Estrutura e desenvolvimento floral no clado Mezilaurus (Lauraceae)

Abstract

Lauraceae has traditional taxonomy based on floral structure, especially androecium characteristics, however, several groups that were never grouped according to traditional taxonomy have emerged in recent phylogenies. One of these groups is the Clado Mezilaurus, which presents variation in the floral structure among the six genera that compose it. The group has representatives with three, six, nine or 12 to 20 stamens; bilocelate or tetralocelate anthers; glands in all series of stamens, glands only in stamens of series III or absent. Anatomy and floral development studies have provided advances in the understanding of the origin, identity of floral organs, and their evolutionary significance in the family, however, they have not contemplated the floral variations present in Lauraceae. Thus, the objective of this study was to analyze the floral structure of species from the six genera that make up the Mezilaurus clade, aiming to elucidate evolutionary questions and identify characteristics that support the relationships in the group. Furthermore, to understand the ontogenetic processes involved in reducing the number of stamen series in Mezilaurus crassiramea. Anthesis flowers of eight species (derived from herbarium specimens) and flower buds of M. crassiramea (collections) were processed and analyzed by scanning electron microscopy-SEM and light microscopy. Most species had outer tepals vascularized by one bundle and the inner tepals by three vascular bundles, except Anaueria brasiliensis and Chlorocardium rodiei; series III stamens with three vascular bundles, except in Williamodendron spectabilis with one bundle and Chlorocardium rodiei with five bundles. The glands present in Chlorocardium rodiei, Clinostemon mahuba and Sextonia rubra are nectaries. The reconstruction of ancestral character states of morphological characters and floral vasculature, based on molecular phylogenetic analysis, supported several subclades in the group. The reduction in the number of series of stamens and nectaries in M. crassiramea results from the non-formation of entire series from the beginning of development, leading to the formation of flowers with only stamens of series III. Our results indicate that possibly the ancestor of the clade had flowers with three series of stamens and a pair of nectaries at the base of the stamens of series III. The absence of stamens of series I and II or of series III, in some species, is a later loss. Furthermore, we concluded that in genera without nectaries, the vascularization of these structures is maintained in the stamens of some species of the clade and completely lost in others.