Neutral and adaptive drivers of genomic change in introduced brook trout (Salvelinus fontinalis) populations revealed by pooled sequencing

Understanding the drivers of successful species invasions is important for
conserving native biodiversity and for mitigating the economic impacts of
introduced species. However, whole-genome resolution investigations of the
underlying contributions of neutral and adaptive genetic variation in
successful introductions are rare. Increased propagule pressure should
result in greater neutral genetic variation, while environmental
differences should elicit selective pressures on introduced populations,
leading to adaptive differentiation. We investigated neutral and adaptive
variation among nine introduced brook trout (Salvelinus fontinalis)
populations using whole-genome pooled sequencing. The populations inhabit
isolated alpine lakes in western Canada and descend from a common source,
with an average of ~19 (range of 7-41) generations since introduction. We
found some evidence of bottlenecks without recovery, no strong evidence of
purifying selection, and little support that varying propagule pressure or
differences in local environments shaped observed neutral genetic
variation differences. Putative adaptive loci analysis revealed
non-convergent patterns of adaptive differentiation among lakes with
minimal putatively adaptive loci (0.001%-0.15%) that did not correspond
with tested environmental variables. Our results suggest that (i)
introduction success is not always strongly influenced by genetic load,
(ii) observed differentiation among introduced populations can be
idiosyncratic, population-specific, or stochastic, and (iii)
conservatively, in some introduced species, colonization barriers may be
overcome by support through one aspect of propagule pressure or benign
environmental conditions.