ORIGINAL ARTICLE Morphological and histological characterization of the osmophores and nectaries of four species of Acianthera (Orchidaceae: Pleurothallidinae) Marcos Cabral de Melo • Eduardo Leite Borba • Elder Anto ˆnio Sousa Paiva Received: 18 February 2009 / Accepted: 13 April 2010 / Published online: 15 May 2010 Ó Springer-Verlag 2010 Abstract Nectar and floral odor are frequently associated with the presence and maintenance of specialized pollina- tion systems in Orchidaceae. We studied flowers of four Acianthera species, a genus of myophilous orchids belonging to the largest fly-pollinated orchid group Pleurothallidinae, in order to characterize the secretory structures related to their pollination mechanism. Flowers at anthesis were sampled to detect volatile compounds and nectar; samples were fixed for light microscopy and scan- ning and transmission electron microscopy. The labellum presents epidermal cells and the first mesophyll layer involved with secretory processes. Cellular characteristics of these regions associated with the occurrence of sugars allowed us to recognize them as nectaries. Some portions of the sepals also shown to be involved with secretory processes and the presence of nitrogenated volatile com- pounds characterize them as osmophores. The production of nectar in these species makes the occurrence of sapr- omyophily questionable, even though these flowers present characteristics of this floral syndrome. The presence of osmophores on the sepals reinforces that this localization is common among the Pleurothallidinae, whilst they occur in a different region (labelum) in the other major fly-polli- nated orchid group (Bulbophyllum). Keywords Acianthera Á Cell ultrastructure Á Myophily Á Nectaries Á Orchidaceae Á Osmophores Introduction The subtribe Pleurothallidinae, with approximately 4,100 species, and the unrelated genus Bulbophyllum Thouars, with approximately 2,100 species are two of the largest fly-pollinated orchid groups. They exhibit some of the most interesting examples of floral convergence due to adapta- tion to the same group of pollinators (Dressler 1993). Although the flowers of these myophilous orchids share a number of morphological characters, floral biology studies have demonstrated that there is still a large diversity of pollination mechanisms associated with fly-pollination, involving both biotic and wind-assisted biotic mechanisms (van der Pijl and Dodson 1966; Borba and Semir 1998). Among them, different adaptations to attract these insects stand out; some of them are associated with the insects’ feeding and/or reproductive instincts (Borba and Semir 1998, 2001; Singer and Cocucci 1999). Evolution of these differences in many myophilous orchids seems to be rela- ted to the great specificity of the plant-pollinator relation- ship to a degree that was not initially expected. Odor and nectar are important elements to maintain the plant-pollinator relationship. Floral odor is responsible for long distance attraction while nectar constitutes a principal floral reward (Proctor et al. 1996). Nectar is the principal reward to pollinators among the Orchidaceae (Dressler 1993) and is commonly present in orchids pollinated by anthophilous species of Diptera. The presence of nectar may result in high pollen transfer, as previously demon- strated in epidendroid orchids (Peter and Johnson 2009). Although nectar is not commonly found among sapromy- ophilic plants, as their pollinators are attracted by ovipo- sition instinct (Faegri and van der Pijl 1979; Proctor et al. 1996), it seems to be an important element in the mechanism of deceit-pollination in some wind-assisted M. C. de Melo Á E. L. Borba Á E. A. S. Paiva (&) Departamento de Bota ˆnica, Instituto de Cie ˆncias Biolo ´gicas, Universidade Federal de Minas Gerais, Av. Anto ˆnio Carlos, 6627, Pampulha, Belo Horizonte, Minas Gerais 31270-901, Brazil e-mail: epaiva@icb.ufmg.br 123 Plant Syst Evol (2010) 286:141–151 DOI 10.1007/s00606-010-0294-1