Seasonal variations in sex‐related gene expression in the housefly,Musca domestica

In cage populations of Musca domestica L., the sex ratio is uniform throughout the year in strains with a sex balance based on the presence‐absence of a single male determinant, either the Y chromosome or an autosomic male determinant M2 or M 3 (5 strains = 48 years’ breeding). The sex ratio displays annual cyclic fluctuations in two strains where the sex of the specimens is due to indirect mechanisms: 1) the arrhenogenic effect of Ag (3 strains = 21 years’ breeding), through a maternal effect Ag/+ heterozygous females produce only male and intersex progeny; 2) the gametic competition of NMSR (3 strains = 18 years’ breeding), males heterozygous for NMSR produce two types of sperm and the shorter are utilized earlier for egg fertilization. Both strains show fluctuations of the sex ratio, with an increased frequency of females during summer. This phenomenon is statistically demonstrated and hypotheses for a possible control mechanism are discussed.