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

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 (Satyrium semiluna). 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.