Esclarecendo o desenvolvimento autônomo do endosperma : um estudo de caso em Melastomataceae

Abstract

The endosperm is the nutritive tissue of the angiosperm seed, generally produced by the fusion of the central cell with a sperm cell. The formation of an autonomous endosperm, independent of fertilization, is uncommon among angiosperms. However, it occurs in some apomictic plants, capable of asexual seed formation. In Melastomataceae, many species are apomictic and entirely independent of pollination for seed production. In this study, we investigated the seed development in the apomictic species Miconia crenata to understand the formation of an autonomous endosperm and the facilitation of this phenomenon within the group. The data were compared with those from its sexually reproducing congener M. tococa. Fruits at different stages were fixed, originating from emasculated buds of M. crenata and flowers subjected to pollination in M. tococa. Mature ovules and seeds were embedded in resin, sectioned, stained, and photographed. Although M. crenata forms embryo and endosperm autonomously, the developmental stages of these components are quite similar between the species. Both exhibit crassinucellate and bitegmic ovules. At the beginning of seed development, nucellar cells expand becoming highly vacuolated, while endosperm and embryo initiate their development. The endosperm produces numerous free nuclei and a dense cytoplasm, concentrated around the globular or heart-shaped embryo. Some endosperm nuclei cellularize in the chalazal pole, but without storing any reserve content. From the globular embryo stage, the accumulation of polysaccharides is observed in the nucellus and around the embryo. This reserve, along with degraded nucellar cells, is consumed as embryo growth progresses. In mature seeds, the embryo is massive and presents reserves, while the nucellus and endosperm have been completely consumed. Miconia crenata differs by forming more than one embryo sac in some ovules, by the occurrence of polyembryony, and by the development of embryos from hypostase cells, conditions associated with apomixis. The conditions observed in M. crenata and M. tococa, such as poorly developed endosperm and the nucellus contributing as a nutrient source for embryo development, seem to be phylogenetically determined and shared with other members of the family. These conditions may have contributed to the evolution of autonomous apomixis in the family, which is the second largest among angiosperms in terms of the number of autonomous apomictic plants.