Parallel recruitment of multiple genes into c4 photosynthesis

During the diversification of living organisms, novel adaptive traits usually evolve through the co-option of preexisting genes. However, most enzymes are encoded by gene families, whose members vary in their expression and catalytic properties. Each may therefore differ in its suitability for recruitment into a novel function. In this work, we test for the presence of such a gene recruitment bias using the example of C4 photosynthesis, a complex trait that evolved recurrently in flowering plants as a response to atmospheric CO2 depletion. We combined the analysis of complete nuclear genomes and high-throughput transcriptome data for three grass species that evolved the C4 trait independently. For five of the seven enzymes analyzed, the same gene lineage was recruited across the independent C4 origins, despite the existence of multiple copies. The analysis of a closely related C3 grass confirmed that C4 expression patterns were not present in the C3 ancestors but were acquired during the evolutionary transition to C4 photosynthesis. The significant bias in gene recruitment indicates that some genes are more suitable for a novel function, probably because the mutations they accumulated brought them closer to the characteristics required for the new function.