The Pollination System And Floral Evolution Of Genus Rhododendron (Ericaceae)

Plant-pollinator interactions have been thought playing a crucial role in the diversification of angiosperm flowers. The importance of pollinator-mediated selection was supported by a large number of studies across the fields of evolutionary ecology and comparative biology. Floral divergence is explained for adaptive to a variety of pollinators, whereas floral convergence has been tied to parallel adaptation to the similar pollination functional groups. However, why some plant groups are superdiverse of species but have similar floral traits? To date, our understanding of how plant-pollinator interactions have influenced floral divergence is still incomplete. Few studies of pollination using phylogenetic information demonstrate that pollination systems are evolutionarily labile but not occur randomly, but very little is known about the association between floral rewards and pollination systems with phylogeny.The genus Rhododendron provides a great opportunity to study the evolution of pollination-system transitions and floral divergence. As a genus which recently has undergone adaptive radiation, there are over 1000 species in the world-wide, including many narrow endemic species that are sympatric in Himalayan region. Although it has a spectacular diversification and some species are horticulturally cultivated worldwide, details of pollination are very scanty for such a well-known genus. We investigate pollination systems of 28 Rhododendron species in the hotspot biodiversity region of Hengduan Mountain in East Himalaya, we found that bees (including bumblebee and honeybee) were the most frequent pollinators, and birds could provide pollination service for over 1/3 Rhododendron species. Among them, Aethopyga ignicauda pollinated for R. haematodes, R. selense, R. rex and R. lacteum, Aethopyga gouldiae pollinated for R. rex, R. oreotrephes. Besides pollination for the five species above, Sylviidae(Heterophasia melanoleuca, Yuhina occipitalis et al.) also pollinated for R. beesianum, R. neriiflorum, R. sinogrande, R. traillianum and R. delavayi. By a caged experiment, we found that birds could be more effective pollinators than bees because they could transfer more pollen in one visit. However, bird visits was unstable that reduced their role in reproductive success in Rhododendron, thus dependent on bee-pollination may be a less-than-ideal alternative for Rhododendron species.We reconstructed the phylogenetic tree through combined the cpDNA sequence (matK) and the internal transcribed spacer (ITS) region of nuclear ribosomal of 28 Rhododendron species. The consensus tree was used to determine the relationship of pollinator shifts and floral trait changes. To determine pollinator-mediated floral evolution,10 floral morphological traits and two combines of tube length and floral opening were selected. Then we use PCA analysis to determine the relationship between floral size or floral size variation and pollination system. We found that neither bird visit nor bee visit was correlated with all the 10 single floral traits. However, bee visit importance was negatively correlated with tube length/opening width, and bird visit importance was positively correlated with tube length/opening width. PCA analysis indicated no correlation between floral size and pollination system, but floral size variation was positively correlated with bird visit importance.Nectar is an important floral reward for pollinator energetics. It is a dilute sugar aqueous solution containing trace amounts of amino acids and secondary metabolites, which has its source in the phloem solution. Although the ecological and evolutionary relationships of sugar concentration in nectar have received considerable attention, the relationship between sugar components and pollination systems has been relatively unexplored. The nectar volume of standing crop and the total sugar concentration in studied Rhododendron species showed great difference among species, the volume ranged from 1 ul (R. simsii) to 70 ul (R. delavayi). The sugar content analysis indicated that bird-pollinated plants produced higher volumes but more diluted nectar than bee-pollinated plants; honey bees preferred more concentrated nectar than bumble bees; bumble bees foraging nectar with no apparent preference.To test hybridization could be as a formation to speciation, we have done artificial interspecific pollination. Our results indicatd that related species had higer ratio of interspecific hybrids than distant species. However, the results of pollination observation showed that interspecific pollination between maternal species occurred occasionally.To conclude, our three continuous years study showed:The preliminary divergence of Rhododendron was related with pollinator shifts; the artificial pollination experiments and interspecific pollination observations in the field indicated postzygotic isolation was incomplete, and reproductive barrier of Rhododendron mainly dependent on prezygotic isolations. It also indicated that pollinator and its behavior promoted the process of speciation of Rhododendron.