Geographic isolation, pollination syndromes, and pollinator generalization in Himalayan Roscoea spp. (Zingiberaceae)

The pollination syndromes have been widely used to predict effective pollinators of plant species and provide the mechanistic explanation of floral diversity. However, in recent years, the predictive applicability of pollination syndromes has been frequently questioned. The accuracy of the syndromes among the closely related plant species may vary temporally and spatially. This suggests the need for the standardized, comprehensive evaluation of factors influencing the matching of a pollination syndrome of a plant species to the predominant pollinator community. We studied the pollination biology of three geographically/phenologically isolated alpine gingers (Roscoea auriculata, Roscoea capitata, and Roscoea tumjensis) exhibiting the correlated suites of floral traits that suggest long-tongued insects as major pollinators, to test for the predictive power of the respective pollination syndrome. We also tested if geographical and temporal isolation affects interspecies breeding system and extent of pollinator generalization. We demonstrate that the three Roscoea species are self-compatible but lack autonomous selfing, and thus completely rely on pollinators for pollination success. Five years of observations demonstrate that only diurnal insect visitors foraged on the flowers of the three Roscoea species. Estimates of the pollinator importance (pollen deposition) of the observed floral visitors indicate that morphologically mismatched bumblebees contribute more than 90% of pollination service in all the three Roscoea species, while long-tongued butterflies and moths function as nectar robbers. We found that geographical isolation and temporal variation in flowering period does not affect the breeding system and pollinator generalization in the three Roscoea species. The three geographically and phenologically isolated Himalayan Roscoea spp., with long-tongued insect pollination syndrome, exhibit generalized pollination system and primarily rely on the morphologically mismatched Bombus species for pollination success. While, for three Roscoea spp., our result suggests that pollination syndromes are a weak predictor of efficient contemporary pollinator community, it also substantiates the role of non-pollinator agents for the evolution of floral traits. The evolution of Bombus pollination in these alpine gingers suggests how the mismatch between plant and pollinator phenology can lead to the emergence of novel reproductive strategies among the congeners, which ultimately seems to play a key role for the speciation and diversification of the genus Roscoea in the Himalayas.