The development of the inflorescence in Setaria italica (foxtail millet), S. verticillata, S. viridis, and S. grisebachii was examined. The inflorescences of the different species differed in numbers of orders of branching, with S. italica having 2 or more orders as compared to one order in the other three species. All species initiate spikelets paired with bristles, but many of the spikelets are suppressed during development. In S. grisebachii pairs of spikelets and bristles appear together on the branches, in S. verticillata and S. viridis a single spikelet at maturity is surrounded by a series of bristles, and in S. italica three or four spikelets and at least twice as many bristles appear on the primary branches. Differences between the species appear early in development and have the potential, along with density of branches in the spike, to be major determinants of yield. The different mature morphologies of the inflorescences are similar to various mutant phenotypes of maize, such as ids1, ts6, and ra1 & 2, which raises the prospect of using genes identified from maize to identify differences in inflorescence morphology in a comparative manner across species groups. The present study is a step towards relating advances in model plant genetics to an understanding of natural morphological differentiation in plants.

Key words: inflorescence development, inflorescence morphology, Poaceae, Setaria