Tendências evolutivas no androceu de Melastomataceae: estudos morfológicos e anatômicos dos estames e anteras em gêneros de Melastomeae

Abstract

Members of Melastomataceae family usually have actinomorphic and bisexual flowers. The most important pollinator of the plants of this family is female bees which are able to vibrate its poricidal anthers and remove pollen, although there are spontaneous self-pollination species and also apomitic species. In addition to this diversity of reproductive systems, variations throughout the development of the anther wall can take place, ensuing in diversification of parietal layer composition. However, most studies have focused on individuals of the Miconieae tribe; hence there is a necessity to look into these variations in other tribes, specially the most representative in Brazil among them, the Melastomeae. Thus, an objective of this study was to describe the morphology and anatomical structure of the stamens in different Melastomeae genera: Acisanthera P.Browne (2 sp.), Comolia DC. (2 sp.), Poteranthera Bong. (1 sp.), Siphanthera Pohl (2 sp.) and Tibouchina Aubl. (5 sp.), attempting to identify and understand, from an evolutionary point of view, the changes found apart from pointing out some features that can assist in the delimitation of these genera and tribes (Chapter 1). This research also aimed to study the floral biology and reproductive system of the smallest species of Melastomataceae known so far, Poteranthera pusilla Bong (Chapter 2). The morphological and anatomical studies, including the vascularization of stamens, were performed according to the usual techniques of plant anatomy; and P. pusilla reproductive biology was studied under control pollination, pollen viability and pollen tube growth. The morphological results have shown that most of the studied species has dimorphic androecium containing ten stamens, which has antesepal stamens larger than antepetals, as well as other members of Melastomeae. But some species have eight stamens of which the four antepetals may be reduced to staminodes, such as S. gracillima. Another modification that has been found is the total loss of one of the cycles of stamens which happens in P. pusilla. During the study about the development of the anther wall, the monocotyledon type was identified on A. genliseoides, A. quadrata, S. foliosa, S. gracillima and T. villosissima and variations of the monocotyledon type have been ascertained on C. lanceiflora, C. sertularia, P. pusilla, T. candolleana, T. herbacea, T. heteromalla and T. stenocarpa. During the whole of microsporogenesis, of all species, the parietal layer of the anther consists of epidermis, endothecium, middle layer and tapetum, differing only in the number of middle layers. At the stage of anthesis just the epidermis is relentless, except for Comolia specieis that also exhibit part of the endothecium. In the course of the vascularization analysis, the trace bundles are larger on antesepals stamens and may be absent on the reduced antepetals, as in A. genliseoides and S. foliosa. These morphological and anatomical studies of the stamens and anthers support the available information up to now about an evolutionary trend towards decreasing the size and number of stamens in Melastomataceae. This decrease, which is apparent on the morphology and the anatomy of anthers and vascularization of stamens, seems to occur separately within each genus and shall be related to a sort of pollinator. The study of reproductive biology has shown some morphological characteristics in P. pusilla that allow spontaneous self-pollination, such as large pores and the arrangement of the stamens of the flower at anthesis. It can be concluded that P. pusilla is self-compatible and that spontaneous self-pollination happens due to the large pore which faces the stigma at anthesis. It is also remarkable, in this species, the independence of biotic factors during pollination and probably over fruits spreading, which culminate in a highly adapted species to survive in environments with scarce pollinators. Considering this scenario, we conclude that all androecium and anthers variations of these species seem to be associated with a general tendency of pollinators in this family, aforementioned for representatives of the Miconieae tribe.