Developmental Gene Evolution and the Origin of Grass Inflorescence Diversity

Grass inflorescences are diverse, developmentally complex, and provide many of the taxonomic characters used to differentiate the estimated 10,000 grass species. Here we review grass inflorescence development in detail and discuss which genes are involved at each developmental stage. We demonstrate that grass inflorescence development is complex, with multiple structures that are not present in Arabidopsis. New and published phylogenetic analyses of genes involved at each developmental stage indicate that the maize FASCIATED EAR2 (FEA2) and Arabidopsis CLAVATA2 (CLV2) genes are the sole remaining co-orthologs following multiple rounds of whole-genome duplication. Analyses of BARREN STALK1/LAX PANICLE (BA1/LAX1), FRUITFULL (FUL), INDETERMINATE SPIKELET1 (IDS1), KNOTTED1 (KN1), LEAFY HULL STERILE1 (LHS1), and RICE CENTRORADIALIS1/2 (RCN1/2) indicate that these genes are members of grass- or monocot-specific small gene families. The complex pattern of gene relationships mirrors a complex pattern of functional evolution. Maize FEA2 and Arabidopsis CLV2 have nonidentical roles, whereas distantly related grass KN1-like and RCN1/2 proteins show functional convergence and conservation, respectively. Duplications near the base of grasses in BA1/LAX, FUL, IDS1 and LHS1 have led to diverse roles in grass inflorescence development. We conclude that developmental gene duplication followed by functional diversification appears to have played a major role in the evolution of novel morphological structures within grass inflorescences.