Abstract
Deeply divergent mitochondrial genomes can reveal hidden diversity within species, however robust assessments of diversity require corroborative divergence in the nuclear genome. Previous phylogeographic analysis of the Common Blue butterfly (Polyommatus icarus) revealed a deeply divergent mitotype in Europe. Here, we reconstruct the phylogeography of this butterfly in the British Isles using both mitochondrial and nuclear DNA. We found strong geographical structuring of three distinct CO1 mitotypes with an ancient lineage restricted to the Outer Hebrides. The short time (c. 20,000 years) available for colonization of the British Isles and an expanded CO1 phylogeography from the species’ entire range supports a hypothesis of at least 3 colonization events with sequential replacement of mtDNA. Analysis of ddRADSeq genomic markers from 184 individuals across the British Isles indicates that genetic clustering is similar but divergence weaker at the nuclear level, consistent with a biogeographic scenario of multiple colonisations. We were also able to use the ddRADSeq to infer the infection status of all individuals sequenced. Interestingly, we found evidence for high frequency (>70%) infections by two different Wolbachia strains, with strong mitotype associations, in the Northern populations of the British Isles. Wolbachia-induced cytoplasmic incompatibility may be sufficient to explain the mito-nuclear discordance observed across an admixture zone on the mainland and the complete dominance of a deeply divergent mitotype on the Outer Hebrides. Moreover, the strain (wIca1), largely restricted to the Outer Hebrides potentially causes phenotypic feminisation of genetic males, a known but rare phenotypic effect of Wolbachia in Lepidoptera. Therefore, the impact of Wolbachia on the biology and genetic diversity of this butterfly merits further attention.
Competing Interest Statement
The authors have declared no competing interest.