Allopolyploidy and diversification in sorghum, maize, and their relatives

What makes some clades produce more species than others? The answers may be genetic, genomic, or morphological. This talk will focus on the grass tribe Andropogoneae, an economically and ecologically important group that includes maize, sugarcane, sorghum and Miscanthus. Most species in the clade are polyploid, and we find that allopolyploidy is a predominant mode of speciation in the group. However, the net speciation rate does not increase after polyploidy. The parental genomes of most Andropogoneae polyploids diverged in the Late Miocene coincident with the expansion of the major C4 grasslands that dominate the earth today. Morphological characteristics include a distinctive seed dispersal unit derived from a pair of spikelets and the associated inflorescence stalk. This structure is usually assumed to be selected for seed dispersal, but also affects breeding system and the investment into pollen and ovules, and is a source of carbon for the developing grain. Thus the structure of the inflorescence may be under selection for pollination, seed filling, or seed dispersal. This may be as important as repeated allopolyploidy in driving speciation.