Wolbachia requirement for oogenesis: occurrence within the genus Asobara (Hymenoptera, Braconidae) and evidence for intraspecific variation in A. tabida

Abstract

Wolbachia are symbiotic bacteria that induce a diversity of phenotypes on their numerous invertebrate host species. In the wasp Asobara tabida (Braconidae), each individual harbours three Wolbachia strains: wAtab3, which is required for host oogenesis, and wAtab1 and wAtab2, that do not have this function but induce cytoplasmic incompatibility. In this study, we surveyed and identified Wolbachia strains in four additional Asobara species. We detected Wolbachia in one of these species, but both the identity (based on wsp gene) and prevalence of the Wolbachia detected in natural population indicate that this host species is not dependent on Wolbachia for oogenesis. We also compared A. tabida lines of different geographical origin for their dependence on Wolbachia. All individuals from 16 A. tabida lines proved to be infected by the three Wolbachia strains wAtab1, wAtab2 and wAtab3, but, interestingly, we found variation among lines in the degree to which females were dependent on Wolbachia to produce their oocytes. In three lines, aposymbiotic females (cured from the three Wolbachia strains by antibiotics) can produce some oocytes. However, these aposymbiotic females produce fewer and smaller oocytes than symbiotic ones, and the larvae they produce die before full development. Thus, depending on which nuclear genotype they have, A. tabida females depend on Wolbachia either because they fail to produce any oocyte or because the few oocytes they do produce generate unviable offspring. We discuss the implications of these findings for the understanding of the physiological and genetic deficiency of aposymbiotic females.