Homozygosity Accumulated Through Genetic Drift and Inbreeding Underpins High Mutation Load in an Endangered Beluga Whale Population.

Isolated populations are susceptible to decline through the accumulation of deleterious mutations and inbreeding depression. As a consequence of habitat alteration, isolated populations are also expected to become increasingly prevalent. Still, some isolated populations have evaded decline through the purging of deleterious mutations. Expanding our knowledge of the scenarios under which purging occurs could therefore improve conservation efforts. Although theoretical and captive studies suggest that genetic purging hinges on a population's demographic history, the factors that shape the probability of purging in natural populations are less resolved. Here, we infer demographic parameters and quantify genetic variation in four Canadian populations of beluga whale to gain insight into purging. In particular, we explore the dynamics of the Saint Lawrence Estuary (SLE), a population that shows signatures of genetic erosion. We find that the SLE has been isolated for hundreds of generations and has accumulated a high mutation load in the absence of purging. We also find evidence of inbreeding from throughout the history of the SLE, excluding the most recent generations. Among deleterious alleles in the SLE, we identify eight clusters unique to neonate mortality-four of which are associated with cellular responses to inorganic substances. Given industrial pollution in the SLE, this finding indicates a relationship between environment, genetic variation and neonate mortality. Our results suggest that the likelihood of purging in natural populations may be reduced when mutation load accumulates via sustained isolation, highlighting the importance of demographic history in shaping conservation risk.