Hybridisation between introduced brown hares and native mountain hares in Sweden

Abstract: To investigate the extent and effect of hybridisation between wild sympatric populations of introduced brown hares (Lepus europaeus) and native mountain hares (L.timidus) in Sweden, the geographic distribution of genetic variation in mitochondrial DNA (mtDNA) and nuclear DNA microsatellite markers were analysed within and between the two species and their presumed hybrids.The mtDNA sequence divergence between the species is approximately 10%,which indicates that the lineages diverged 2-7 million years ago. However, the genetic distance in nuclear genes, calculated from microsatellite data is comparable to would be expected between two different populations of the same species (FST=0.20). The geographic substructuring of mtDNA haplotypes among mountain hares indicates a postglacial recolonisation of Scandinavia both from the south and the north-east. The high mtDNA diversity among brown hares (0.893+/-0.002) is explained with an admixture of introduced individuals with different geographic origins in Europe.Approximately 10% of the brown hares have mountain hare mtDNA, whichindicates extensive hybridisation between the two species and a biased introgression of mtDNA. The distribution of mountain hare mtDNA haplotypes introgressed to brown hares was, however, significantly different from that among pure mountain hares. Possibly, hybridisation and introgression was common during the initial phase of contact between the species. Subsequently, the mtDNA haplotype distribution evolved differently among mountain hares and brown hares with mountain hare mtDNA. The frequency of transmitted mountain hare mtDNA is significantly lower among brown hares in areas where mountain hares occurred less than 100 years ago. Presumably, brown hares with mountain hare mtDNA have a reduced fitness caused by cytonuclear incompatibility between the mountain hare mtDNA and the brown hare nuclear DNA.A population assignment test, along with a neighbor-joining tree based ongenetic distances between individuals, enabled determination of species identity and detection of possible F1 hybrids. Simulations of synthetic hybrids by randomised allocation of alleles from the separate species, combined with the assignment test, strengthened the identification of hybrids and provide new means to evaluate the accuracy of individual assignments.Hybridisation with mountain hares and subsequent introgression may be a waywhereby brown hares achieve new genetic variation and, thus, adapt to earlierunfavourable habitats. However, mountain hares may disappear from south and central Sweden as a result of the interspecific competition induced by hybridisation.