Polyploid evolution, intercontinental biogeographical relationships and morphology of the recently described African oat genus Trisetopsis (Poaceae)

Abstract

AbstractWe examined the morphology and molecular phylogeny of several genera of the traditional grass tribe Aveneae, with focus on Helictotrichon and the recently described genus Trisetopsis from tropical and subtropical Africa. Altogether we surveyed 34 taxa for structural characters of the spikelets including lemmas, lodicules and ovaries. Sequences of the nuclear single–copy gene topoisomerase 6 (Topo6) spanning two introns, nuclear ribosomal ITS and the plastid matK–psbA region were generated for 81 taxa and analyzed by maximum parsimony and Bayesian methods. The phylogenetic results show African Trisetopsis in all molecular markers investigated clearly distinct from Helictotrichon, which in its new circumscription encompasses only Northern Hemisphere species. Morphological data support exclusion of the African species from Helictotrichon as a separate genus. The Topo6 sequences reveal a strong geographical pattern of variation which allows for comprehensive phylogeographic analyses. Two copy types of Topo6 are present in Trisetopsis (A and B). They are strongly different from that found in the Eurasian species of Helictotrichon. Copy type A probably descended from South African T. longa or T. rogerellisii, which appear to be the only African species with only type A. All other species of Trisetopsis contain both types A and B. Unexpectedly, type B is widespread in the New World (Calamagrostis, Graphephorum, Peyritschia, Sphenopholis) but, apart from the Trisetopsis allopolyploids, is absent from Africa. Trisetopsis imberbis is introduced as a new combination.