Post-glacial expansion and evolution of North American goldenrods (Solidago subsect. Humiles)

Abstract

Aim We analysed range-wide chloroplast DNA (cpDNA) variation in a clade of North American goldenrods (Solidago subsect. Humiles) to infer its biogeographical history and evolution. Our objectives were to: (1) examine the structuring of cpDNA diversity in this widespread species complex, (2) reconstruct Pleistocene refugia and post-glacial migration of the study species, and (3) test hypotheses relating to the frequency of polyploidization. We expected the glacial history of Solidago to differ markedly from that of temperate trees and forest understorey plants. Location North America (Canada, continental USA, Mexico). Methods 1466 bp of chloroplast intergenic spacer DNA (cpDNA) were sequenced from 368 individuals representing 72 populations of subsect. Humiles, which consists of the widespread Solidago simplex and four geographically restricted species. Estimates of NST and GST were compared as a test of phylogeographical structure, and spatial analysis of molecular variance (SAMOVA) was used to examine cpDNA variation. Rarified haplotype diversity and chromosome diversity (ploidy levels) were used to infer locations of glacial refugia and post-glacial expansion, and to determine origins of polyploidy, respectively. Results A total of 46 haplotypes were recovered. While there was significant phylogeographical structure (NST > GST), cpDNA variation was not strongly partitioned across species boundaries, geography or ploidy levels, and six haplotypes were shared among species. The highest haplotype diversity was located in western North America, followed by the south-eastern USA and the formerly glaciated Great Lakes region. Main conclusions Solidago simplex recolonized formerly glaciated eastern North America from refugia in western North America and near the perimeter of the ice margin. The south-eastern USA had only limited involvement in recolonization of these northern regions. The geographical disjunction and scattered positions of polyploids in the haplotype network provide evidence of multiple polyploid origins within S. simplex, and the restriction of endemic, polyploid taxa to post-glacial habitats provides evidence of Holocene polyploid speciation. The results highlight polyploidization as a source of adaptive genetic variation and speciation in novel and post-glacial habitats.