Traversing the Great Lakes: Post-glacial colonization by a widespread terrestrial salamander

Aims: Glacial retreat at the end of the Pleistocene epoch opened vast expanses of emergent habitat in the northern hemisphere that were colonized by opportunistic taxa. However, species that undergo post-glacial expansion may have originated from one or several glacial refugia. We inferred the post-glacial expansion history of the Eastern Red-backed Salamander (Plethodon cinereus), a fully terrestrial species with a vast distribution despite severe dispersal limitations. Previous studies indicated populations south of the glacial boundary at the eastern and western limits of the distribution were closely related, suggesting either multiple refugia or an extraordinary post-glacial expansion event.

Location: Eastern North America.

Taxon: Plethodon cinereus (Green, 1818), Caudata: Plethodontidae.

Methods: We collected ddRAD-seq data from 106 individuals throughout the distribution of P. cinereus. We estimated phylogeographic structure, including finer-scale structure among the post-glacial populations. To test the origins and routes of colonization, we used ecological niche modeling, population trees, and analyses of directional range expansion.

Results: Analyses supported our hypothesis of a southeastern glacial refugium, with northward expansion along the Eastern Seaboard prior to westward invasion into the Great Lakes region, including southwestern expansion into unglaciated areas at the western end of the distribution. However, a distinct subgroup in the northwestern portion of the range raises the possibility of a second refugium near the ice-free Driftless Area.

Main conclusions: Based on our results, we hypothesize a southeastern refugium from which most of today’s northern populations undertook extensive post-glacial colonization. Our results indicate a geographically non-linear colonization history for P. cinereus.

Methods
Phylogeographic data represent ddRAD-seq for 106 samples of Plethodon cinereus.

See Materials and Methods of Waldron et al. (2024) for details.