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