The evolution of grasses: phylogeography of C4 photosynthesis

The temperature niches of grasses of the world overlaid onto their phylogenetic relationships.

The two great datasets in biology are the tree of life and the global biogeographic distributions. The first describes the phylogenetic relationships among organisms. The second describes their distributions on our planet. In a rich and well-nuanced paper, Edwards and Smith have brought the two together to shed light on one of the most fundamental questions regarding the evolutionary ecology of plants, namely the origin of C4 photosynthesis. The authors first use an expanded grass phylogeny to describe the origins of C4 photosynthesis in more detail than has done before. They then determine the current distribution of the grass species to determine the climates they occupy.

With regard to the evolution of C4 photosynthesis, the authors conclude that shifts from C3 to C4 photosynthesis did not involve shifts to warmer macroclimates, but instead to drier macroclimates. This results comes as a bit of a surprise--it is less clear that C4 photosynthesis is a response to low water availability as much as high temperatures. Their next logical step is a bit of a leap--namely that these modern geographic differences can be associated with habitat shifts in the past.

As important as the insights into the phylogeography of C4 photosynthesis is that the evolution of cold-tolerance in grasses is more difficult evolutionarily. Cold-tolerance apparently evolved vary early on in the grass radiation and has not been repeated to the degree that C4 photosynthesis has.

In all, this hardly seems like the last word on the topic. The biogeographic data needs to be improved, climatic ranges rather than centers will likely be used, and the grass phylogeny is still relatively unresolved. Also, we still have little understanding of why C4 photosynthesis would benefit plants in dry environments. That said, there is a lot of insight for many types of researchers and a solid step in understanding the strategies of plants to resource scarcity.

Edwards, E. J. and S. A. Smith. 2010. Phylogenetic analyses reveal the shady history of C4 grasses. Proceedings of the National Academy of Sciences 107:2532-2537.