Spatio-temporal genetic changes – the role of a local introduction
In a previous study, Couchoux et al. (2016) suggested some ongoing genetic mixing between the H. horticola parasitoid population in Sottunga and the neighbouring northern population inhabiting the Seglinge-Kumlinge islands. Many genotypes found in the original mainland population of Finström were evident in wasps from both Sottunga and Seglinge-Kumlinge islands. The movement of genotypes over long distances such as the one separating the islands from the mainland might occur over several generations of ongoing gene flow through interbreeding (Couchoux et al., 2016; DiLeo, Husby, & Saastamoinen, 2018; Slarkin, 1985), but it is simply more likely that, instead, the genotypes introduced on Sottunga in 1991 have persisted over time on the island, and that dispersal between Sottunga and the islands of Seglinge-Kumlinge in the North has occurred. In contrast, the mainland genotypes are not found in H. horticola from the southern islands of Föglö, which is about the same distance from Sottunga as Seglinge is, but further away from the mainland populations.
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 islands (Ojanen et al., 2013). The parasitoid H. horticola is more dispersive than its butterfly host (van Nouhuys & Hanski, 2002), commonly flying over 1km distance, and potentially travelling up to 7.5km over land 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 interest 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), and is thus thought unlikely.
While the butterfly was known to occupy Seglinge-Kumlinge prior to year 2000, there is no local 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 resembles to the introduced parasitoids from Sottunga, suggesting that H. horticola from Sottunga may have colonized Seglinge-Kumlinge. If this is true, prior to the H. horticolaintroduction, the Seglinge-Kumlinge butterfly population may have been free of the parasitoid. This is significant for the butterfly because where present, H. horticola parasitizes 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 may have 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 and 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 and of Seglinge-Kumlinge, show slightly lower observed heterozygocity (Ho ≈0.3) than the large mainland Åland populations (Ho ≈0.4). This is probably due to loss of genetic diversity in the islands following local population crashes. However, heterozygosity remains relatively high compared to studies from other similarly isolated animal populations (Fountain et al., 2016; O’Brien et al., 1988; Sarhan & Kokko, 2007), which may be due to the relatively large founding populations of 71 larval hosts nests. Most of these nests would have contained H. horticola from different families (Couchoux et al., 2015a). The processes of species invasion as well as intentional introduction for biological control are often hindered by the genetic consequences of small founding populations (Fauvergue et al., 2012; Hufbauer et al., 2004; Hufbauer, Rutschmann, Serrate, Vermeil de Conchard, & Facon, 2013). Furthermore, while there is some evidence of occasional strong inbreeding (Fis values appendix 1c), all five populations show similar degrees of overall inbreeding without strong differences between mainland and island populations (Gis =0.18 and 0.23 in the mainland populations, whileGis values vary between 0.21 and 0.24 in the three island populations). 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), our study might represent such example of a clear ancestry being still visible despite local bottlenecks and long-term isolation of some populations.
Spatio-temporal genetic changes – the effect of Wolbachia