Female characteristics in the Drosophila melanogaster-sigma virus system in natural populations from Languedoc (southern France).

The Drosophila melanogaster-sigma virus system has been extensively analysed for some years in Languedoc. The sigma virus, endemically present in natural populations of the fly, is not contagious but vertically transmitted. The frequency of infected flies dramatically increased from 1983 to 1988 in Languedoc and the increase was clearly correlated with some viral characteristics. From 1988 onwards, this frequency steadily decreased. Three zones were identified in Languedoc, from North to South, in each of which the virus evolved differently. Transovarial transmission and other parameters on the female side in the host virus system were investigated in samples collected in 1992. There were differences in values between the three zones. The contribution of these parameters to the observed differential frequencies of infected flies is discussed.