Taxonomy, phylogeny, and inflorescence development of the genus Ixophorus (Panicoideae: Poaceae)

Patterns of morphological variation within the grass genus Ixophorus have led to uncertainty in its phylogenetic position and also to disagreement about the number of species in the genus. We use molecular phylogenetic, developmental, and morphometric approaches to address taxonomic and evolutionary problems in the genus. DNA sequence data for the gene ndhF place Ixophorus within the "bristle grass" clade, which includes Setaria and Pennisetum, and data for the trnL intron plus the trnL-F intergenic spacer show low levels of variation within the genus. Inflorescence development of Ixophorus was compared with that of several Setaria species to identify the stages of development that make Ixophorus so distinctive. Ixophorus is distinguished by (1) abaxially oriented tertiary axes that develop as spikelets, (2) a fixed, low number of orders of branching, (3) synchronous development of spikelets within an inflorescence, and (4) uniform elongation of primary and secondary axes late in development. However, these developmental character states are also shared with various other bristle grass clade members, making them unsuitable for circumscribing the genus. Specimens representing the geographical and morphological ranges of Ixophorus were used to detect groupings based on morphological variation. Principal components analyses, as well as many separate variables, weakly correlated morphological variation with latitude but supported recognition of a single species, Ixophorus unisetus (J. Presl) Schltdl. Common garden experiments show that much morphological variation results from plasticity. A formal taxonomic revision of the genus is presented.