Out of Place: Three Eurasian Taxa Shed Light on Origin of Thermopsideae in North America

Abstract

The North American Thermopsideae, a monophyletic group comprising the North American endemic Baptisia, and the paraphyletic Eurasian-North American disjunct Thermopsis, is nested within the Sophoreae (Fabaceae: Papilionoideae). Previous phylogenetic studies have identified two East Asian taxa within the North American Thermopsideae, suggesting two independent dispersal events between North America-East Asia. More recent studies have also placed a third taxon, Vuralia turcica, an endemic species from Turkey, among the North American Thermopsideae. The presence of three geographically distant Eurasian taxa within a relatively young clade of North American origin is perplexing, and the biogeographic implications of this observation are not clear. To investigate this matter, 1540 low-copy nuclear genes and complete plastomes were obtained from 36 taxa across the core genistoids, including 26 newly sequenced taxa. Nuclear and plastome based maximum likelihood (ML) and ASTRAL analyses were conducted based on varying degrees of taxon coverage and read mapping consensus threshold values. Additional analyses were performed to estimate divergence times and to reconstruct biogeographic history. The results strongly support a relictual Old World clade, presently composed of V. turcica and T. chinensis, which diverged from the ancestor of the North American lineage during the mid to late Miocene. A single and recent North America-East Asia dispersal involving T. lupinoides is reported. Furthermore, the traditional inclusion of the genus Ammopiptanthus among Thermopsideae, and the recent recircumscription of T. turcica to the monotypic Vuralia, are not supported. A relatively high degree of cytonuclear discordance is reported within each sub-clade of the North American Thermopsideae. This finding is likely attributable to the high degree of interspecific hybridization reported within these groups and raises the need for more rigorous genome-scale testing to disentangle their complex phylogenies. Subjects: Biodiversity, Biogeography, Evolutionary Studies, Genetics, Plant Science