Spatio-temporal genetic changes – the role of a local introduction
Couchoux et al. (2016) had already suggested some geneflow between theH. horticola parasitoid population in Sottunga and the neighbouring northern population inhabiting the Seglinge-Kumlinge islands. Many genotypes found in the original population of Finström were also characterized from wasps from Sottunga and Seglinge-Kumlinge. The movement of genotypes over long distances such as the one separating the islands from the mainland might occur over several generations of gene flow through interbreeding (Couchoux et al., 2016; DiLeo, Husby, & Saastamoinen, 2018), but it is simply more likely that, instead, the genotypes introduced in Sottunga in 1991 have persisted over time on the island, and have dispersed to colonise the islands of Seglinge-Kumlinge in the North. In contrast, the mainland genotypes are not found inH. horticola from the southern islands of Föglö, which is about the same distance from Sottunga than Seglinge is, suggesting restricted gene flow towards Föglö.
The coast-to-coast distances between Sottunga and the neighbouring islands of Seglinge or Föglö are at least 6,5km, with about 12km as the shortest distances between known suitable habitat patches on the two islands (Ojanen et al., 2013). The parasitoid wasp H. horticolais a much better disperser than its butterfly host (van Nouhuys & Hanski, 2002), commonly flying over 1km distance, and travelling up to 7,5km within a breeding season (Couchoux et al., 2016). The flight capacity of the parasitoid allows it to disperse across unsuitable habitats on the mainland, but is most likely not sufficient to cross stretches of open water separating two islands, or the 30km separating the mainland from the shores of Sottunga. Insects can however move across large unsuitable habitats under prevailing winds (Compton, 2002; Pasek, 1988). In the Baltic Sea, a dominant wind blowing from the South (Bierstedt, Hünicke, & Zorita, 2015) could transport wasps from Sottunga to Seglinge-Kumlinge, as well as inhibit southward movements towards Föglö. Insects can also be moved by humans intentionally or incidentally (Kritani & Yamamura, 2003). One of the host plants of the Glanville fritillary butterfly, Veronica spicata (Kuussaari, van Nouhuys, Hellmann, & Singer, 2004) produces indigo blue flower spikes that may be of interests to gardeners on the different islands. The human-assisted migration of H. horticola within their host caterpillars feeding on plants (Carlsson, Hæggström, & Sundberg, 2014), although possible, has not been suggested from any population genetic studies conducted on the Åland butterfly populations (Fountain et al., 2018; Fountain et al., 2016).
While the butterfly was known to occupy Seglinge-Kumlinge prior to year 2000, there is no historical record of the parasitoid H. horticola , and we did not find any unique genotypes nor mitotypes in those islands. Rather, the current parasitoid population on Seglinge-Kumlinge bares resemblance to the introduced parasitoids from Sottunga, suggesting that H. horticola may have been absent from Seglinge-Kumlinge prior to its introduction to Sottunga and subsequent colonization of Seglinge-Kumlinge. If this is true, prior to theH. horticola introduction, the Seglinge-Kumlinge butterfly population may have been free of the parasitoid, which is significant for the butterfly population because where present, H. horticolaparasitizes about 30% of the host larvae (Montovan et al., 2015). To date, it remains unclear whether the introduction of the parasitoid to Seglinge-Kumlinge, where the butterfly had persisted without a specialist parasitoid, has had any influence on the eco-evolutionary dynamics of this local butterfly population.
There are many examples of the effect of isolation on the genetics of introduced Island populations (Hufbauer et al., 2004; Mattila et al., 2012; Miller, Eldridge, Morris, Zenger, & Herbert, 2011; Szucs et al., 2014; Urquia et al., 2019), including a study of human population on the island of Sottunga (O’Brien, Jorde, Rönnlöf, & Eriksson, 1988). Generally, these small isolated populations show low allelic diversity, low heterozygosity and high inbreeding values (Fauvergue et al., 2012; Mattila et al., 2012; Nei, Marutama, & Chakraborty, 1975). The Sottunga population of the parasitoid wasp H. horticola , as well as the isolated populations of Föglö (South) and of Seglinge-Kumlinge (North), show slightly lower heterozygocity (Ho ≈0.3) than the large mainland populations (Ho ≈0.4). This is probably due to loss of genetic diversity in the islands during the founder event and/or following local population crashes. However, heterozygosity remains relatively high compared to studies from other similarly isolated animal populations (O’Brien et al., 1988). Furthermore, all five populations show similar degrees of inbreeding without strong differences between mainland and island populations (Fis =0.18 and 0.23 in the mainland populations, while Fis values vary between 0.21 and 0.24 in the three island populations). This pattern of genetic variability contrasts our original expectations. However, rapid population growth after bottleneck and high dispersive ability are known to counteract the effect of small population size and isolation on both the loss of heterozygosity and inbreeding (Nei et al., 1975).
Spatio-temporal genetic changes – the effect of Wolbachia