Gene duplication and dosage effects during the early emergence of C-4 photosynthesis in the grass genus Alloteropsis

The importance of gene duplication for evolutionary diversification has been mainly discussed in terms of genetic redundancy allowing neofunctionalization. In the case of C-4 photosynthesis, which evolved via the co- option of multiple enzymes to boost carbon fixation in tropical conditions, the importance of genetic redundancy has not been consistently supported by genomic studies. Here, we test for a different role for gene duplication in the early evolution of C-4 photosynthesis, via dosage effects creating rapid step changes in expression levels. Using genome- wide data for accessions of the grass genus Alloteropsis that recently diversified into different photosynthetic types, we estimate gene copy numbers and demonstrate that recurrent duplications in two important families of C-4 genes coincided with increases in transcript abundance along the phylogeny, in some cases via a pure dosage effect. While increased gene copy number during the initial emergence of C-4 photosynthesis probably offered a rapid route to enhanced expression, we also find losses of duplicates following the acquisition of genes encoding better- suited isoforms. The dosage effect of gene duplication might therefore act as a transient process during the evolution of a C-4 biochemistry, rendered obsolete by the fixation of regulatory mutations increasing expression levels.