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