Wolbachia-mediated parthenogenesis induction in the aphid hyperparasitoid Alloxysta brevis (Hymenoptera: Figitidae: Charipinae)

Abstract

Thelytokous parthenogenesis (thelytoky), in which females can produce female offspring without mating, can be caused by parthenogenesis-inducing endosymbiotic bacteria in the genus Wolbachia. This interaction is well known in hymenopteran parasitoids, where unfertilized eggs typically develop as males via haplo-diploidy in the absence of parthenogenesis-inducing bacteria. We report on a case of thelytoky in Alloxysta brevis (Thomson) (Hymenoptera: Figitidae), a globally widespread aphid hyperparasitoid. A previous study had shown that sex ratios of this species collected in Minnesota (USA) were extremely female biased, and we found here that unmated females reared from field-collected hosts produced female offspring without exposure to males. This result demonstrated thelytoky, and we tested for the role of bacterial endosymbionts by comparing offspring production of unmated females fed the antibiotic rifampicin to offspring production of control females not fed antibiotics. Antibiotic-fed females produced almost exclusively male offspring, and control females produced mainly females. This result showed that antibiotic treatment facilitated male production by unmated Alloxysta brevis females, thus implicating bacterial symbiosis in the expression of thelytoky. We then used molecular analyses to determine the identity of the symbiont. These analyses identified a Wolbachia strain from supergroup B, and excluded other bacteria known to mediate parthenogenesis induction, such as Cardinium and Rickettsia. While Wolbachia had been previously detected by molecular analysis in this species, these are the first experiments demonstrating Wolbachia-mediated parthenogenesis in the figitid subfamily Charipinae. IMPORTANCEParthenogenesis induction in insects can have important environmental and economic consequences. This is especially true if pests or their natural enemies are affected. The case of Alloxysta brevis is of particular interest, as this species is a hyperparasitoid of aphids, meaning that they attack and kill primary parasitoids of aphids. The populations of many species of pest aphids are controlled by primary parasitoid species, and hyperparasitoids thus have the potential to interfere with this mechanism of control. The role of hyperparasitoid parthenogenesis in the suppression of aphids by primary parasitoids remains unexplored. Thus, the results of this set of studies provides a starting point for determining whether parthenogenesis-inducing Wolbachia in hyperparasitoids should be expected to improve or hinder biological control of pest aphids by primary parasitoids. The focus on Alloxysta brevis as a model for these questions could be particularly instructive, since it is a species of worldwide distribution that is involved in numerous economically important aphid-parasitoid interactions.