No evidence for Wolbachia-induced parthenogenesis in the social Hymenoptera

The typical pattern in hymenopteran species is that females are produced from fertilized eggs but that males arise parthenogenetically from unfertilized eggs (arrhenotoky). In some species, however, unfertilized eggs can yield diploid female offspring through the process of thelytokous parthenogenesis. Indeed, it is puzzling why such asexual mutants have not outreproduced their sexual counterparts more often (Williams, 1975; Maynard Smith, 1978; Bell, 1982). A recent theory poses that asexual reproduction should be opposed by the male sex, as males can gain ®tness bene®ts through sexual reproduction only (Hurst & Peck, 1996; Hurst, 1997). Perhaps not surprisingly then, maternally derived plasmagenes have been discovered that adopt the opposite strategy: induction of asexual reproduction, hence escaping the two-fold cost of sex (Hurst, 1993). Most notably, in some parasitoid wasps infection by the micro-organism Wolbachia leads to asexual reproduction and all-female broods (reviewed in Stouthamer, 1997). Being maternally transmitted, these microbes make their host produce asexually broods of mostly female offspring. This phenomenon is reversible upon treatment by antibiotics or excessive heat (Stouthamer et al., 1990). In addition, manipulative strategies have evolved several times and are not limited to parthenogenesis induction. Effects of sex ratio distortion also occur through feminization of genetic males (Rigaud, 1997) or killing of male offspring (Hurst et al., 1999), and in most insect species the bacteria spread by sterilizing competing uninfected females (reviewed by O'Neill et al., 1997; Werren, 1997). The limits of distribution of each of these phenotypes have not yet been established. Parthenogenesis induction has been found to be the most common phenotype in chalcidoid and cynipoid parasitoid wasps (Stouthamer, 1997), but its exact distribution within the Hymenoptera is unknown. As a ®rst systematic survey of an aculeate group, we investigated whether Wolbachia is involved in parthenogenesis induction in any of the known thelytokous social hymenopteran species (six ant species and the cape bee, Table 1) using a PCR-based assay. Of the handful of species where thelytoky has been documented, only P. pungens and C. biroi show obligatory thelytoky (Itow et al., 1984; Tsuji & Yamauchi, 1995); in the other species thelytoky is thought to occur only in orphaned colonies. In striking contrast to the apparent rarity of thelytoky in the social Hymenoptera, Wolbachia seems to be extremely common in ants, with at least 50% of the species being infected (Wenseleers et al., 1998). Indeed, several authors have suggested that Wolbachia might have some importance in explaining the reproductive patterns found in social insects (Crozier & Pamilo, 1993,