Collaborative Research: The saboteur's tools: mechanisms for host reproductive manipulation by the bacterial arthropod endosymbiont Cardinium hertigii

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Most insects carry bacteria that live within their cells; these bacteria are inherited from their mothers. The bacteria may manipulate their insect hosts’ reproduction in ways that improves the health or number of female hosts carrying the bacterium. Relevant to this study, these bacterial “symbionts” can sabotage host sperm such that fertilized eggs laid by females without the bacterium die early in life (“cytoplasmic incompatibility,” or “CI”). Currently, two common, unrelated bacteria are known to cause CI in insects: Wolbachia, and Cardinium hertigii, the focus of this study. The main goal of this project is to discover the molecular mechanism by which Cardinium causes CI in insects. The benefits of this work are at least two-fold. Bacteria such as Cardinium target animal cell division, a fundamental process that can be understood better when examining how Cardinium interferes with it. Secondly, CI-causing bacteria may be used for pest or vector management. The CI agent Wolbachia reduces the susceptibility to viruses of insects that carry it, and is currently being introduced around the world to mosquito populations, with major implications for reduction of vector-borne disease and the global bioeconomy. The project will also provide engaging science education targeted for under-represented groups through outreach programs for elementary school (U Arizona Insect Discovery and Insect Festival), high school (production of a book “Animal Manipulating Parasites at NC State, and Iowa State George Washington Carver Internship program) and undergraduate students (via research opportunities at all three institutions). For Cardinium strains that infect Encarsia spp., parasitic wasps of whiteflies, genomic and transcriptomic studies have identified candidate genes that are likely to be important in CI and/or symbiosis. The central hypotheses are that (1) Cardinium effector proteins involved in modifying host DNA during spermatogenesis are associated with the testes, and other Cardinium effectors involved with rescue of modified sperm are present in the ovaries of infected wasps, (2) the role of these proteins can be inferred by identifying the host proteins they interact with and (3) that the B vitamin biotin has a critical role in CI and/or the symbiosis. The project presents a unique opportunity to test these hypotheses with a diverse array of approaches, in four objectives: (1) Compare Cardinium localization and CI candidate gene expression in male pupal Encarsia, the “modification” stage, with patterns observed in adults.(2) Identify Cardinium and Encarsia proteins involved in CI with differential proteomics. (3) Use heterologous expression to identify interacting host proteins of CI candidate proteins. (4) Investigate the role of Cardinium synthesized biotin in modification of host proteins. This project will shed light on the sophisticated CI reproductive manipulation by Cardinium. Cardinium has received comparatively little attention, but produces a virtually identical CI phenotype to Wolbachia with what appears to be completely independent eukaryote-interacting genes, a plasmid and not a phage, and an unusual type 6 secretion system. What is learned about Cardinium candidate gene function, expression and host targets will illuminate CI and symbiosis in both systems.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.