Contrasting genetic structure of rear edge and continuous range populations of a parasitic butterfly infected by Wolbachia

Background: Climatic oscillations are among the long-term factors shaping the molecular features of animals and 9 plants and it is generally supposed that the rear edges (i.e., the low-latitude limits of distribution of any given 10 specialised species) situated closer to glacial refugia are vital long-term stores of genetic diversity. In the present 11 study, we compared the genetic structure of several populations of an endangered and obligate myrmecophilous 12 butterfly (Maculinea arion) from two distinct and geographically distant parts of its European distribution (i.e., Italy 13 and Poland), which fully represent the ecological and morphological variation occurring across the continent. 14 Results: We sequenced the COI mitochondrial DNA gene (the ‘barcoding gene’) and the EF-1α nuclear gene and 15 found substantial genetic differentiation among M. arion Italian populations in both markers. Eleven mtDNA 16 haplotypes were present in Italy. In contrast, almost no mtDNA polymorphisms was found in the Polish M. arion 17 populations, where genetic differentiation at the nuclear gene was low to moderate. Interestingly, the 18 within-population diversity levels in the EF-1α gene observed in Italy and in Poland were comparable. The genetic 19 data did not support any subspecies divisions or any ecological specialisations. All of the populations studied were 20 infected with a single strain of Wolbachia and our screening suggested 100% prevalence of the bacterium. 21 Conclusions: Differences in the genetic structure of M. arion observed in Italy and in Poland may be explained by 22 the rear edge theory. Although we were not able to pinpoint any specific evolutionarily significant units, we 23 suggest that the Italian peninsula should be considered as a region of special conservation concern and one that is 24 important for maintaining the genetic diversity of M. arion in Europe. The observed pattern of mtDNA 25 differentiation among the populations could not be explained by an endosymbiotic infection