Whole genome re-sequencing uncovers significant population structure and low genetic diversity in the endangered clouded apollo (Parnasssius mnemosyne) in Sweden

The clouded apollo (Parnassius mnemosyne) used to have a wide distribution in Fennoscandia. Recent population declines have, however, led to regional extinctions and in Sweden it is currently one of the most endangered butterflies, confined to three geographically separated metapopulations: Blekinge, Roslagen and Vasternorrland. Especially the Blekinge population has declined dramatically and few imagines have been observed during recent census efforts (< 10 in some localities). The clouded apollo is subject to a species action plan which includes both habitat restorations and captive breeding to produce individuals for release and reintroductions. Here, we apply whole-genome resequencing of clouded apollo individuals collected in the three natural populations and the captive population in Sweden and apply population genomic approaches to get a better understanding of the genetic structure and levels of genetic diversity in the species. We find that the clouded apollo populations in the different geographic regions have similar, but comparatively low levels of genetic diversity and we find evidence for significant genetic differentiation between the northernmost population and the populations in southern Sweden. Additional analysis, including previously available mitochondrial data, unveil that a bi-directional re-colonization of Fennoscandia after the latest glacial maximum most likely is the explanation for the considerable differentiation between some Swedish populations. Finally, we find evidence for population sub-structure in one of the Swedish populations. The results provide insights into the genetic consequences of population size declines and fragmentation in general and provide important information for direct conservation actions for the clouded apollo in Sweden in particular.

Automated summary

The clouded apollo butterfly in Sweden is currently endangered and only found in three geographically separated populations. Conservation efforts include habitat restoration and captive breeding. By using whole-genome resequencing and population genomic approaches, we found that the different populations in Sweden have similar but comparatively low levels of genetic diversity, with significant genetic differentiation between the northernmost and southern populations. We also discovered population sub-structure in one of the populations. These findings provide insights into the genetic consequences of population declines and fragmentation, and important information for the conservation of clouded apollo butterflies in Sweden.