Our cytological analysis revealed high proportions of XX males in some housefly populations of Turkey. In contrast to earlier findings in Europe, on the British Islands, and in Japan, the XX male frequency in Turkey was not correlated to the geographical latitude. In general, the frequency of XX males was lower in populations from the central and eastern Anatolian highlands than from coastal regions, demonstrating a correlation of the sex-determining mechanisms to geographical altitude and perhaps other geographical characteristics.
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 Y, on the X, or on any of the five autosomes. By analyzing a Y-autosomal translocation and a ring-shaped, truncated Y chromosome, we could show that M on the Y consists of at least two regions with M activity: One of them can be assigned to the short arm of the Y chromosome (MYS), which is largely C-banding negative, the other region lies on the C-banding positive long arm of the Y, including the centromeric part (MYL). Each region alone behaves as a hypomorphic M factor, causing many carriers to develop as intersexes of the mosaic type instead of as males. When introduced into the female germ line by transplantation of progenitor germ cells (pole cells), the MYS shows an almost complete maternal effect that predetermines 96% of the genotypic female (NoM) animals to develop as males. In contrast, the MYL has largely lost its maternal effect, and most of the NoM animals develop as females. Increasing the amount of product made by either of the two hypomorphic M factors (by combining the MYS and MYL or two MYS) leads to complete male development in almost every case. We thus assume that the Y chromosome carries at least two copies of M, and that these are functionally equivalent.
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