The Reproductive Ecology Of Distylous Clonal Aquatic Plant--Nympphoides Peltata (Gel.) O. Kuntze

Plants must depend on vectors to transfer pollen because of their immobility. Diversified sexual systems and mating stragidies have been evolved in flowering plants in order to achieve a high reproductive seccucess. Heterostyly, one of the distinctive reproductive system in animal pollinated flowering plants, has fansinated biologist for more than a century since the publishing of Darwin's book, The Different Forms of Flowers on Plants of the Same Species. Heterostyly is characterized by the reciprocal herkogamy between the flowers of different morphs in one plant species. Heterostylous system includes distylous and tristylous systems. Tristylous reproductive system is more complex than distylous system and tristylous plants are much rarer than distylous plants in nature.The adaptive significance of hetetrostyly is usually interpreted as promoting cross-pollinating between individuals of different morphs and thus reducing the harmful effect of inbreeding. Under the natural selection in different environments, heterostylous reproductive system can evolve towards dioecy or self-compatible homostylous reproductive system symbolizing the breakdown of distyly. Many experimental and theoretical work has been done during the past decades in exploring the selective mechanism responsible for the maintenance and breakdown of heterostylous system.~Nymphoid.es peltata (Gmel.) O. Kuntze is a distylous clonal aquatic plantwidely distributed in the temperate area of north hemisphere. Earlier research has revealed that the incompatibility system in this species appear to be weak and illegitimate pollinating can produce a certain number of seeds per fruit. It was further suggested that under the environment where selective force strongly favors complete outbreeding, the selective pressure would result in either the eventually extinction of the species or in the development of mechanism that reinforce outcrossing. A probably example is the morphological modification in the direction of dioecism, as has occurred in Rubiaceae and Menyanthaceae. At the same time, due to the strong colonability in N. peltata, biased morph ratios are common in the natural populations, and monomorphic populations are not rare. Given that the incompatibility system is strong in N. peltata, the function of distyly is frustrated for lacking of illegitimate pollen grains. Furthermore, there is also a strong pollen limitation in the central part of the monomorphic patch due to the "pollen dilution effect". It is possible that under the condition where there is a shortage of legitimate pollen grains, self-compatible homostylous variants may be selected to ensure reproductive success and breakdown of distyly can happened, an example of Baker's rule. 30 natural populations of Nymphoides peltata from different area of China were investigated. The main objective is to investigate (a) the range of the variation of the basic morphological components of distyly in N. peltata. (b) the variation of the mating system in natual populations, (c) the interaction between the colonal propagation anddistylous reproductive system.The results show that N. peltata is a typical distylous species with all basic morphological components characterizing distylous reproductive system. There is a evident dimorphism in stigma-anther height, pollen grain size, pollen production and stigma morphology. All individuals can be seperated as pins or thrums by visually access the population in the field. The stigma-anther heights of both morphs are variable among different populations or individuals of the same population. Further investigations are needed to identify the pattern of this variation and the selective force responsible for the maintenance of stigma-anther height dimorphism. The stigma of the long-styled morph is bigger than that of short-styled morph. The papillae of the receptive surface are longer in the long-styled flowers than in the short-styled flowers. The pollen grains of the short-styled morph are bigger than those of long-styled morph, while the long-styled morp...