The degeneration of three of four meiotic products is a very common process in the female gender of oogamous eukaryotes. In Tillandsia (and many other angiosperms), the surviving megaspore has a callose-free wall in chalazal position while the other three megaspores are completely embedded in callose. Therefore, nutrients and signals can reach more easily the functional megaspore from the nucellus through the chalazal pole with respect to the other megaspores. The abortion of three of four megaspores was already recognized as the result of a programmed cell death (PCD) process. We investigated the process to understand the modality of this specific type of PCD and its relationship to the asymmetric callose deposition around the tetrad. The decision on which of the four megaspores will be the supernumerary megaspores in angiosperms, and hence destined to undergo programmed cell death, appears to be linked to the callose layer deposition around the tetrad. During supernumerary megaspores degeneration, events leading to the deletion of the cells do not appear to belong to a single type of cell death. The first morphological signs are typical of autophagy, including the formation of autophagosomes. The TUNEL positivity and a change in morphology of mitochondria and chloroplasts indicate the passage to an apoptotic-like PCD phase, while the cellular remnants undergo a final process resembling at least partially (ER swelling) necrotic morphological syndromes, eventually leading to a mainly lipidic cell corpse still separated from the functional megaspore by a callose layer.

The authors thank Ente Cassa di Risparmio di Firenze for support under grant n°2002.1076 to J.L.V. Teresa Ghiglione and Gabriela Venturi for English corrections.

The ultrastructure and ontogeny of bicellular pollen in Tillandsia seleriana Mez (Bromeliaceae) before anther dehiscence has been here investigated. The development, after the first mitosis, is described to consist in three stages. The aim of the present report, which is part of a more extensive study on the reproductive mechanisms in the genus, is to compare the development of the male gametophyte of Tillands with information available from other angiosperms. Aspects on water, starch and soluble carbohydrate contents of the pollen ofT. seleriana are discussed in the light of the current ecological classification of pollens. The generative cell did not show any plastid in its cytoplasm, hence plastid inheritance in Tillandsia is confirmed as totally maternal (Lycopersicon type). Contrarily to other pollen grains, those of T. seleriana maintain a considerable amount of starch in the vegetative cell until dehiscence. The inulin test was negative, indicating that T. seleriana pollen lacks of Fructans. Pollen only with starch is subjected to faster dehydration and, probably, cannot be transported at long distance. The ecological adaptive plasticity of genus Tillandsia, is confirmed also by morphological characters that appear to be typical both of the partially dehydrated and partially hydrated pollen in T. seleriana. This situation is, probably, a plesiomorphic character that permitted to genus Tillandsia to differentiate species well adapted to arid environments and others living in mesic forests.