Abstract
Cynipids are known to use various reproductive modes (arrhenotoky, thelytoky and strict cyclical parthenogenesis), but the mechanism remains unknown. We have studied the reproductive system of a rose gallwasp, Diplolepis spinosissimae, which was found to exhibit two different reproductive systems according to the population. In eight out of the ten populations studied, all along the Atlantic coast, D. spinosissimae is thelytokous. Males are extremely rare, and all females are homozygous at three microsatellite loci. 'Obligate homozygous parthenogenesis' was found to be strictly associated with the presence of the endosymbiotic bacterium Wolbachia sp. In the two remaining populations, deprived of Wolbachia, D. spinosissimae reproduced by arrhenotoky as indicated by the larger frequency of males and heterozygosity of females. Allelic diversity, although not zero, was highly reduced in the coastal populations, as a consequence of thelytoky and gamete duplication. We hypothesize that mating of uninfected males with infected females during the first generations after an infection event could explain the small but significant amount of polymorphism observed in those populations. The high level of differentiation indicates a low gene flow, even between geographically close coastal populations.