Sexual Differentiation In Eurya Obtusifolia H. T.Chang And Comparison Of Pollination Characteristics Between The Sexes

The genus Eurya Thunb. was described as strictly dioecious in most of the literatures, however, rare gender variation have been found, which leaves some unsolved problems. But gender variant plants are satisfactory materials for the study of ecological adaptation mechanism and dioecy evolution in this genus.Gender variation has been found in Eurya obtusifolia H. T. Chang during investigation. In order to accumulate some data to recognize the law of sex expression in this genus, and to provide some references for the study of ecological adaptation and dioecy evolution in this genus, research was applied in three aspects:1. the sex expression law in Eurya obtusifolia,2. floral morphology and flowering dynamics in Eurya obtusifolia,3. pollination characteristics in Eurya obtusifolia. Based on the field investigation, marking and observing at particular spots, sampling detection, visual study, taking photographs with digital camera, observation under stereo microscope, observation under optical microscope, observation under electron microscope, assessment of pollen viability and stigma receptivity, pollen culture, assessment of pollen amount and nectar volume, as well as bagging experiment were applied, the main findings were as below:1. Typical dioecious plants are common, while gender variant plants are rare. Typical dioecious populations, both on single flower and plant level, only have two exclusionary states:the male and the female. However, populations which contain gender variant plants, both on single flower and plant level, exhibit transitionary sexual characteristics. There are six types of flowers in Eurya obtusifolia:pistillate flower (PF), staminate flower (SF), pistillate flower with vestigial stamens (PVS), staminate flower with vestigial pistils (SVP), hermaphrodite (HF), and infertile flower (IF). Typical dioecious plants only bear PF and SF, while gender variant plants bear various types of flowers. Not all the gender variant plants are bisexual, and a plant could be regard as bisexual when bears PFs or bears both PFs and SFs. As abortive pistils (or stamens) were often observed on gender variant plants, and the maleness and femaleness of bisexual plants tended to neutralize each other, these gender variant plants have less fitness than the dioecious plants.2. The male and the female plants have something in common, but also have many obvious differences in respect of the floral morphology and the pollination characteristics. However, many of these characteristics implied its adaption to dioecious sexual system. Sexual variant plants have the characteristics between the two except floral longevity. Regardless its sex, Eurya obtusifolia is a species with short florescence, small but massive blossoms and highly-synchronous flowering, the color and the smell of different sexes of which are the same. Besides its essential differences of the sexual organs, the male and the female still differ in:1) the flowers on male plants are lantern-like with non-reflexed petals, while the flowers on female plants are radial with reflexed petals.2) The males possesses significantly bigger flowers compared to the females, and the males have heavier sigle flower dry weight than that of females.3) The flower apex of the males tends to vertically downwards, while which of the female tends to obliquely downward to the terminal of the branch.4) The flowers on female plants live longer than those on the male ones.3. Entomophilous pollination is relatively dominating in Eurya obtusifolia compared to wind pollination, which plays a tiny role. There are various insect visitors in Eurya obtusifolia at Jinyuan Mt., include insects in Apidae, Syrphidae, Muscidae, Sphingidae, Chrysomelidae and Formicidae. The main pollinators were Apis mellifera and a kind of bumble bee. They sucked nectar on female plants, collected pollen on male plants, visited the male more often and acted more continuous on the female plants.4. The pollen grains of Eurya obtusifolia can germinate in pure water with a highly germination rate, and lose its viability quikle after germination. That mean, rain would somehow destroy pollen grains of Eurya obtusifolia. The floral morphology and its flower orientation would be some kinds of protection mechanism.In sum, there are bisexual plants in Eurya obtusifolia, but they do not have the potential to evolutionary replace the dioecious ones. On the contrary, the dioecious plants have a series of adaption characteristics. Much evidence has proved that dioecy evolved from hermaphroditism many times independently. Evidence based on morphology, embryology, and molecular biology has proven that Eurya is closely related to Adinandra, Cleyera, and most especially, Euryodendron, and most of the plants in the last three genera are hermaphroditic. From this gender variation phenomenon and its phylogenic position, Eurya plants should have evolved from a hermaphrodite ancestor. But, till now, the sexual genetic background and the factors affect sex expression are absolutely unknown. This short-term and regional study is not enough to demonstrate if the sex would change with time and environment. Additional study should be applied. The differences between sexes reflect its adaption to dioecy and they are also the result of natural selection. The evolution of these sex related differences main close related to the evolution of dioecy. Pollinator and rain selection might play important roles as selective pressure in the evolution of dioecy. As the sex-determing mechanism and the origin of sex related difference are still unknown, and these differences may appear before or after the evolution of sex, related issues remain to be studied.