Wolbachia penetrance and its relationship to bacterial density in parthenogenetic Trichogramma

An understanding of sex determining processes and the factors that modulate them is extremely important for effective use of parasitoids as biological control agents. Members of the genus Trichogramma are tiny parasitoid wasps used in the biological control of lepidopteran pests. Hymenopterans generally exhibit haplodiploid reproduction in which males are haploid, developing from unfertilized eggs, and females are diploid, arising from fertilization events. However, several members of Trichogramma are infected with intracellular parthenogenesis-inducing Wolbachia bacteria that cause the production of all-female broods from unfertilized eggs through a process involving diploidization and feminization. Factors such as offspring production and heat can affect penetrance of the parthenogenesis phenotype and result in the formation of intersexes and males. This study investigates the role of bacterial density in the penetrance of the parthenogenesis-inducing phenotype in Wolbachia infected Trichogramma. Offspring produced by heat-treated Wolbachia infected Trichogramma females were visually examined and their phenotypes characterized as male, female, or intersex. The genetic sex of each wasp was established using flow cytometry to determine ploidy. Multiplex real-time quantitative PCR was then used to examine Wolbachia density in each individual. Comparisons between phenotypic and genotypic sex and corresponding bacterial densities will provide valuable information regarding the mechanisms by which Wolbachia induce parthenogenesis in their insect hosts. The results of this study lie at the interface between basic and applied entomology, offering critical insight into the fundamental processes of sex determination in Trichogramma which can immediately and directly contribute to their use as more effective biological control agents.