Abstract

Phylogenetic relationships of the grass family inferred from the sequences of the internal transcribed spacer (ITS) region of the nuclear ribosomal DNA (rDNA) are generally in accord with the boundaries of the six commonly recognised subfamilies: Bambusoideae, Pooideae, Arundinoideae, Centothecoideae, Chloridoideae and Panicoideae. ITS phylogeny recognises the herbaceous bamboo allies, Streptochaeta and Pharus, as the basal taxa to all grasses. The core grass lineage is resolved into three major clades: the basal Bambusoideae is sister to the monophyletic Pooideae and the Panicoideae–Arundinoideae–Centothecoideae–Chloridoideae (PACC) clade. Several genera with uncertain taxonomic affinities, Lygeum, Nardus, Brachyelytrum, Diarrhena, Anisopogon, Ampelodesmos, and the tribe Stipeae, are all clustered with a broadly defined Pooideae, and may be the ‘missing links’ between the Pooideae and the Bambusoideae. Relationships of the PACC clade indicate that C₄ photosynthesis evolved independently among and within the PACC subfamilies. ITS phylogeny of the grass family, and evidence from the chloroplast genome, cytogenetics, fossil records, biogeography, and plate tectonic theory, suggest that the origin of the grasses is probably ‘out of South America’.