Reduction in post-invasion genetic diversity in Crangonyx pseudogracilis (Amphipoda: Crustacea): a genetic bottleneck or the work of hitchhiking vertically transmitted microparasites?
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Parasites can strongly influence the success of biological invasions. However, as invading hosts and parasites may be derived from a small subset of genotypes in the native range, it is important to examine the distribution and invasion of parasites in the context of host population genetics. We demonstrate that invasive European populations of the North American Crangonyx pseudogracilis have experienced a reduction in post-invasion genetic diversity. We predict that vertically transmitted parasites may evade the stochastic processes and selective pressures leading to enemy release. As microsporidia may be vertically or horizontally transmitted, we compared the diversity of these microparasites in the native and invasive ranges of the host. In contrast to the reduction in host genetic diversity, we find no evidence for enemy release from microsporidian parasites in the invasive populations. Indeed, a single, vertically transmitted, microsporidian sex ratio distorter dominates the microsporidian parasite assemblage in the invasive range and appears to have invaded with the host. We propose that overproduction of female offspring as a result of parasitic sex ratio distortion may facilitate host invasion success. We also propose that a selective sweep resulting from the increase in infected individuals during the establishment may have contributed to the reduction in genetic diversity in invasive Crangonyx pseudogracilis populations.
KeywordsBiological invasions Enemy release Emergent disease Microsporidia Sex ratio distortion Vertical transmission
This research was carried out at The University of Leeds. JSG was supported by an ORS award and a University of Leeds John Henry Garner scholarship. Samples were collected with the assistance of an Ann Bishop Travel Award from the British Society for Parasitology. JS and AD acknowledge support from the Natural Environment Research Council and the Biotechnology and Biological Sciences Research Council (NE/DO12937/1). We thank Beth McClymont, Genie White, Lucas Terracina, Joan Jass, William Brooks and Alex Douglas for technical support and two anonymous referees for helpful suggestions.
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