On the maintenance of the polymorphism at the ref(2)P locus in populations of Drosophila melanogaster

Polymorphism for two alleles of the ref(2)P locus is a very constant feature of French natural populations of Drosophila melanogaster. One of these alleles interferes with the multiplication of the hereditary sigma virus in the fly. An equilibrium, quite similar to the natural one, has been observed previously in experimental populations, whether the sigma virus is present or not. Evidence is given that one of the selection components involved in the maintenance of this equilibrium affects adult stages when flies have not suffered severe larval competition. In conditions of severe larval competition, a maternal effect seems to be involved in the differential egg-to-adult viability of heterozygotes.