A molecular phylogeny of the genus Alloteropsis (Panicoideae, Poaceae) suggests an evolutionary reversion from C4 to C3 photosynthesis

Abstract

The grass Alloteropsis semialata is the only plant species with both C₃ and C₄ subspecies. It therefore offers excellent potential as a model system for investigating the genetics, physiology and ecological significance of the C₄ photosynthetic pathway. Here, a molecular phylogeny of the genus Alloteropsis is constructed to: (a) confirm the close relationship between the C₃ and C₄ subspecies of A. semialata; and (b) infer evolutionary relationships between species within the Alloteropsis genus. The chloroplast gene ndhF was sequenced from 12 individuals, representing both subspecies of A. semialata and all four of the other species in the genus. ndhF sequences were added to those previously sequenced from the Panicoideae, and used to construct a phylogenetic tree. The phylogeny confirms that the two subspecies of A. semialata are among the most recently diverging lineages of C₃ and C₄ taxa currently recognized within the Panicoideae. Furthermore, the position of the C₃ subspecies of A. semialata within the Alloteropsis genus is consistent with the hypothesis that its physiology represents a reversion from C₄ photosynthesis. The data point to a similar evolutionary event in the Panicum stenodes–P. caricoides–P. mertensii clade. The Alloteropsis genus is monophyletic and occurs in a clade with remarkable diversity of photosynthetic biochemistry and leaf anatomy. These results confirm the utility of A. semialata as a model system for investigating C₃ and C₄ physiology, and provide molecular data that are consistent with reversions from C₄ to C₃ photosynthesis in two separate clades. It is suggested that further phylogenetic and functional investigations of the Alloteropsis genus and closely related taxa are likely to shed new light on the mechanisms and intermediate stages underlying photosynthetic pathway evolution.