The C-4 pathway is found in 62 lineages of land plants. We assess evidence for parallel versus convergent evolution of C-4 photosynthesis from three approaches: (i) studies of specific genes and cis-elements controlling their expression; (ii) phylogenetic analyses of mRNAs and inferred amino acid sequences; and (iii) analysis of C-3 and C-4 genomes and transcriptomes. Evidence suggests that although convergent evolution is common, parallel evolution can underlie both changes to gene expression and amino acid sequence. cis-elements that direct cell specificity in C-4 leaves are present in C-3 orthologues of genes recruited into C-4, probably facilitating this parallel evolution. From this, and genomic data, we propose that gene duplication followed by neofunctionalisation is not necessarily important in the evolution of C-4 biochemistry.
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