Oligocene CO sub(2) Decline Promoted C sub(4) Photosynthesis in Grasses

C sub(4) photosynthesis is an adaptation derived from the more common C sub(3) photosynthetic pathway that confers a higher productivity under warm temperature and low atmospheric CO sub(2) concentration. C sub(4) evolution has been seen as a consequence of past atmospheric CO sub(2) decline, such as the abrupt CO sub(2) fall 32-25 million years ago (Mya). This relationship has never been tested rigorously, mainly because of a lack of accurate estimates of divergence times for the different C sub(4) lineages. In this study, we inferred a large phylogenetic tree for the grass family and estimated, through Bayesian molecular dating, the ages of the 17 to 18 independent grass C sub(4) lineages. The first transition from C sub(3) to C sub(4) photosynthesis occurred in the Chloridoideae subfamily, 32.0-25.0 Mya. The link between C0 sub(2) decrease and transition to C sub(4) photosynthesis was tested by a novel maximum likelihood approach. We showed that the model incorporating the atmospheric CO sub(2) levels was significantly better than the null model, supporting the importance of CO sub(2) decline on C sub(4) photosynthesis evolvability. This finding is relevant for understanding the origin of C sub(4) photosynthesis in grasses, which is one of the most successful ecological and evolutionary innovations in plant history.