Post glacial reticulate evolution: phylogeography of Packera in western North America

点击翻译按钮获取中文摘要

Packera, a genus of 50+ species in the Asteraceae (Daisy) family, is an ideal group for studies aimed at understanding the evolutionary history of the North American flora because (i) it is widespread in North America, with species occurring in all the major biomes (ii) many of the species are poorly defined due to apparent hybridization and introgression so that patterns of hybridization and resulting gene flow can both be examined in a range of historical and geographical contexts and (iii) high levels of intrapopulational chloroplast DNA (cpDNA) haplotype diversity provide a source of molecular markers than can be analyzed in a phylogenetic context. Packera originated in Mexico and has diversified there, in the Great Plains, the arctic and along the North American west coast. We will examine evolutionary patterns in all these regions to better assess the role of geographical isolation, natural selection, hybridization, refugial survival and species migration in the evolution of the flora. We will examine molecular divergence of both nuclear and cpDNA from the two Mexican mountain ranges where Packera diverged to determine whether this, followed by northern migration and widespread hybridization would explain the basis for the cpDNA diversity observed in many northern taxa. In the arctic, we will assess the importance of refugial survival followed by migration and recent hybridization, while in the Great Plains, we believe widespread species show patterns of high haplotype diversity that may be interpretable in the context of post-glacial migration patterns and resultant hybridization among newly overlapping species. Along the west coast two factors appear important; (i) serpentine soils have shaped the evolution of numerous plant groups including Packera by exerting selection for edaphic tolerance among colonizers and (ii) various coastal refugia may have provided a source of genetic diversity for post-glacial colonizing species throughout this region. Our phylogeographic studies of Packera over the next five years, will improve our understanding of the evolutionary importance of all of these factors and thereby greatly improve our understanding of factors affecting the evolution of the North American flora.