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 populations of cold-adapted avian sister species from the Holarctic: rock ptarmigan (Lagopus muta) and willow ptarmigan (Lagopus lagopus). We reconstruct the demographic histories of the populations and determine their nucleotide diversity, past and present inbreeding, and mutation load. Genomic vulnerability to future climate change scenarios (also known as offset) is predicted for the 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 environments are rock ptarmigan populations in East Greenland, Iceland, and Svalbard, while among willow ptarmigan, subspecies residing on the British Isles are the most vulnerable.### Competing Interest StatementThe authors have declared no competing interest.