A role for neutral variation in the evolution of C4 photosynthesis
Convergent trait evolution is a recurrent phenomenon in all domains of the tree of life. While some convergent traits are caused by simple sequence changes, many are associated with extensive changes to the sequence and regulation of large cohorts of genes. It is unknown how organisms traverse this expansive genotype space to assemble such complex convergent phenotypes. C4 photosynthesis is a paradigm of large-scale phenotypic convergence. Conceptual and mathematical models propose that C4 photosynthesis evolved from ancestral C3 photosynthesis through sequential adaptive changes. These adaptive changes could have been rapidly assembled if modifications to the activity and abundance of enzymes of the C4 cycle was neutral in C3 plants. This neutrality would enable populations of C3 plants to maintain genotypes with expression levels of C4 enzymes analogous to those in C4 species and thus enable rapid assembly of a functional C4 cycle from naturally occurring genotypes given shared environmental selection. Here we show that there is substantial natural variation in expression of genes encoding C4 cycle enzymes between natural accessions of the C3 plant Arabidopsis thaliana. We further show through targeted transgenic experiments in the C3 crop Oryza sativa, that high expression of the majority of C4 cycle enzymes in rice is neutral with respect to growth, development, biomass and photosynthesis. Thus, substantial variation in the abundance and activity of C4 cycle enzymes is permissible within the limits of operation of C3 photosynthesis and the emergence of component parts of this complex convergent trait can be facilitated by neutral variation.