Format: ORAL

Dmitry D. Sokoloff1,2, Constantin I. Fomichev2,3, Paula J. Rudall4, Terry D. Macfarlane5, Margarita V. Remizowa2

1 School of Plant Sciences and Food Security, Faculty of Life Sciences, Tel Aviv University, Tel Aviv, Israel 2 Faculty of Biology, M.V. Lomonosov Moscow State University, Moscow, Russia 3 Faculty of Biology, Shenzhen MSU-BIT University, Shenzhen, China 4 Jodrell Laboratory, Royal Botanic Gardens, Kew, UK 5 Western Australian Herbarium, Biodiversity and Conservation Science, Department of Biodiversity, Conservation and Attractions, Perth, Australia

Jean-Baptiste Payer (1857) was among the pioneering researchers of grass flower development. He nicely demonstrated that, taken in isolation, data on gynoecium development in grasses do not contradict its interpretation as a solitary carpel. This conclusion created a need for an explanation of the occurrence of more than one stigma in most grasses. Some scattered angiosperms do have two stigmas per carpel, but a number of grasses possess as many as three (and rarely even more) stigmas. Grass stigmas and their stalks develop as ab initio terete and filiform structures. They differ in this respect from the typical plicate condition of the angiosperm stigma. As first illustrated by Payer (but not documented in full detail with SEM before the present study), at least some PACMAD grasses have stigmas attached to the outer surface of the gynoecium wall, well below the margin of the gynoecium orifice. Consequently, some researchers concluded that the grass stigma is not homologous to the stigma of other angiosperms. Broad comparative analysis in a modern phylogenetic context demonstrates that the grass gynoecium, contrary to the view of Payer, is pseudomonomerous. Each stigma, at least in the hypothetical ancestral grass condition, belongs to its own carpel. A bistigmatic grass gynoecium has two sterile carpels, each producing a stigma, and a fertile carpel that lacks a stigma. Comparison with outgroups is highly important in resolving the morphological interpretation of the grass stigma. Ecdeiocoleaceae are the closest specialised wind-pollinated outgroup of grasses. Our data show that the stigmas of Ecdeiocoleaceae are flat and bifacial, and this condition is apparently plesiomorphic in grasses. The peculiar stigma position found in some PACMAD grasses can be explained in terms of formation of a secondary carpel margin in the region of closure of the gynoecium orifice.