Comparisons of Multiple Indepedent Evolutionary Origins of C4 Photosynthesis

C4 photosynthesis is an powerful model to address the basic question how do new complex traits evolve? The evolution of the C4 cycle required the co-option of enzymes from unrelated biochemical pathways, as well as significant changes in the leaf developmental program to create a morphology more suitable for the new photosynthetic cycle. C4 photosynthesis evolved independently multiple times providing something quite rare in both comparative genomics and evolutionary biology: biological replication. This study focused on the PACMAD grasses a clade estimated to include more than 20 independent evolutionary origins of C4 photosynthesis. The two C4 origins represented by species with sequenced genomes were compared using both cell type specific and developmental gradient RNA-seq data as were two species (maize and sorghum) predicted to be descended from a common C4 ancestor. In the absence of additional sequenced genomes we surveyed changes across species representing at least nine of the estimated 20+ independent origins of C4 photosynthesis within the PACMAD grasses as well as C3 PACMAD species. While the same genes were often co-opted in parallel between independent evolutionary origins, co-option of different members of the same gene family is also observed.