Evidence of long-term purging of mutation burden in killer whale Orcinus orca genomes

Genomes of high latitude killer whales (Orcinus orca) harbour signatures of post-glacial founding and expansion. Here, we investigate whether reduced efficacy of selection increased the predicted mutation burden in founder populations with low N_e, or whether recessive deleterious mutations exposed to selection in homozygous genotypes were purged. Comparing the accumulation of synonymous and non-synonymous mutations across pairs of globally sampled genomes reveals that high latitude North Atlantic genomes have accumulated proportionally fewer non-synonymous mutations than other populations. The genome of a 7.5-Kyr-old North Atlantic killer whale, inferred to be closely related to the direct ancestors of the present-day Icelandic and Norwegian populations, was used to calibrate the timing of the action of selection on non-synonymous mutations predominantly to the Holocene. Non-synonymous mutations purged in modern Norwegian killer whale genomes are found as globally shared standing variation in heterozygote genotypes more often than expected, suggesting associative- or pseudo-overdominance. Taken together, our findings are consistent with purging of recessive non-synonymous mutations exposed to selection in founder-associated homozygous genotypes.