Gondwanan evolution of the grass alliance of families (Poales)

Abstract

Phylogenetic interrelationships among all 18 families of Poales were assessed by cladistic analysis of chloroplast DNA rbcL and atpB sequences from 65 species. There are two well-supported main clades; the graminoid clade with Poaceae (grasses), Anarthriaceae, Centrolepidaceae, Ecdeiocoleaceae, Flagellariaceae, Joinvilleaceae, and Restionaceae; and the cyperoid clade with Cyperaceae, Juncaceae, and Thurniaceae. A sister group relationship between Poaceae and Ecdeiocoleaceae is identified with strong support. The sister group of this pair is Joinvilleaceae. These relationships help in elucidating the evolution of grasses and the grass spikelet. Dating of the tree was done by nonparametric rate smoothing of rbcL molecular evolution. Most Poales families date back to the Cretaceous >65 million years ago (mya). Dispersal-vicariance analysis indicates that the Poales originated in South America, the cyperoid clade in West Gondwana (South America or Africa), and the graminoid clade in East Gondwana (Australia). The Trans-Antarctic connection between South America and Australia, and its breakup about 35 mya, probably influenced the evolution of the Poales and the graminoid clade in particular, leading to vicariance between the continents, but the separation of Africa from the other Gondwanan areas, completed about 105 mya, is too old for such a relation.