The phylogenetics and evolutionary history of the northern latitude plant genus Therorhodion (Maxim.) small (Ericaceae)

Abstract

Taxonomic uncertainty in the Arctic-alpine flowering plant genus Therorhodion (Maxim.) Small (Ericaceae) can be attributed to two distinctly different viewpoints representing the taxonomic diversity. Russian taxonomists recognize two species, one with two subspecies, whereas three distinct species are recognized in North America following a broader species concept. Therorhodion redowskianum Hutch. is restricted to Asia, and is unambiguously recognized by both viewpoints. Therorhodion camtschaticum Small and T. glandulosum Standl. ex Small have an amphiberingian distribution in eastern Asia and Alaska with T. glandulosum sometimes recognized as a subspecies of T. camtschaticum. Investigating this taxonomic disagreement creates an opportunity to learn more about the diversification of Beringian taxa and how past glacial events have influenced speciation and the exchange of biota between the continents. I set out to unravel the taxonomic relationships within Therorhodion and the likely dispersal route/s of these amphiberingian taxa through the measurement of macromorphological characteristics from voucher specimens, phylogenetic analyses using plastid and nuclear DNA markers, and divergence time analyses. A comparison of age estimates was also performed based on secondary constraints versus fossil constraints. Although leaf length and width measurements were not reliable delimiting characters, there is strong molecular support for Therorhodion as the sister clade to Rhododendron, and within Therorhodion three strongly supported monophyletic clades representing three species were recovered. The use of secondary constraints in the divergence time analyses resulted in younger age estimates than when fossil constraints were applied, corroborating previous studies. Using fossil constraints I inferred a divergence of Therorhodion from Rhododendron in the late Paleocene with the Asian-restricted species diverging first from the T. camtschaticum/ T. glandulosum clade during the middle Miocene, supporting an Asian origin for the genus. Subsequently, the remaining two species are inferred to have diverged in the middle to late Miocene and further dispersed throughout the Pliocene and Pleistocene as suitable habitat became available through a cooling climate.