Phylogeny and Taxonomy of Genus Physaria in North America

Physaria is a genus of ~108 species belonging to family Brassicaceae that is predominantly distributed in Western North America, but one species occurs in Arctic Russia and Northern Canada and several species occur in South America. Regardless of the vast number of species in the genus, the genus lacks a well-resolved phylogeny representing many taxa, partially because phylogeny reconstruction is complicated by the fact that many species of Physaria vary in chromosome numbers and ploidy levels. In chapter 1, we review how polyploids are formed and become established and summarize what is known about variation in chromosome number and ploidy in Brassicaceae and in the genus Physaria. In Chapter 2, we extracted DNA representing 84 species of Physaria species and employed a 2b-RAD sequencing technique to generate data for phylogeny reconstruction. The specific goals of the study were 1) to reconstruct the phylogeny of Physaria and assess whether species relationships proposed in early monographs by Payson (1921) and Rollins and Shaw (1973) based on morphology correspond to the current species relationships revealed through the molecular phylogeny representing 86 species of Physaria, 2) to investigate the monophyly of species represented by multiple accessions in the resulting phylogeny, and 3) to investigate how the inclusion of polyploid taxa affects the topology of the phylogeny, which may help shed light on the origins of polyploid taxa. The resulting phylogeny had species from Mexico and Texas at the base of the tree, suggesting that the genus originated in southern North America, although additional outgroups and a formal biogeographic analysis are needed to confirm this result. The species in the phylogeny were grouped into two main clades, one containing species predominantly from eastern North America, and one containing species predominantly from western North America. Except for the group 1 species proposed by Rollins and Shaw in 1973, none of proposed groups of species in monographs formed clades. Instead, the resulting phylogeny grouped species collected from nearby locations irrespective of their taxonomic placement, suggesting a strong biogeographical affinity towards species groupings, possibly due to hybridization within geographic locations. Only a handful of species were monophyletic; in the eastern clade, P. recurvata, P. gracilis, P. angustifolia, P. globosa, and a new species, P. ouachitensisformed monophyletic species in phylogenies in which the polyploid species were present and absent. In the western clade, P. brassicoides, P. pruinosa, P. valida, P. parvula, P. pulvinata and P. intermedia formed monophyletic species when the polyploids were removed. The topology varied depending on whether polyploids were included, suggesting that some species may be hybrids or allopolyploids. Conducting additional chromosome counts, identifying hybrid and allopolyploid taxa, and reconstructing the evolution of ploidy levels is an important area for future studies to understand how they have affected diversification in the group. Overall, the current study resulted a well resolved phylogeny with many taxa of Physaria, which is useful for future studies on understanding the evolutionary history, character evolution, and biogeography of the genus.

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