Floral gland diversity in Pleurothallidinae (Epidendroideae-Orchidaceae)

Pleurothallidinae are a diverse Neotropical subtribe of Orchidaceae pollinated by Diptera. Here, our goal was to map how myophily and deceptive sapromyophily appears in the Pleurothallidinae' lineages, analyzing the morphological and topological variations of the floral glands in a phylogenetic framework. Scanning electron and light microscopy were employed to examine the glands located on the ovary, sepals, and labellum of twenty-eight species belonging to the subtribe. Colleters could be considered as a new morphological synapomorphy due to their widespread occurrence on the external ovary wall, while multicellular and unicellular oil-producing glands were also identified on this organ for the first time. Myophilous or nectariferous species usually have unicellular trichomes on the labellum and this trait appears in four lineages restricted to the basal nodes of the phylogenetic tree. Sapromyophilous species normally possess sticky-exudate glands and/or osmophores, the former being considered as an evolutionary novelty in Pleurothallidinae lineages. Sepaline osmophores are commonly found in myophilous and sapromyophilous species. Osmophores constituted by unicellular secretory trichomes mostly occur on the labellum and probably have a unique relationship with directional evolution related to sapromyophily. The diversity in the secretion of the labellum's glands includes monosaccharides, polysaccharides, and lipophilic compounds, which lead us to propose that heterochronic events are involved to the synthesis of these compounds by pleurothallidineaen species.

Automated summary

This study examined the morphological and topological variations of floral glands in Pleurothallidinae species and found associations between these gland characteristics and pollination methods. Differences in gland morphology and secretions were observed between myophilous and sapromyophilous species, which may be related to their pollination methods and evolution.