A Preliminary Study On Pollination Ecotype At Different Altitudes In Elaeagnus Umbellate(Elaeagnaceae)

The interaction between plants and pollinators is generally believed to be one of the main drivers of the diversity and evolution of angiosperm,and it is widely believed that flower traits are partly the result of pollinator selection.This evolutionary model is known as the "Grant-Stebbins" model.According to this model,pollinator-mediated selection may result in divergence of floral traits when plants adapt to local pollination system.Local effective pollinators shape the evolution of flower traits.Verne Grant and Karen A.Grant couples in the monograph?Flower Pollination in the Phlox Family?noted that pollinators and flowers appear obvious differentiation when the same kind of plants under different habitats,proposed the pollination ecotype concept.This concept could be supported by the classic example that same plants adapted to different altitudes habitats and produced different genotypes.However,little is known whether altitudinal variation in floral traits could be mediated by local pollinator functional groups,particularly a common shift from bees to birds.In related species,the evolution of pollination systems from bees to birds occurs many times.What are the ecological factors that driving the transition from bee pollination to bird pollination?In order to confirm"Grants-Stebbins" model or pollination ecotype concept,we conducted a comparative study on the pollination ecology of Elaeagnus umbellate at three different altitudes in Tangjiahe National Nature Reserve at the northern edge of Hengduan Mountains in China.E.umbellate is a shrub of the family Elaeagnaceae.We observed that the pollination phenomenon of insect pollination and bird pollination was different at different altitudes in the field,which provided a rare opportunity for us to explore the pollination ecotypes at different altitudes.The main results of three years field observations and experiments are summarized as follows:First of all,we compared the differences in vegetative(plant)and reproductive(floral)traits of E.umbellate at three altitudes.The floral traits(corolla length)of E.umbellate at high-altitude were longer than that at low-and mid-altitudes,while the plant traits(plant height,stem diameter,branches of plant base,leaf length and leaf width)at high-altitude were lower or smaller than that at low-and mid-altitudes.The results indicated that corolla tube became longer,while plants became smaller with the increasing of altitude.The nectar volume of E.umbellate at high-altitude was significantly higher than that at low-and mid-altitudes,while the nectar concentration at high-altitude was significantly lower than that at low-and mid-altitudes.Nectar sugar components(fructose,glucose and sucrose)were examined by high-performance liquid chromatography(HPLC)and the results showed that the content of fructose and glucose were higher than sucrose at low-and mid-altitudes,while sucrose was higher at high-altitude.Changes in floral size and nectar properties may be influenced by pollinator selection rather than environment.Secondly,the main effective pollinators of E.umbellate at different altitudes were determined.Artificial pollination experiments showed that the fruit/seed set of cross-pollination was 54.0±0.1%and self-pollination produced no fruit/seed.The results indicated that the fruit/seed set of E.umbellate depended on pollinators.The results of pollinator observation showed that the functional pollinator groups of the three populations were similar,but the visitation frequency of different visitors at different altitudes was different.At low-and mid-altitudes,the visitation frequency of bees was significantly higher than sunbirds,while the visiting frequency of sunbirds was significantly higher than other insect visitors at high-altitude.The pollination system changed from bees to birds with altitude.After a single visit,the stigmatic pollen load of sunbirds was significantly higher than other visitors,and the pollen brought by syrphid flies was the lowest.In order to compare the pollination contribution of sunbirds at different altitudes,we set up an experiment to exclude sunbird visiting flowers in three populations;the results showed that insect visitation frequency did not differ between caged and open in each population,while the stigmatic pollen load and fruit set in caged were significantly reduced.Combined with the visitation frequency and pollination efficiency,indicated that sunbirds played a more important role at high-altitude and insects participated in pollination,and the opposite is true for low-and mid-altitudes.Finally,the possible reasons for sunbirds to be the main pollinators of E.umbellate at high-altitude were investigated.According to investigating the species,the traits and distribution of flowering plants at different altitudes,found that the species and distribution of plants available for sunbirds to choose as nectar source at low-altitude were more than that at high-altitude.The visits of insects and sunbirds differed significantly and showed a negative correlation relationship between insects and sunbirds at the different period at low-and mid-altitudes,the visits of sunbirds decreased with the increase of the visits of insects.However,the visits of insects and sunbirds did not differ significantly in different time periods at high-altitude.The nectar standing crop at high-altitude was significantly higher than that at low-and mid-altitudes.Compared with the open group,the visitation frequency of sunbirds in the flower-removed group did not decrease significantly at high altitude,however,the duration time,the number of flowers visited by sunbirds,and the fruit set were significantly reduced.These results indicated that the visit of sunbird to E.umbellate was affected by the species and distribution of nectariferous plants in the community,the competition among pollinators,the nectar standing crop,and the number of flowers of E.umbellate.Our investigations of plant reproductive biology in E.umbellate indicated that altitudinal variation in both relative frequency of major pollinators and floral traits,the pollination ecotype was formed at different altitudes,supporting the pollination ecotype concept.These quantitative changes in floral traits were corresponding to the shift of functional pollinators from bees to sunbirds.The differentiation of flower traits at different altitudes might reflect the transition of pollinator functional groups.Variations in pollinator abundance were probably crucial to the evolution of the floral traits in E.umbellate.The results also confirmed the "pollinator-shifts" hypothesis and "Grant-Stebbins"model.Through the investigations and experiments indicated that the possible reasons for sunbirds to become the main pollinator of E.umbellate at the high-altitude were as follows:food sources,number of flowers,nectar standing crop,and competition for nectar among different pollinators.The construction of the interaction between sunbird and E.umbellate might be the results of the integrated impacts of different factors.