Phylogeographic analyses of the southern leopard frog: the impact of geography and climate on the distribution of genetic lineages vs. subspecies

Abstract

The southeastern United States is a major phylogeographic break hotspot for amphibians, but the processes underlying this hotspot remain to be explicitly tested. We test the correlation of genetic lineages with subspecies breaks in the southeastern United States and the association of such breaks with climate, using Rana sphenocephala as a case study, and place our results in the broader context of the Alabama-Appalachian suture zone (AL-Appalachian SZ). We use genetic and ecological methods to (i) determine whether genetic lineages are coincident with the AL-Appalachian SZ or the subspecies and (ii) test the correlation of major climatic breaks with genetic structure and morphological variation in R.sphenocephala. Bayesian phylogenetic analyses of the ND1 mtDNA gene and microsatellite cluster analyses revealed two distinct lineages with over 4% sequence divergence. The geographic distributions of the two lineages are concordant with the AL-Appalachian SZ but do not correspond to the ranges of the subspecies based on morphology. Mantel tests revealed that isolation by distance and historical barriers to gene flow, rather than climate, are the major drivers of genetic divergence at neutral loci. Examination of climate breaks across the Southeast revealed a pattern incongruent with suture zone hotspots, suggesting that phylogenetic structure has been driven primarily by historical factors, such as isolation, the Appalachian Mountains and the Apalachicola/Chattahoochee/Flint River Basin. However, climate breaks are consistent with the geographic distribution of the subspecies of R.sphenocephala, suggesting that environmental pressures may be driving divergence in morphological traits that outpaces molecular evolution.