Pollen carryover, geitonogamy, and the evolution of deceptive pollination systems in orchids

Geitonogamy (transfer of pollen among flowers on the same plant) may lead t o reduced outcrossing and interfere with sex function. Orchids with pollen packaged into pollinaria would be expected to be particularly vulnerable to the loss of cross-mating opportunities imposed by geitonogamy. We tested t he hypothesis that the absence of floral rewards in many orchid species is a means of reducing geitonogamy. Experiments with the deceptive species Orc his mascula and Orchis morio showed that queen bumble bees probe more flowe rs and stay longer on plants when artificial nectar is added to the hewers. Overall, the data indicated that the evolution of nectar production in dec eptive Orchis species would result in moderate to high levels of geitonogam y, as a consequence of the greater number of flowers probed and longer visi t duration (similar to 60 s) by pollinators. However, the estimated levels of geitonogamy were less than expected, due both to a time delay before fre shly withdrawn pollinaria bend into the correct position to strike a stigma and to extensive carryover of pollen. The time elapsed before a freshly wi thdrawn pollinarium is in the correct position to strike the stigma was fou nd to vary 30-80 a, depending on the orchid species. Since pollinators usua lly spend <30 s on an inflorescence, we estimate that natural populations o f the study species are highly outcrossed. The fraction of the pollen load carried over from flower to flower was found to be 0.67 in O. mascula. Sele ction should favor longer delays in pollinaria bending and extensive pollen carryover in nectar-producing orchids. This is corroborated by the nectari ferous orchid Platanthera chlorantha, which we found to have a pollinaria b ending delay of 80 s and a high pollen carryover fraction (0.87). In genera l, selection for traits that prevent geitonogamy should occur only when pol linators are abundant. Since fruit set of orchids is usually pollinator lim ited, additional explanations may have to be sought to explain deception. T he most plausible complementary hypothesis is that resources in pollinator- limited orchids are invested in advertising display, rather than nectar pro duction.