Morphology, anatomy and development of floral nectaries associated to pollinator shifts in neotropical gesneriaceae

Secretions produced by floral nectaries play a key role in plant-pollinator interactions in angiosperms. Multiple and rapid changes of pollination syndromes have occurred in the tribe Sinningieae, and several aspects of nectar including sugar concentration, volume, chemical composition and timing of secretion could have promoted flower specialization on certain types of pollinators. However, few studies have documented how nectary morpho-anatomy and development have changed during pollinator shifts. This question was addressed in the tribe Sinningieae, a group of Brazilian Gesneriaceae displaying floral and nectar features adapted to different functional group of pollinators such as bees, hummingbirds and bats. Morpho-anatomy of nectaries was observed at different floral developmental stages using light and scanning electronic microscopy and compared between two independent pairs of closely related species with contrasting mode of pollination (bats/hummingbirds, bats/bees) and nectar characteristics. Data on nectary arrangement were used in combination with phylogenetic information to reconstruct nectary evolution in the tribe. Our results show that floral nectary in Sinningieae is composed of 5 conspicuous, histologically similar glands that developed as an enation of the ovary base. In all four species studied, the supply to nectariferous tissue derived from lateral bundles of the ovary wall, and nectar is secreted through modified stomata concentrated on the uppermost region of the glands. Nectary fusion and arrangement vary among species, but five free glands represent the ancestral state in Sinningieae. The structural changes observed between our selected species and the variation of nectary arrangement within the tribe were not correlated with pollinator shifts, and nectary size appears as the main factor explaining the abilities of bat-pollinated flowers to produce a larger amount of nectar than other species. Additional studies at cellular level could improve our understanding of the mechanisms implied in nectar secretion