Evolution of natural populations in the Drosophila melanogaster sigma virus system I. Languedoc (southern France).

In natural populations of Drosophila melanogaster, sigma virus is usually present in a minority of individuals. The virus is transmitted transovarially but is not contagious from fly to fly. Two viral Types (I and II) are found in populations. One of them (Type II) is better adapted to an allele for resistance to the virus, present as a polymorphism in fly populations. Previous observations have led to the hypothesis that a viral Type II originating in central France might be invading populations. The study of Languedoc populations was undertaken to examine this hypothesis. Two striking phenomena were observed. The strong increase in Type II clones frequency (from 0.53 to 0.91) confirmed that there was invasion in this region. The frequency of infected flies also increased dramatically, at levels never observed elsewhere yet, which indicates that Languedoc should present some unusual characteristics. The epidemiological consequences of such a burst, in the case of a pathogenic virus would have to be taken into consideration. Significant changes in other viral characteristics, from 1983 to 1987, in Languedoc populations have also been documented.