Whole-genome evaluation of genetic rescue: The case of a curiously isolated and endangered butterfly

<p>Genetic rescue, or the translocation of individuals among populations to augment gene flow, can help combat genetic erosion, inbreeding depression, and loss of adaptive potential in small and isolated populations. Genetic rescue is currently being considered for an endangered butterfly in Canada, the half-moon hairstreak (<em>Satyrium semiluna</em>). A small, unique population persists in Waterton Lakes National Park, Alberta, isolated from other populations by >350km. However, whether genetic rescue would actually be helpful has not been evaluated. Here, we generate the first chromosome-level genome assembly and whole-genome resequence data for the species. We find that the Alberta population’s genetic diversity is extremely low and very divergent from the nearest populations in British Columbia and Montana. Runs of homozygosity suggest this is due to a long history of inbreeding, and coalescent analyses show that the population has been small, isolated, yet stable for up to 40k years. When a population like this maintains its viability despite inbreeding and low genetic diversity, it has likely undergone purging of deleterious recessive alleles and could be threatened by their reintroduction via translocations. Ecological niche modelling indicates that the Alberta population also exhibits environmental associations that are atypical of the species. Together, these results suggest that translocations are likely to result in outbreeding depression. We infer that genetic rescue has a unique potential to be harmful rather than helpful at present. However, due to reduced adaptive potential, this population may still benefit from future genetic rescue as climate conditions change, and experimental population crosses should be completed.</p>