UC Riverside

In Trichogramma parasitoid wasps, infection with the intracellular bacterium, Wolbachia, alters host reproduction, inducing gamete duplication and thus the parthenogenetic production of female offspring from unfertilized eggs. Little is known about the mechanisms governing sex determination in Trichogramma. Beyond outlining the basic mechanism of gamete duplication, few studies have investigated the interactions between Wolbachia and its host which allow parthenogenesis induction. This dissertation manipulates Wolbachia infection in Trichogramma kaykai to investigate the mechanisms involved in sex determination and the extent of Wolbachia's role in parthenogenesis induction. The production of males and intersexes by parthenogenetic females, and the factors contributing to their formation, provide valuable insight into mechanisms of sex determination and Wolbachia manipulation. Males and intersexes regularly appeared among the offspring of aging infected females as a result of incomplete parthenogenesis induction (Chapters 3-6). Intersexes ranged from very feminine to very masculine (Chapter 2) and flow cytometry confirmed that all were of a single genetic constitution (haploid or diploid) and therefore, not mosaics (Chapter 4). Infected females also produced occasional diploid males (Chapters 4-6). Together, these findings suggest a two-step mechanism of Wolbachia-induced parthenogenesis, requiring both gamete duplication and feminization. The complete lack of diploid males and intersexes in the absence of Wolbachia infection strongly supports this mechanism and points to a method of sex determination consistent with the imprinting model recently proposed for Nasonia vitripennis. Here, female development relies on a paternally derived sex allele to compensate for an imprinted maternal allele. The production of triploid daughters by diploid males strengthens this finding (Chapter 5). Factors modulating expression of Wolbachia-induced parthenogenesis in T. kaykai were found to include host age, host genetic background, and to a lesser extent heat (Chapter 3). Quantification of Wolbachia density (Chapter 6) revealed a clear relationship between bacterial density and wasp ploidy, with diploids harboring 7 times more than haploids. However, the relationship between Wolbachia density and sexual phenotype was not clear.