Floral specialization for different pollinators and divergent use of the same pollinator among co-occurring Impatiens species (Balsaminaceae) from Southeast Asia

摘要

Floral variation among closely related species is thought to often reflect differences in pollination systems. Flowers of the large genus Impatiens are characterized by extensive variation in colour, shape and size and in anther and stigma positioning, but studies of their pollination ecology are scarce and most lack a comparative context. Consequently, the function of floral diversity in Impatiens remains enigmatic. This study documents floral variation and pollination of seven co-occurring Impatiens spp. in the Southeast Asian diversity hotspot. To assess whether floral trait variation reflects specialization for different pollination systems, we tested whether species depend on pollinators for reproduction, identified animals that visit flowers, determined whether these visitors play a role in pollination and quantified and compared key floral traits, including floral dimensions and nectar characteristics. Experimental exclusion of insects decreased fruit and seed set significantly for all species except I.muscicola, which also received almost no visits from animals. Most species received visits from several animals, including bees, birds, butterflies and hawkmoths, only a subset of which were effective pollinators. Impatiens psittacina, I.kerriae, I.racemosa and I.daraneenae were pollinated by bees, primarily Bombus haemorrhoidalis. Impatiens chiangdaoensis and I.santisukii had bimodal pollination systems which combined bee and lepidopteran pollination. Floral traits differed significantly among species with different pollination systems. Autogamous flowers were small and spurless, and did not produce nectar; bee-pollinated flowers had short spurs and large floral chambers with a wide entrance; and bimodally bee- and lepidopteran-pollinated species had long spurs and a small floral chamber with a narrow entrance. Nectar-producing species with different pollination systems did not differ in nectar volume and sugar concentration. Despite the high frequency of bee pollination in co-occurring species, individuals with a morphology suggestive of hybrid origin were rare. Variation in floral architecture, including various forms of corolla asymmetry, facilitates distinct, species-specific pollen-placement on visiting bees. Our results show that floral morphological diversity among Impatiens spp. is associated with both differences in functional pollinator groups and divergent use of the same pollinator. Non-homologous mechanisms of floral asymmetry are consistent with repeated independent evolution, suggesting that competitive interactions among species with the same pollination system have been an important driver of floral variation among Impatiens spp.