Linnéa Smeds

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Any random genetic change is more likely to impair than improve fitness, a situation that owes to the fact that contemporary genotypes bear a history of having been shaped by natural selection for a very long time. Most mutations are thus deleterious and generate a genetic load that can be difficult to handle in small populations and increase the risk of extinction. We used functional annotation and evolutionary conservation scores to study deleterious variation in 200+ genomes from the highly inbred Scandinavian wolf population, founded by only three wolves and suffering from inbreeding depression, and neighboring populations in northern Europe. The masked load was high in Russia and Finland with deleterious alleles segregating at lower frequency than neutral variation. Genetic drift in the Scandinavian population led to the loss of ancestral alleles and fixation of deleterious variants. The per-individual realized load increased with the extent of inbreeding and reached several hundred homozygous deleterious genotypes in protein-coding genes, and a total of more than 50,000 homozygous deleterious genotypes in the genome. Arrival of immigrants gave a temporary genetic rescue effect with ancestral alleles re-entering the population and moving deleterious alleles into heterozygote genotypes. However, in the absence of permanent connectivity inbreeding has then again led to the exposure of deleterious mutations. These observations provide genome-wide insight into the character of genetic load and genetic rescue at the molecular level, and in relation to population history. They emphasize the importance of securing gene flow in the management of endangered populations.