Genotypes and mitochondrial haplotypes
We successfully genotyped 7 to 14 microsatellite markers from of 324
samples (Table S1). Although the Bayesian clustering of individuals
analysis (Npop=1) resolved to 31 genetic clusters (k=31,
p=0.999, Appendix 1), both the spatial Bayesian clustering analysis of
five localities (NPop=5, k=4, p=0,999; Appendix 2), and
the spatial Bayesian clustering analysis of 12 spatio-temporal groups
(Npop=12), detected only four genetically differentiated
population clusters (Npop=12, k=4, p=0.999; Appendix 3).
Additionally, spatial Bayesian analyses including all genotyped female
individuals based on both the five localities and the 12 spatio-temporal
groups, suggest restricted gene flow between the four clusters, and
across generations.
Both the five localities and the 12 spatio-temporal groups analyses
suggest that Sottunga and Seglinge-Kumlinge are genetically similar to
one another (Appendix 3). In the Bayesian clustering of localities
(NPop=5) analysis, Sottunga and Seglinge-Kumlinge
populations made up one cluster (FST=0.014, p=0.042, Table 3), and
Föglö, Finström and Saltvik each made up the three other clusters
(Figure 1c). Although the clustering analysis of the 12 spatio-temporal
groups in general support these same clusters, it also reveals that the
early samples from Föglö and Finström, and all samples from Saltvik,
group in the same cluster (red). The two island populations (Sottunga
and Seglinge-Kumlinge) are always found to be genetically different from
Föglö (FST=0.113, p=0.001, Table 3). The shared ancestry of the Sottunga
and Finström samples is visible through detailed screening of the
individual cluster dataset (Npop=1, Appendix 1). The
samples collected in Sottunga belong to 13 genotypes, five of which
(genotypes #9, 11, 14, 17 and 26) also include samples from Finström
collected in 1992 and 1993. These same five genotypes include most of
the samples from Kumlinge-Seglinge (N=32/42, 76%), and are also found
in other localities (Appendix 1), suggesting shared ancestry within each
of the populations.
In general, the admixture analyses revealed little gene flow and
interbreeding between the four clusters (Figure 3). We observe some gene
flow between Föglö and Sottunga, between Föglö and Saltvik (possibly as
a result of dispersal through several generations, or stepping stone
events), as well as between the two mainland populations of Saltvik and
Finström (as a result of dispersal between the two neighbouring regions)
(Figure 3). Additionally, consistent with Couchoux et al. (2016),
inbreeding occurs in Åland at similarly low levels in each of the five
localities, despite the mainland populations being of more connected,
and of larger sizes. The inbreeding coefficient (Fis ), generally
about 0,222 across Åland, ranges from 0,174 in Saltvik, and 0,258 in
Föglö (Table 3); while reaching 0,206 in Sottunga.
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. The genotype
characterizing Föglö after the population crash of 2010 (Figure 1) 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 affect the genotypic clusters of these populations.
Unfortunately, we lack data to directly address the impact of
bottlenecks that occurred in both Seglinge-Kumlinge and Sottunga.
Finally, in agreement with a previous study by Duplouy et al. (2015),
the majority of the H. horticola parasitoids carry the C-mitotype
(N=145, 67%), while the remaining specimens carry the T-mitotype (Table
1). Noticeably, the C-mitotype is prevalent in all localities (57% in
Saltvik, 71% in Sottunga, 84% in Finström, 95% in Föglö) except
Seglinge-Kumlinge (28%) (Table 1). These general patterns hold across
the different spatio-temporal groups (Table 2).
Wolbachia infection status
Wolbachia was detected in samples from all five localities. The
mean infection rate across our samples is 50%, which is consistent with
previous work from Duplouy et al. (2015) across the entire Åland
archipelago. Among the localities, Seglinge-Kumlinge shows the highest
infection rate (95%), greatly contrasting with the lower infection
rates of Finström (42%), Föglö (32%), Saltvik (59%), and Sottunga
(23%) (Table 1).
Among all Wolbachia -infected wasp sampled for the present study,
57% carry the T-mitotype, which is also consistent with the
intermediate infection rate from across the entire Åland archipelago, as
characterized by Duplouy et al. (2015). However, the proportion of
infected wasps carrying the T-mitotype differed among localities. In
Saltvik, Finström and Föglö, only 60%, 33% and 0% ofWolbachia -infected wasps carry the T-mitotype, respectively. In
contrast, despite the two islands showing contrasting proportions of
specimens carrying the T-mitotype (see above), Sottunga and
Seglinge-Kumlinge are once again more similar to each other than to any
other population, with the majority of Wolbachia- infected wasps
carrying the T-mitotype (75% and 79%, respectively) rather than the
C-mitotype. Finally, the four most common genotypes in Seglinge-Kumlinge
(Genotype -5 (N=13); -6 (N=6); -9 (N=6); and -17 (N=6)) are found in
strict associations with Wolbachia- infected wasps, but all mainly
carry the T-mitotype (mean of 77%; genotype -5 (54%), -6 (83%), -9
(100%), -17 (100%), respectively). This contrasts with the rest of
Åland, including Sottunga, where these four genotypes are not strongly
associated with Wolbachia , and more rarely in association with
the T-mitotype than with the C-mitotype (mean of 21%; genotype -5
(11%), -6 (0%), -9 (29%), -17 (50%)). All other genotypes in the
Seglinge-Kumlinge sample are represented by less than 3 samples, and
thus it is difficult to determine patterns from them.