The distribution of sex-determinants in field populations of Musca domestica domestica L. was studied in 62 samples of flies collected at 53 sites (animal farms) between 1975 and 1981 in an area stretching North–South from Denmark (+ Iceland) to Sicily.
Karyological observations and genetic analyses demonstrated the existence of three types of population along a latitudinal cline. Populations of Northern Europe were of the standard type (XX females and XY males) with the Y chromosome determining sex. Those of Central and Southern Italy from sites below 100 m.a.s.l. (metres above sea level) were autosomal (XX females and males), sex in them being determined by autosomal sex-determinants for both femaleness and maleness. In the large intermediate zone the populations were mixed and had several karyotypes in both sexes. In this zone an altitudinal gradient was also observed, with autosomal determinants less common at higher altitudes. Genetic tests showed, in the autosomal and in the mixed populations, the presence of two autosomal male factors: M III, the most common, on autosome III and M II, on autosome II.
The gradient in sex determinants found in flies of Western Europe appears to be a dynamic phenomenon of relatively recent origin. Both climatic influence and selective pressure with insecticides have probably contributed towards the micro-evolution of populations with different sex-determinants in the houseflies of the area studied.