Genomic vulnerability to climate change and mutation load are affected by past declines in effective population size in two sedentary arctic bird species

Using whole genome re-sequencing data we study the effects of climate influenced declines in effective population size on the accumulation of deleterious mutations and the response to future climate change in set of populations of two cold-adapted sedentary ptarmigan bird sister species from the Holarctic: rock ptarmigan and willow ptarmigan. We use reconstruct the demographic histories of the studied populations and determine their nucleotide diversity, past and present inbreeding and mutation load. We predict genomic vulnerability (sometimes referred to as offset), defined as the mismatch between current and predicted future genomic variation based on genotype-environment relationships given future climate change scenarios in contemporary populations. We show that relatively small and isolated populations have reduced nucleotide diversity, higher signatures of past and present inbreeding and higher estimates of mutation load. Among the studied populations, the most vulnerable to a mismatch between current and predicted future environmentare rock ptarmigan populations in East Greenland, Iceland and Svalbard in the North Atlantic and among willow ptarmigan, the subspecies residing on the British Isles, red grouse, is the most vulnerable. ### Competing Interest Statement The authors have declared no competing interest.