Angiosperm Responses to a Low-CO2 World: CAM and C4 Photosynthesis as Parallel Evolutionary Trajectories
Crassulacean acid metabolism (CAM) and C4 photosynthetic syndromes have much in common: they employ a shared biochemical pathway that enables the concentration of CO2 inside plant cells, they are both considered to be adaptations to stressful environments, and they are both arguably among the most convergent of complex traits, having each evolved multiple times in various plant lineages. They are also both signature elements of stress-adapted floras the world over and play fundamental roles in the ecological success of flowering plants. In spite of these similarities, the obvious phenotypic and ecological differences between certain groups of fully optimized C4 and CAM plants have led us to generally view these syndromes as very distinct ecological adaptations. A broad look at the distribution of CAM and C4 plants across a very large phylogeny of angiosperms highlights that while CAM photosynthesis seems to have evolved more often, both CAM and C4 origins show tight and overlapping clustering in many regions of the tree, suggesting that certain plant lineages are prone to evolve either pathway. Additionally, recent phylogenetic analyses revealed that the origins and diversification of many CAM and C4 lineages were recent and contemporaneous in time. We postulate that the evolutionary “starting points” for CAM and C4 pathways could be much more similar than typically acknowledged. Using species with C3-C4 and CAM-like intermediate phenotypes as models of CAM and C4 evolution has been productive, but the distinct advantages that each affords may have promoted rapid ecological divergence that subsequently masked any shared ancestral characteristics between the two pathways. Focusing on newly discovered phylogenetic “hotbeds” of CAM and C4 evolution will allow for inclusion of relevant C3 taxa and a finer evaluation of the possible environmental and organismal traits that would strongly favor the evolution of one syndrome over the other.