Using genomic data to inform genetic rescue of endangered butterflies

Genomic factors are important considerations in the conservation of small and isolated populations, where inbreeding depression and reduced adaptive potential increase extinction risk. Here, we generate a chromosome-level genome assembly and whole-genome resequence data for an endangered butterfly, the half-moon hairstreak (Satyrium semiluna). A small and presumably unique population persists in Waterton Lakes National Park, Alberta, isolated from all other populations by >350km. Genetic rescue via translocations was recently proposed to ameliorate genetic erosion, but whether this would actually be helpful has not previously been evaluated. Our analyses indicate the Alberta population’s genetic diversity is very low and divergent relative to the nearest populations in British Columbia and Montana. Runs of homozygosity suggest this is due to a long history of inbreeding. Coalescent-based analyses show that the population has been small, completely isolated, yet very stable for ~40k years. When a population like this maintains its viability despite inbreeding and low genetic diversity, it has likely undergone genetic purging and could be threatened by reintroduction of deleterious recessive alleles via translocations. Ecological niche modelling indicates that the Alberta population also exhibits environmental associations that are atypical of the species. Together, niche and genomic divergences 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 here, and recommend that experimental population crosses be completed before action is taken. Due to reduced adaptive potential, this population may still benefit from future genetic rescue as climate conditions change.