Abstract
In Fennoscandia, mountain hare (Lepus timidus) and brown hare (Lepus europaeus) hybridize and produce fertile offspring, resulting in gene flow across the species barrier. Analyses of maternally inherited mitochondrial DNA (mtDNA) show that introgression occur frequently, but unavailability of appropriate nuclear DNA markers has made it difficult to evaluate the scale- and significance for the species. The extent of introgression has become important as the brown hare is continuously expanding its range northward, at the apparent expense of the mountain hare, raising concerns about possible competition. We report here, based on analysis of 6833 SNP markers, that the introgression is highly asymmetrical in the direction of gene flow from mountain hare to brown hare, and that the levels of nuclear gene introgression are independent of mtDNA introgression. While it is possible that brown hares obtain locally adapted alleles from the resident mountain hares, the low levels of mountain hare alleles among allopatric brown hares suggest that hybridization is driven by stochastic processes. Interspecific geneflow with the brown hare is unlikely to have major impacts on mountain hare in Fennoscandia, but direct competition may.
Highlights
Species boundaries are frequently challenged by lineage divergence and hybridization [1]
No phylogenetic dating of Lepus and Oryctolagus divergence has been made, comparative evidence from Ig gene sequences suggests that the two genera could be as distant as mice and rats [40], which split almost 12 Ma ago [41]
SNPs that have remained variable between the species during long evolutionary histories, could be enriched in loci under balancing selection, only 277 loci in brown hare and 109 in mountain hare were not in Hardy-Weinberg equilibrium (S3 File)
Summary
Species boundaries are frequently challenged by lineage divergence and hybridization [1]. Diverged lineages (i.e. species) are maintained by barriers to gene flow that vary in strength over time, space, or the genome [2]. The barrier may be permeable [3,4,5]. Behavior, population dynamics and distribution may all result in increased levels of spatial and temporal sympatry between closely related species, leading to an increased frequency of hybridization events. These often have profound effects on a wide range of individual- and population level processes. Hybridization may affect fitness by creating novel combinations of traits adapted to different environments [6,7,8]
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