A naked grass in the "bristle clade'':: A phylogenetic and developmental study of Panicum section Bulbosa (Paniceae: Poaceae)

Recent molecular phylogenies of the grasses (Poaceae) have revealed unexpected relationships among species with disparate inflorescence morphologies. "Panicum" bulbosum (Panicum sect. Bulbosa) was recently placed in the "bristle clade," a monophyletic clade in the Paniceae whose members have inflorescences containing both bristles and spikelets, even though "P." bulbosum appears to lack bristles. This placement of "P." bulbosum also indicates that it is not related to true Panicum (Panicum sect. Panicum). Here, the placement of "P." bulbosum is confirmed using chloroplast markers ndhF and trnL. The closest relatives of the species are thus morphologically quite different from "P." bulbosum, and conversely the morphologically similar species of true Panicum are unrelated. To investigate the morphological differences more carefully, we used scanning electron microscopy to detail the development of inflorescence structures of "P." bulbosum and to compare it with bristle-bearing grasses and true Panicum. We find that inflorescence development of "P." bulbosum closely resembles that of the type species of true Panicum, Panicum miliaceum. Thus "P." bulbosum is a morphological anomaly, having lost the synapomorphy of its clade and sharing many characters with more distant relatives.