Floral development and the formation of unisexual spikelets in the Andropogoneae (Poaceae)

We investigated spikelet development in four distantly related species of t he grass tribe Andropogoneae to determine whether spikelet development and the formation of unisexual florets are uniform throughout the tribe. We stu died development in Bothriochloa bladhii, Coelorachis aurita, Heteropogon c ontortus, and Hyparrhenia hirta, and compared these with Panicum, a member of the sister tribe Paniceae. Many aspects of spikelet development in the s pecies we have studied correlate with what is already known for Tripsacum a nd maize (both Andropogoneae), despite variation in how unisexual florets a re distributed on the plant. The formation of unisexual spikelets is also u niform. All florets initiate both pistil and stamen primordia. In florets d estined to be male, cell death occurs in the subepidermal layers of the gyn occium after the formation of a gynoecial ridge. In florets destined to be female, there is no apparent cell death in the stamens, but growth ceases a fter anther formation. The similarity in spikelet development and the forma tion of unisexual florets point to a common genetic mechanism for sex deter mination throughout the Andropogoneae and possibly the entire Panicoideae. Use of a cell death pathway to cause gynoecial abortion may be the basis of one morphological character that defines the subfamily.