Diversification and Phylogeography of Synthyris (Plantaginaceae) in Northwestern North America

Abstract

Both climatic and geologic activity can shape the distributions of species by altering the habitat in which they live and necessitating populations to either migrate, adapt, elsewise they may go extinct. Migration offers the opportunity for species to encounter new habitats and diverge and landscape elements can either facilitate migration through corridors of suitable habitat or impede migration and cause population fragmentation and isolation with subsequent genetic divergence. Here we seek to examine how past environmental changes, such as mountain uplifts, glaciations, and glacial lake formation and flooding, have shaped the distributions within Synthyris (Plantaginaceae), a genus of approximately 20 species of perennial herbaceous plants found largely found in the American west. By using a phylogenetic framework, we reconstruct the divergence times of the genus, infer the biogeographic history of the group, and reconstruct the ancestral habitat preferences for the genus. We then further examine how specific events of the Pleistocene impacted the phylogeographic history of Synthyris in the Pacific Northwest, specifically of S. rubra and the laciniate corolla clade (S. platycarpa and S. schizantha). Synthyris rubra is found in rocky grasslands and ponderosa pine savannas in the arid Inland Northwest. Phylogeography of taxa in the Inland Northwest has not been previously examined and has been shaped by the complex landscape, glacial/interglacial cycles, and the repeated formation and subsequent flooding of glacial Lake Missoula that covered large areas of Western Montana with water before the floods swept across the Channeled Scablands of Eastern Washington. Synthyris schizantha and S. platycarpa are both rare species that occur in mesic disjunct habitats of Washington/Oregon and Idaho, respectively. Extreme genetic differentiation between the two species suggests an ancient vicariance model of disjunction where the uplift of the Cascade Mountains created a rain shadow and isolating historic populations to moist habitats where they have since become rare.