Chloroplast phylogeography of Dirca palustris L. indicates populations near the glacial boundary at the Last Glacial Maximum in eastern North America

Abstract

AbstractAimLittle is known about the distribution of temperate forest shrubs in eastern North America during the Last Glacial Maximum (LGM), and how far they subsequently migrated to occupy previously glaciated landscapes. We assessed whether populations of Dirca palustris L. persisted near the glacial margin during the LGM. Such populations might explain how the species colonized northern latitudes, despite scant evidence for contemporary long‐distance seed dispersal.LocationEastern North America, Mexico and California.MethodsWe conducted ecological niche modelling (ENM) and sequenced five non‐coding regions of the chloroplast genome from 390 plants among 104 populations representing the four species of Dirca. We used a Bayesian approach to identify relationships among individuals and to estimate timing of lineage splits. The spatial genetic structure of D. palustris was evaluated qualitatively and by statistical methods, including Monmonier's algorithm and analysis of molecular variance (AMOVA).ResultsEcological niche modelling indicated suitable climate at the LGM in the Gulf Coastal Plain, but also north to the Ozark Plateaus Province (northern Arkansas), Interior Low Plateaus and Appalachian Plateaus Provinces (portions of Tennessee and Kentucky), and the Piedmont and Coastal Plain Provinces of the Carolinas and Virginia. Sequence variation was extensive and indicated deep lineage splits within D. palustris. Numerous haplotypes, many private, were identified at middle latitudes (c. 35–40° N), Monmonier's algorithm identified multiple genetic discontinuities within this region, and northern and southern regions differed by AMOVA.Main conclusionsEcological niche modelling and genetic analyses indicate that populations likely existed within several hundred km of the Laurentide ice sheet at the LGM. Deep Pleistocene diversification, along with weak geographical fidelity of haplotypes, probably resulted from repeated isolation, differentiation, and secondary contact during Quaternary glacial cycles. The absence of many northern haplotypes south of 35° N supports the hypothesis that temperate understorey shrubs accompanied temperate trees of eastern North America in populations near the glacial margin at the LGM.