Molecular phylogenies disprove a hypothesized C4 reversion in Eragrostis walteri (Poaceae)

Background and Aims The main assemblage of the grass subfamily Chloridoideae is the largest known clade of C-4 plant species, with the notable exception of Eragrostis walteri Pilg., whose leaf anatomy has been described as typical of C-3 plants. Eragrostis walteri is therefore classically hypothesized to represent an exceptional example of evolutionary reversion from C-4 to C-3 photosynthesis. Here this hypothesis is tested by verifying the photosynthetic type of E. walteri and its classification. Methods Carbon isotope analyses were used to determine the photosynthetic pathway of several E. walteri accessions, and phylogenetic analyses of plastid rbcL and ndhF and nuclear internal transcribed spacer DNA sequences were used to establish the phylogenetic position of the species. Results Carbon isotope analyses confirmed that E. walteri is a C-3 plant. However, phylogenetic analyses demonstrate that this species has been misclassified, showing that E. walteri is positioned outside Chloridoideae in Arundinoideae, a subfamily comprised entirely of C-3 species. Conclusions The long-standing hypothesis of C-4 to C-3 reversion in E. walteri is rejected, and the classification of this species needs to be re-evaluated.