infection
While the Wolbachia strain w Hho occurs at an intermediate prevalence across the Åland Islands (Duplouy et al., 2015), on the islands of Seglinge-Kumlinge, it has been almost at fixation since the year 2000, and potentially earlier. As shown by Duplouy and colleagues (2015), this particular Wolbachia strain appears not to affect dispersal capacity of H. horticola , thus the highWolbachia prevalence in Seglinge-Kumlinge is unlikely to be related to differential dispersal of infected wasps to these islands. In contrast, w Hho-infected wasps are more susceptible to hyperparasitism by the specialist hyperparasitoid wasp Mesochoruscf. stigmaticus (van Nouhuys et al., 2016). Mesochorus cf.stigmaticus is common across the Åland mainland (Nair et al., 2016), and is also present in Sottunga and Föglö (van Nouhuys & Hanski, 2005). The hyperparasitoid wasp restrains the spread of w Hho inH. horticola by keeping the infection at lower prevalence when highly abundant (van Nouhuys et al., 2016), while the absence ofM. cf. stigmaticus in Seglinge-Kumlinge (Nair et al., 2016; van Nouhuys & Hanski, 2005; van Nouhuys et al., 2016) releases the selection pressure on w Hho-infected H. horticolawasps, and allows the spread of the symbiont in this isolated wasp population (van Nouhuys et al., 2016).
The spread of a maternally inherited symbiont, such as Wolbachia , may lead to a simultaneous increase in prevalence of the mitochondrial genotypes carrying the symbiont, as selection on the symbiont will promote the fitness of the infected genotypes (Charlat et al., 2009; Duplouy et al., 2010; Schuler et al., 2016). In H. horticola in Åland, C- and T-mitotypes associate with the w Hho infection, but the C-mitotypes are less associated with the infection, potentially because the trans-generation transmission of this Wolbachiastrain is less efficient in females carrying the C- over the T-mitotype (Duplouy et al., 2015). Consequently, we expected that the T-mitotype would be found at low frequency in Seglinge-Kumlinge, as it was in the original population of Finström (16%), as well as in Sottunga (29%). Instead, we found that the T-mitotype is prevalent in Seglinge-Kumlinge (72%). The spread of the wasps carrying the T-mitotype in Seglinge-Kumlinge could result from: (I) the selective sweep of the T-matriline during the spread of Wolbachia in Seglinge-Kumlinge in the absence of pressures from the hyperparasitoid, (II) strong bottlenecks randomly selecting for individuals from the T-matriline over the C-matriline in Seglinge-Kumlinge during migration events from Sottunga; and (III) uncharacterized fitness benefits associated with the T- mitotype. According to our data, all genotypes in Seglinge-Kumlinge are found in the rest of Åland, which suggests that several migration events have occurred between the two populations over the 22 years period of our study. This rate of migration considerably reduces the probability of a high frequency of the T- mitotype in Seglinge-Kumlinge due to bottlenecks, especially because the C-mitotype is significantly more prevalent in the source population of Sottunga. It is then more likely that selection acts on the wasps after migration in Seglinge-Kumlinge. Additionally, there is currently no evidence that the T-mitotype provides any benefit to its host, as it is generally rare across Åland (Duplouy et al., 2015). Consequently, the high prevalence of the T-mitotype in Seglinge-Kumlinge is most likely due to the spread of w Hho under relaxed predation pressures in this island.