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.