Graduate Thesis Defense
|Title:||The phylogenetics and evolutionary history of the northern latitude plant genus Therorhodion (Ericaceae)|
|Date:||Friday, 28 April 2017|
|Location:||Murie Bldg, rm 107|
|Major Professor:||Stefanie Ickert-Bond|
Taxonomic uncertainty in the arctic-alpine flowering plant genus Therorhodion (Ericaceae) has led to two distinctly differing view points representing the taxonomic diversity. Russian taxonomists recognize two species, one with two subspecies, while three distinct species are recognized in North America following a broader species concept. The consistently recognized species T. redowskianum is restricted to Asia, and is unambiguously recognized by both traditions. Therorhodion camtschaticum and T. glandulosum are considered subspecies in Russia and are disjunctly distributed in the Russian Far East and Alaska, making Therorhodion an ideal case study to learn more about the diversification of Beringian taxa and how past glacial events have influenced the exchange of biota between the continents. I set out to better understand the taxonomic relationships within Therorhodion and the likely exchange route/s of the amphiberingian species through the measurement of macromorphological characteristics from voucher specimens, phylogenetic analyses using plastid and nuclear DNA markers, and divergence time analyses. A comparison was also performed looking at the effects secondary constraints versus fossil constraints have on age estimates and confidence intervals. 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 the 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. Following this the remaining two taxa are inferred to have diverged in the middle to late Miocene with further dispersal throughout the Pliocene and Pleistocene as suitable habitat became more available through a cooling climate.