely lost its maternal effect, and most of the NoM animals develop as females. Increasing the amount of product made by either o f the two hypomorphic,M factors (by combining the M-YS and M-YL or two M-YS) leads to complete male development in almost every case. We thu s assume that the Y chromosome carries at least two copies of M and th at these are functionally equivalent.