Patterns of inflorescence development of three prairie grasses (Andropogoneae, Poaceae)

Premise of research: The tribe Andropogoneae (Poaceae) is a morphologically diverse clade that is distributed worldwide; members of the tribe are ecological dominants in some of the world’s most expansive grasslands. Inflorescence morphology in the tribe is variable and does not indicate relationship, creating problems for systematic treatments of morphological characters. Closely related taxa may have different pollination syndromes and ecological roles, correlating with substantially different morphology.

Methodology: To determine when morphological differences arise in developmental time, we investigated three species endemic to North American tallgrass prairies: big bluestem (Andropogon gerardi), little bluestem (Schizachyrium scoparium), and the inbreeding broomsedge bluestem (Andropogon virginicus). These species are widespread, common, and often ecologically dominant, but their development has never been studied. We used SEM to sample inflorescences from initiation to anthesis.

Pivotal results: Both early and late development differ among the three species. Despite the close phylogenetic relationship of the two Andropogon species, many aspects of their development differed from each other and from those of Schizachyrium. Early in development, A. gerardi forms two long branches, A. virginicus forms one, and Schizachyrium does not form any. Andropogon gerardi and S. scoparium form similar trichomes and awns, whereas both structures are larger and more prominent in A. virginicus. The former two species develop three anthers per fertile flower, whereas the latter forms only one. In contrast, formation of spikelet pairs during middevelopment was quite similar among all three species.

Conclusions: Differences among the three species appear early and continue throughout development. Only spikelet pair formation is shared among all three species. Developmental patterns do not reflect phylogeny, and early developmental events are not more similar among the two species of Andropogon.