Datasets for Mixed support for gene flow as a constraint to local adaptation and contributor to the limited geographic range of an endemic salamander

Understanding mechanisms that underlie species range limits is at the core
of evolutionary ecology. Asymmetric gene flow between larger core
populations and smaller edge populations can swamp local adaptation at the
range edge and inhibit further range expansion. However, empirical tests
of this theory are exceedingly rare. We tested the hypothesis that
asymmetric gene flow can constrain local adaptation and thereby species’
range limits in an endemic US salamander (Ambystoma barbouri) by
determining if gene flow is asymmetric between the core and peripheries of
the species’ geographic distribution and testing whether local adaptation
is swamped at range edges with a reciprocal transplant experiment. Using
putatively neutral loci from populations across three core-to-edge
transects that covered nearly the entire species’ geographic range, we
found evidence for asymmetric, core-to-edge gene flow along western and
northern transects, but not along a southern transect. Subsequently, the
reciprocal transplant experiment suggested that northern and western edge
populations are locally adapted despite experiencing asymmetric gene flow
yet have lower fitness in their respective home regions than center
populations do. Conversely, southern populations exhibit low deme quality,
experiencing high mortality regardless of where they were reared, likely
due to harsher edge habitat conditions. Consequently, we provide rare
species-wide evidence that local adaptation can occur despite asymmetric
gene flow, though migration from the core may prohibit range expansion by
reducing fitness in edge populations. Further, our multi-transect study
shows that multiple, non-mutually exclusive mechanisms can lead to range
limits within a single species.