Ecology, evolution, and genetics of parasitism and symbiosis in insects

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Infection by pathogens and parasites is one of the most powerful threats that all organisms face, and is a major driver of innovation and rapid evolution. Indeed, organisms have evolved diverse defenses to deal with infection, including sophisticated immune systems and pathogen avoidance behaviours. My lab studies the evolution and ecology of host-parasite interactions, using insects as model hosts, and in this proposal, I will study the role of two pervasive but cryptic players defensive symbionts and selfish genetic elements. Recently, we have come to appreciate that many organisms harbour beneficial microbes that protect them against harmful infections. Yet we do not have a good understanding of how important symbiont-mediated protection is in the wild, or how it interacts with other modes of defense, such as the immune system. Nor do we have a good understanding of how natural enemies counter-resist symbionts. We recently found that the common woodland fly, Drosophila neotestacea, harbours a strain of the bacterial symbiont Spiroplasma, that protects it against a virulent parasitic nematode. The benefit conferred by the symbiont is so great that symbiont-infected flies are rapidly spreading across N. America, and we will determine how this has affected the parasite and the various fly species it infects. Have nematodes evolved resistance to symbionts? Spiroplasma also protects hosts against parasitic wasps, and in parallel, we will determine whether this occurs and is important in the field. We have also discovered that the parasitic nematode itself harbours an obligate bacterial endosymbiont and we will determine its role in the fly-nematode interaction. Organisms also commonly harbour genomic parasites also known as selfish genetic elements. These break the basic rules of Mendelian inheritance in order to gain a transmission advantage, often with negative fitness consequences. Selfish genetic elements are pervasive burdens on host genomes but they are much less studied because they are cryptic and often suppressed by the rest of the genome. One of the most striking types of selfish genetic elements are selfish X chromosomes, whereby males that carry a selfish X chromosome produce a gene product that destroys Y-bearing sperm, siring almost exclusively daughters. This sex ratio distortion can have drastic population consequences. How do selfish X chromosomes persist over evolutionary time? Do they have other fitness consequences? How does the rest of the genome suppress them? What is the mode of action of selfishness and resistance to it? We recently discovered a selfish X chromosome, as well as an autosomal gene that suppresses it, in D. testacea (the sister species of D. neotestacea) and we will answer these questions, using both ecological and genomic approaches.