Andropogoneae Evolution and Generic Limits in Sorghum (Poaceae) Using ndhF Sequences

Abstract

DNA sequences of the chloroplast gene ndhF were used to estimate the phylogeny of the grass tribe Andropogoneae. Previous hypotheses of relationships in the tribe were based on cytological and mor- phological characters such as the presence/absence of awns, monoecy vs. andromonoecy, or inflorescence characteristics. Classifications were subsequently proposed based on those ideas of relationships, and these are examined in the context of the molecular data. The notion that the base number of the tribe is x=5 is tested using the phylogeny. Diploids with n=5 are not conclusively the earliest diverging members of the group but are nested high in the tree and the earliest diverging lineages in the tribe have n=10. Subtribal designations are not informative due to the lack of resolution between clades in the strict consensus tree. There are well-supported clades in the tribe, including three Sorghum lineages and a core Andropogoneae clade, even though relationships among clades are poorly supported. The topological pattern suggests a rapid radiation for the group. A preliminary assessment of the generic limits of the genus Sorghum show it to be paraphyletic, including the genera Cleistachne, Miscanthus, and a species of Microstegium. Subtribe Sorghinae is represented here by Sorghum, Sorghastrum, Cleistachne, Bothriochloa, Capillipedium, Dichanthium, and Chry- sopogon and does not form a monophyletic group.