Cyclopogon guayanensis is an Unusual Orchid with a Generalistic Pollination System and Hexose Dominant Nectar

Abstract

Cyclopogon is a large Neotropical orchid genus pollinated by halictid bees that offers nectar as a reward. In a recent phylogenetic tree, Brachystele guayanensis emerged nested within Cyclopogon and was transferred to that genus. The hypothesis for this study was that C. guayanensis would show similar floral biology to Cyclopogon, although distinctive in its small, congested white flowers. Data on floral biology, pollinators, micromorphology, histochemistry, and nectar sugar composition of C. guayanensis from samples from the the Distrito Federal, Brazil were gathered. C. guayanensis is pollinated by at least four species of bees belonging to the genera Exomalopsis, Nomada, Tetrapedia (Apidae), and Dialictus (Halictidae) foraging for nectar. Nectar is produced in visually imperceptible quantities by papillae on the inner surface of the labellum; similar papillae occur in other species of Cyclopogon but C. guayanensis nectar is hexose dominant (< 10% sucrose). This type of nectar is class 4 in the traditional Baker and Baker system and our record is the second record of this nectar class in the Orchidaceae in the literature. Pollinia are dorsally adhesive and probably attach to the underside of the bee labrum, as in other Cyclopogon. The inflorescence rachis, bracteoles, and outer surfaces of the base of the sepals are covered with lipid-secreting glandular trichomes; sepals and petals have numerous raphid-rich idioblasts. This is the first record of papillae on a spurless labellum shown to produce nectar in the Orchidoideae. We suggest that hexose dominant nectars in the Orchidaceae are associated with a geophytic habit, small pale flowers, exposed nectaries, visually imperceptible quantities of nectar, and a generalistic pollination system, and coin the term 'modest pollination strategy' for this suite of characters.

Keywords: Apidae; Floral histochemistry; Halictidae; Hexose; Nectar; Raphids; Spiranthinae.