C4 photosynthesis evolved in grasses via parallel adaptive genetic changes

Phenotypic convergence is a widespread and well-recognized evolutionary phenomenon. However, the responsible molecular mechanisms remain often unknown mainly because the genes involved are not identified. A well-known example of physiological convergence is the C-4 photosynthetic pathway, which evolved independently more than 45 times [1]. Here, we address the question of the molecular bases of the C-4 convergent phenotypes in grasses (Poaceae) by reconstructing the evolutionary history of genes encoding a C-4 key enzyme, the phosphoenolpyruvate carboxylase (PEPC). PEPC genes belong to a multigene family encoding distinct isoforms of which only one is involved in C-4 photosynthesis [2]. By using phylogenetic analyses, we showed that grass C-4 PEPCs appeared at least eight times independently from the same non-C-4 PEPC. Twenty-one amino acids evolved under positive selection and converged to similar or identical amino acids in most of the grass C-4 PEPC lineages. This is the first record of such a high level of molecular convergent evolution, illustrating the repeatability of evolution. These amino acids were responsible for a strong phylogenetic bias grouping all C-4 PEPCs together. The C-4-specific amino acids detected must be essential for C-4 PEPC enzymatic characteristics, and their identification opens new avenues for the engineering of the C-4 pathway in crops.