No abstract

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: Mill, the most common, on autosome III and Mil, 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.

Houseflies collected from eight pig-breeding farms were used to investigate the nature of sex determinants in fly populations of South-East England. Earlier observations had shown that their sex determination mechanism was not of the standard (XX females, XY males) type.Most flies of both sexes were XX; the male determining Y chromosome of standard populations was rare. Test-crosses to females of standard multimarked strains and crosses using aneuploid (OX) flies identified two dominant male determinants, one on autosome 3 (M III) and another on the X chromosome (X m ), and provided the first demonstration in this species of an active involvement of the X chromosome in sex determination. A small secondary constriction on X appeared to indicate reliably the presence of X m . Most individuals in field populations were X m homozygotes, implying the presence of an unlocated female determinant F,f epistatic to X m and Jf III.M III was less common and differed in frequency between samples. Its increased frequency in a strain selected in the laboratory with the pyrethroid insecticide permethrin might be due either to genetic drift, or to linkage between M III and a gene on autosome 3 that confers resistance to pyrethroids in houseflies.

Genetic and cytological analyses of house-flies collected from 12 pig-breeding farms throughout the British Isles demonstrated that the non-standard sex determination mechanism prevailing in South-East England, involving a dominant female determinant (F) and virtual homozygosity for a male determinant on the X chromosome (X m , both males and females morphologically XX), was not typical of the country as a whole. Instead there was a gradual decrease in the frequency of F, X m and a rarer male determinant M III, and a concomitant increase in the standard male determining Y chromosome, on moving north, east and west of this region. Only the Scottish and probably the Irish populations were fully standard {XX females XY males), although one from the East Anglian coast in which non-standard determinants were rare was predominantly of this type. Populations from intermediate areas possessed complex multifactorial mechanisms in which Y, F X m and M III coexisted. It is hypothesized that this radial cline in sex determinants, like the latitudinal cline known from mainland Europe, represents a transient polymorphism caused by the recent and continuing invasion of non-standard determinants into originally standard populations. The cause(s) of this apparently rapid evolutionary change, however, remain unclear. 2-2* X™ is used here to denote both the AMinked male determinant and an X chromosome that bears it. X denotes a standard X chromosome lacking X m .

In the housefly, mosaics appear spontaneously but rarely. Sexual mosaics or gynandromorphs also appear in strains in which sex determination is based on autosomal sex factors. Rare cases of recombination in the male have been reported by some authors. In field and laboratory populations, mitotic plates with figures indicating exchange of chromatid segments are regularly observed in tissues of individuals of both sexes and at all stages of development. All these anomalies are interpreted as outward manifestation of the same phenomenon: mitotic recombination. The cytological basis of mitotic recombination, its relative frequency, its influence on linkage and genetic variability are discussed.