A phylogenetic study of Danthonia DC. (Poaceae) in North America
The Danthonioideae (Poaceae) has been the focus of several systematic studies. Previous studies have shown the subfamily is monophyletic, but relationships within several groups, including Danthonia, have not been examined in depth. To address the question of the monophyly in North American representatives of Danthonia, this study utilized a holistic approach, examining both molecular and morphological features. A molecular phylogeny was constructed based on the trnL-F region of the chloroplast genome, and micromorphological characters of the leaf epidermis, caryopsis, and lodicules were examined by scanning electron microscopy. This study involved a phylogenetic examination of the subfamily Danthonioideae, with emphasis on North American Danthonia. The objectives were twofold: 1) to determine whether representative species of Danthonia in North America form a monophyletic assemblage based on molecular and morphological characters, and 2) to examine relationships of the North American Danthonia species to putative related species in the Southern Hemisphere and Europe. The survey included the genera Austrodanthonia, Cortaderia, Danthonia, Merxmuellera, Notodanthonia, Rytidosperma, Tribolium, and eight North American Danthonia species. South American representatives of Danthonia were included for comparative purposes. Several nicromorphological epidermal features for danthonioid grasses were examined, including macrohairs, bicellular microhairs, prickle hairs, silica bodies, and stomatal complexes. Macrohairs are large, unicellular basifixed structures. Bicellular microhairs are of three types: 1) microhairs with a long basal cell relative to the terminal cell, 2) microhairs with basal and terminal cells approximately equal in length, and 3) microhairs with a short basal cell relative to the terminal cell. Prickle hairs in the costal regions of the leaf epidermis of four species of North American Danthonia are reported for the first time. Epidermal silica bodies in danthonioid grasses are dumbbell-shaped, tall and narrow, or cross-shaped. The stomatal complexes are paracytic with two dome-shaped subsidiary cells. No distinguishing characters were found at the subfamily level. Danthonia is characterized by the absence of abaxial stomata, presence of bicellular microhairs with basal and terminal cells of equal length, as well as microhairs with long basal cells relative to terminal cells. These findings provide a new framework useful for interpretating and re-evaluating taxonomic and phylogenetic relationships in the Danthonioideae. Caryopsis features were useful in reconstructing the phylogeny of the Danthonioideae. Three features associated with the caryopsis, hilum, and surface pattern of Danthonia included: 1) ovoid to obovoid caryopsis shape, 2) linear hilum, and 3) undulating or straight reticulate surface pattern. No other taxon examined in this study possesses this combination of characters. Secondly, Rytidosperma is characterized by 1) ovoid caryopses that are generally smaller than the caryopses in Danthonia, 2) punctate hila, and 3) undulating reticulate or substriate caryopsis surface patterns. Finally, Tribolium has 1) small obovoid caryopses ¡Ü1.2 mm in length, 2) punctate hila, and 3) a substriate caryopsis surface pattern. Though Cortaderia shares the linear hilum and undulating reticulate surface pattern with Danthonia, the lanceolate caryopsis differs from the ovoid to obovoid caryopsis of Danthonia. The subfamily Danthonioideae is monophyletic. Within this subfamily two monophyletic clades, i.e. Danthonia and Rytidosperma, were identified. Cortaderia selloana is basal to the aforementioned clades. Although the Danthonia clade is monophyletic, this study does not support the separation of North and South American and European species, and there is poor resolution of terminal branches within the clade. Interspecific relationships within Danthonia are not clear, but evidence suggests the taxonomic separation of Danthonia from Rytidosperma, two genera that were previously considered to be closely related. The Rytidosperma clade is composed of Austrodanthonia, Notodanthonia, Rytidosperma, Tribolium, and Merxmuellera. Though taxonomic sampling of South American Rytidosperma only included one species, the trnL-F strict consensus tree shows strong support for its inclusion in the Rytidosperma clade, demonstrating that South American representatives of Rytidosperma are distinct from Danthonia. Within the Rytidosperma clade, the genus Rytidosperma may be paraphyletic.
Danthonia, Poaceae, phylogeny
Master of Science (M.Sc.)