RESEARCH PAPER Genomic evidence of a widespread southern distribution during the Last Glacial Maximum for two eastern North American hickory species

Abstract

Aim: Phylogeographical studies of temperate forest taxa often infer complex histories involving population subdivision into distinct refugia during the Last Glacial Maximum (LGM). However, some temperate deciduous trees may have been broadly distributed in southeastern North America during the LGM. We investigate genome-wide genetic structure in two widespread eastern North American tree species to determine if range expansion from genetically isolated refugia or from a broader, less genetically subdivided region better explains their post-glacial history. Location: Eastern North America (ENA). Taxa: Bitternut hickory (Carya cordiformis [Wangenh.] K.Koch) and shagbark hickory (Carya ovata [Mill.] K.Koch). Methods: Genetic diversity and differentiation indices were calculated from >1,000 nuclear SNP loci genotyped in ca. 180 individuals per species sampled across ENA. Genetic structure was investigated using principle component analysis and genetic clustering algorithms. As an additional tool for inference, areas of suitable habitat during the LGM were predicted using species distribution models (SDMs). Results: Populations across all latitudes showed similar levels of genetic diversity. Most genetic variation was weakly differentiated across ENA, with the exception of an outlier population of Carya ovata in Texas. Genetic structure in each species exhibited an isolation-by-distance pattern. SDMs predicted high LGM habitat suitability over much of the southeastern United States. Main conclusions: Both hickory species likely survived the LGM in low-density populations that were broadly distributed across southeastern North America and not highly genetically differentiated, except that the range-edge Texas population of Carya ovata may represent a separate glacial refugium. Over most of ENA, genetic structure in both species is best explained by simple latitudinal range shifts and high gene flow among populations, rather than expansions from multiple, genetically isolated refugia as is characteristic of taxa from other Northern Hemisphere temperate regions of the world.