Pollen Removal From Tristylous Pontederia Cordata: Effects of Anther Position and Pollinator Specialization

Abstract

Stamens produce male gametophytes and expose them to the pollination process, so that stament characteristics should reflect selection on male function. For animal—pollinated plants, restricting removal in favor of using all available pollinators should generally promote pollen dispersal, so that staminal characteristics should limit pollen removal by individual pollinators. Here we describe the influences of anther position on pollen removal from Pontederia cordata, a tristylous plant with anthers at three distinct positions: exserted from the flower (long—level anthers), at the mouth of the flaring tubular perianth (mid—level anthers), and near the perianth based (short—level anthers). To measure pollen removal, we presented unvisited flowers to freely foraging bees and counted the number of pollen grains left in flowers after 1—4 visits. The pollinators included two generalist bumble bees (Bombus impatiens and B. vagans: Apidae) and Melissodes apicata (Anthophoridae), a specialist with morphological and behavioral adaptations for collecting pollen from P. cordata. Analysis of pollen removal incorporated the effects of bee species, anther position, pollen size, flower length, and the number and cumulative duration of bee visits. Anther position and bee species significantly influenced pollen removal, although the differences between bee species were not related to pollinator specialization. Long—level anthers dehisced early, lost an average of 39% of their pollen during their first visit, and did not consistently contact small pollinators, such as M. apicata. Short—level anthers restricted pollen removal more (28% lost) than long—level anthers, but dehisced more slowly and were subject to unpredictable pollen removal. Mid—level anthers combine the advantages of more exserted and inserted positions without suffering the corresponding disadvantages as they restricted removal (25% lost), dehisced relatively early, and seemed to consistently contact all pollinators. The overall benefits of positioning anthers within the perianth mouth, the position occupied by mid—level anthers in P. cordata, may explain the prevalence of this morphology among bee—pollinated plants with flaring, tubular, monomorphic flowers.