Morphology and movement of Bulbophyllum hortorum (Orchidaceae) flowers enable selection of optimal-sized Dacini fruit fly males as pollinators

Abstract

Bulbophyllum is the largest orchid genus with > 2000 species. There are many daciniphilous Bulbophyllum species with unique lip mechanisms for fly pollination. Their flowers exclusively attract pest and non-pest Dacini fruit fly species via fragrant floral synomones containing at least one major male attractant. Bulbophyllum hortorum bears non-resupinate and solitary flowers. The four-year study showed that anthesis normally started at dawn; and every flower closed nightly and reopened the next morning (if pollinarium is not removed previously). The flowers attracted many raspberry ketone-responsive fruit fly species. In Malaysia, eleven Dacini species were attracted, of which six species with light-weight males (< 13 mg) were unable to open the small close-spring lip (CSL), thereby, they are not potential pollinators but mere floral visitors. The other five species had individuals sufficiently heavy to overcome the CSL’s torsional force (equivalent to a mean weight of 13 mg) to open the CSL; and also with some lighter males unable to pry open the CSL, thereby, rendering them floral visitors and not potential pollinators. The CSL moved slowly towards the column in the afternoon before the flower closed for the night. Besides selecting for an adequately heavy male fly to be a potential pollinator, the CSL has the vital role of catapulting the fly in to the column cavity, thereby, enhancing spontaneous pollinarium removal. The unique dynamic lip mechanism and the ‘stopper’ (the tip of column-foot which maintains the temporarily opened CSL in a horizontal position) may be pre-adaptations to floral resupination in B. hortorum.