Results
Genotyping Hyposoter horticola across Åland
After two rounds of genotyping, we successfully genotyped 7-14
microsatellite markers from 313 samples (Table S1), with majority of the
samples in each spatio-temporal group being genotyped with at least 11
markers. The spatial Bayesian clustering analysis of 12 spatio-temporal
groups (Npop=12, k=4, posterior probability=0.999,
Appendix 1A), and the spatial Bayesian clustering analysis of five
localities (NPop=5, k=4, posterior probability=0.999;
Appendix 3A), detected only four genetically differentiated population
clusters; while the Bayesian clustering analysis of female individuals
(Npop=1) resolved to 16 genetic clusters (k=16,
posterior probability=0.982, Appendix 2). Additionally, spatial Bayesian
analyses including all genotyped female individuals based on either the
five localities and the 12 spatio-temporal groups, show little signature
of shared history (e.g. restricted gene flow) between the four clusters,
and across generations, with little admixture between the different
groups.
Both the five localities and the 12 spatio-temporal groups analyses
suggest that Sottunga and Seglinge-Kumlinge are genetically similar to
one another (Figure 3). In the Bayesian clustering of localities
(NPop=5) analysis, Sottunga and Seglinge-Kumlinge
populations made up one cluster (F ST=0.014,
p=0.045, Table 3), and Föglö, Finström and Saltvik each made up the
three other clusters (Figure 3,F ST>0.05, p=0.001, Table 3).
Although the clustering analysis of the 12 spatio-temporal groups
generally supports the formation of these same four clusters, it also
reveals more detailed spatio-temporal changes, especially in the early
samples from Föglö and FinströmAll samples from Saltvik, group in the
same cluster (Figure 3). The two island populations of Sottunga and
Seglinge-Kumlinge are always found to be genetically different from
Föglö (F ST=0.113, p=0.001, Table 3). The shared
ancestry of the Sottunga and Finström female samples is visible, the
samples collected in Sottunga belong to six genotypes, most of which are
also found in samples collected from Finström in 1992 and 1993 (Table
S1), in Seglinge-Kumlinge, and/or in other localities. For example, the
genotype ‘8’ is found in two samples from Sottunga, five from Finström,
eight from Saltvik, and eight from Föglö, while the genotype ‘15’ is
found in 14 samples from Sottunga, 11 from Seglinge-Kumlinge and two
from Föglö (Table S1). In general, the admixture analyses revealed
little interbreeding between Föglö and Sottunga or between Föglö and
Saltvik, which would have been possible through dispersal through
several generations, or steppingstone events, as well as dispersal over
water. In contrast the analysis showed ongoing geneflow between the two
mainland populations of Saltvik and Finström (Figure 2).
Consistent with Couchoux et al. (2016), we show that inbreeding occurs
in Åland at similarly low levels in each of the five localities, despite
the mainland populations being more connected, and of larger sizes. The
inbreeding coefficient (Gis ), generally about 0.215 across Åland,
ranges from 0.165 in Saltvik, to 0.270 in Föglö (Table 1; Appendix 3B);
while reaching 0.206 in Sottunga (based on the diploid females data
only). The values of heterozygosity are the lowest in Föglö and
Seglinge-Kumlinge (Ho =0.277 & 0.284, Hs =0.379 & 0.383,
respectively) and the highest in Finström and Saltvik (Ho =0.405
& 0.408, Hs =0.523 & 0.488, respectively), with Sottunga showing
intermediate heterozygosity values (Ho =0.345, Hs= 0.410)
(Table 1). At the level of the 12 spatio-temporal groups, the three
temporal groups from Finström and the three temporal groups from Saltvik
also show the highest Ho and Hs values (i.e. generallyHo >0.31 and Hs >0.47; Table 2),
but in that case, the multi-locus dataset for each spatio-temporal group
show that they are not all at Hardy-Weinberg equilibrium (Appendix 1C).
The parasitoid H. horticola in Åland has experienced both global
and local population crashes through the years (Figure 2), some of which
resulted in detectable changes in local genetic structure. For example,
the genotype characterizing Föglö after the population crash of 2010
(Figure 2) is from a different genetic cluster than any of the three
other clusters characterized from the other four populations. In
contrast, other local crashes, for example in 1999 and 2006 in Finström,
and in 1999 in Saltvik, did not seem to affect the genotypic clusters of
these populations, maybe due to their expected larger population sizes.
Unfortunately, we lack data to directly address the genetic consequences
of the population bottlenecks that occurred in both Seglinge-Kumlinge
and Sottunga.