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

Background and Aims: The grass Alloteropsis semialata is the only plant species withboth C₃ and C₄ subspecies. It therefore offers excellent potentialas a model system for investigating the genetics, physiologyand ecological significance of the C₄ photosynthetic pathway.Here, a molecular phylogeny of the genus Alloteropsis is constructedto: (a) confirm the close relationship between the C₃ and C₄subspecies of A. semialata; and (b) infer evolutionary relationshipsbetween species within the Alloteropsis genus. Methods: The chloroplast gene ndhF was sequenced from 12 individuals,representing both subspecies of A. semialata and all four ofthe other species in the genus. ndhF sequences were added tothose previously sequenced from the Panicoideae, and used toconstruct a phylogenetic tree. Key Results: The phylogeny confirms that the two subspecies of A. semialataare among the most recently diverging lineages of C₃ and C₄taxa currently recognized within the Panicoideae. Furthermore,the position of the C₃ subspecies of A. semialata within theAlloteropsis genus is consistent with the hypothesis that itsphysiology represents a reversion from C₄ photosynthesis. Thedata point to a similar evolutionary event in the Panicum stenodes–P.caricoides–P. mertensii clade. The Alloteropsis genusis monophyletic and occurs in a clade with remarkable diversityof photosynthetic biochemistry and leaf anatomy. Conclusions: These results confirm the utility of A. semialata as a modelsystem for investigating C₃ and C₄ physiology, and provide moleculardata that are consistent with reversions from C₄ to C₃ photosynthesisin two separate clades. It is suggested that further phylogeneticand functional investigations of the Alloteropsis genus andclosely related taxa are likely to shed new light on the mechanismsand intermediate stages underlying photosynthetic pathway evolution.