Sexual dimorphism in Catasetum orchids: forcible pollen emplacement and male flower competition
Sexual Dimorphism in Catasetum Orchids: Forcible Pollen Emplacement and Male Flower Competition
FLOWERS OF THE NEOTROPICAL ORCHID genus Catasetum are sexually dimorphic (Fig. 1). Plants are typically unisexual (1), and the sexes are so different that they once were assigned to different genera (2). Female flowers in closely related species are morphologically similar, whereas male flowers usually are quite different (3). Catasetum flowers produce no nectar and are pollinated exclusively by male euglossine bees, especially Euglossa and Eulaema, that collect aromatic hydrocarbons and volatile terpenes (such as cineole) (4). The strong sexual dimorphism of these orchids and the greater interspecific differences among males are apparently related to their interactions with euglossine bees and to competition among male flowers.
Euglossa gaianii and E. modestior were observed foraging at terrestrial Catasetum ochraceum plants near Puerto Ayacucho, Venezuela (5). A bee typically hovered at a male flower (Fig. 1A), landed right side up on the flower's hooded lip, collected chemical attractants from inside the lip with the front legs, and then hovered while transferring the chemicals to the hollow hind legs. After several such sequences, the bee fully entered the lip and pushed the flower's antenna (Fig. 1A), triggering the rapid ejection [at up to 323 cm/sec in C. fimbriatum (6)] of the pollinarium and anther cap (Fig. 1). These struck the bee's dorsum where the pollinarium adhered by the viscidium (Fig. 1E). The anther cap later dried and dropped. Discharged male flowers wilted within a day. In their approach, bees behaved similarly at female flowers. However, as female flowers are inverted relative to male flowers (Fig. 1B), the bees foraged upside down inside the females' lips. Pollination could occur when a pollinarium-bearing bee (loaded bee) exited the female upside down. In this position the pollinarium would pivot down and a pollen mass could lodge in the stigmatic cleft (Fig. 1B) (1, 5).
Bees responded negatively to pollinarium attachment. We observed 13 C. ochraceum pollinarium emplacements. The bees left immediately in seven cases, left after unsuccessfully trying to remove the pollinarium in five, and died after wing-fouling in one (5). In contrast, no beest ceased foraging at male flowers before pollinarium emplacement (except after...