The Chloridoideae is one of the most difficult grass subfamilies with a history of unsettled taxonomic problems at the tribal and generic levels. A phylogeny based on matK sequence data demonstrated the monophyly of the Chloridoideae, showed its sister group relationship to the arundinoid genus Centropodia, and resolved four major lineages: Triraphis, clade A (comprising the Pappophoreae, Uniolinae, and three genera of Eragrostideae), clade B (including most members of subtribe Sporobolinae, Spartina, Zoysia, and two Eragrostis species), and clade C (encompassing the remaining species). However, a prominent polytomy at the base of the phylogeny prevented the determination of the basal lineages of the Chloridoideae and an assessment of character evolution in the subfamily. In this study, we added sequence data from the trnT-trnF region for representative genera of the four clades and outgroup taxa to provide insight into the topology at the base of the Chloridoideae. Parsimony analysis of trnT-trnF sequences, with Anomochloa as the outgroup, resolves relationships among the four clades and shows Triraphis as the most basal lineage in the Chloridoideae. Combined analysis of matK and trnT-trnF data further support Triraphis as sister to the remaining Chloridoideae followed by a grade comprising clade A, then clades B and C. The Chloridoideae, as traditionally defined, is strongly supported with Centropodia plus Merxmeullera as sister group. However, the clade including these two genera and the Chloridoideae has only modest support. The implications of this assessment on discerning trends in structural character evolution will be discussed.

Key words: Chloridoideae, grasses, matK, phylogeny, Poaceae, trnT-trnF