Long-term spatio-temporal genetic structure of an accidental introduction, and the population dynamics of its associated Wolbachia symbiont

Population bottlenecks associated with founder events strongly impact the establishment and character of populations. Founding individuals bring along symbionts, which can affect the host establishment, dynamics and evolution. Thus, to understand introductions and invasions, we should identify the roles played by accompanying symbionts. In 1991, a parasitoid wasp and its associated hyperparasitoid were accidentally introduced from the main Åland islands to an isolated island in the archipelago, along with their host, the Glanville fritillary butterfly. The species have persisted as small populations since. A strain of the endosymbiotic bacterium Wolbachia has an intermediate prevalence across the main Åland parasitoid population. The infection increases susceptibility of the parasitoid to hyperparasitism. We investigated the establishment and dynamics of the parasitoid, along with pattern of prevalence of the symbiont using 324 specimens collected between 1992 and 2013, from five localities across Åland, including the source and introduced populations. Using 14 microsatellites and one mitochondrial marker, we suggest that occasional migration between the islands might have facilitated the persistence of all isolated populations, despite local population crashes. We also show local near-fixation of Wolbachia, where the hyperparasitoid is absent, and selection against infected host genotypes is relaxed.