Evolutionary history of the grass gynoecium

Abstract

The grass family (Poaceae) includes cereal crops that provide a key food source for the human population. The food industry uses the starch deposited in the cereal grain, which develops directly from the gynoecium. Morphological interpretation of the grass gynoecium remains controversial. We re-examine earlier hypotheses and studies of morphology and development in the context of more recent analyses of grass phylogenetics and developmental genetics. Taken in isolation, data on gynoecium development in bistigmatic grasses do not contradict its interpretation as a solitary ascidiate carpel. Nevertheless, in the context of other data, this interpretation is untenable. Broad comparative analysis in a modern phylogenetic context clearly demonstrates that the grass gynoecium is pseudomonomerous. A bistigmatic grass gynoecium has two sterile carpels, each producing a stigma, and a fertile carpel that lacks a stigma. To date, studies of grass developmental genetics and developmental morphology have failed to fully demonstrate the composite nature of the grass gynoecium because its complex evolutionary history is hidden by extreme organ integration. It is problematic to interpret the gynoecium of grasses in terms of normal angiosperm gynoecium typology. Even the concept of a carpel becomes misleading in grasses; instead, we recommend the term pistil for descriptive purposes.