Demographic history shaped geographical patterns of deleterious mutation load in a broadly distributed Pacific Salmon

A thorough reconstruction of historical processes is essential for a
comprehensive understanding the mechanisms shaping patterns of genetic
diversity. Indeed, past and current conditions influencing effective
population size have important evolutionary implications for the efficacy
of selection, increased accumulation of deleterious mutations, and loss of
adaptive potential. Here, we gather extensive genome-wide data that
represent the extant diversity of the Coho salmon (Oncorhynchus kisutch)
to address two objectives. We demonstrate that a single glacial refugium
is the source of most of the present-day genetic diversity, with
detectable inputs from a putative secondary micro-refugium. We found
statistical support for a scenario whereby ancestral populations located
south of the ice sheets expanded in postglacial time, swamping out most of
the diversity from other putative micro-refugia. Demographic inferences
revealed that genetic diversity was also affected by linked selection in
large parts of the genome. Moreover, we demonstrate that the recent
demographic history of this species generated regional differences in the
load of deleterious mutations among populations, a finding that mirrors
recent results from human populations and provides increased support for
models of expansion load. We propose that insights from these historical
inferences should be better integrated in conservation planning of wild
organisms, which currently focuses largely on neutral genetic diversity
and local adaptation, with the role of potentially maladaptive variation
being generally ignored.