Characterisation of the historic demographic decline of the British European polecat population

The European polecat ( Mustela putorius ) has a widespread distribution across many countries of mainland Europe, but is documented to be declining within these ranges. In Britain, direct persecution led to a severe decline of the polecat population during the 19 ^th century. Unlike European mainland populations, it is now recovering across much of its former range. The genomic and conservation implications of such a severe demographic decline, followed by the current recovery, have still to be characterised in the European polecat in Britain. Here we carry out population-level whole-genome analyses of 65 polecats from Britain (Wales, England) and the European mainland. Welsh polecats show a unique genetic structure compared to English and European populations. We also reconstructed the demographic history of the Welsh polecat to quantify the magnitude of the bottleneck. Our analyses confirmed the drastic decline of the Welsh polecat’s effective population size, with a severe genetic bottleneck around 40 generations ago (1854-1872). We investigated whether genomic signatures reflected this demographic event and found that Welsh polecats had significantly less genetic diversity than English ones, but not European polecats. Runs of homozygosity and the genetic load present in Welsh polecat genomes also indicated recent historic inbreeding. We show that the increase of the British polecat population could be due to admixture events, but also that the Welsh polecat serves as a distinct genetic population, which could be crucial for the overall conservation of European polecats.