The Interspecific Hybridization In Sagittaria And Conspecific Pollen Precedence

Alismataceae (Alismataceae) is the more primitive groups of monocots, Sagittaria as one of the most important components has been extensively studied. As we know, Sagittaria are widely distributed around the world, China is the distribution center in Asia. In China, Sagittaria includes eight species, two variants and a subspecies.The significant differentiation of morphologies among intra-species and inter-species within the genus Sagittaria resulted in the taxonomic problems which have long been plagued to plant taxonomists. To solve these questions, Chen et al. (1989) made systematical in-depth researches about the genus Sagittria distributed in China with the help of the morphological, cytological methods and quantitative genetics and other means. Natural interspecific hybridization event is considered to be an important way to enlarge plant taxa. Under the base of the previous researches related to Sagittaria on morphology, systematics as well as reproductive physiology and ecological researches, we did some work on the inter-specific hybridization among common genus, and then comprehended the process of species formation, which extended our knowledge of their evolutionary history.1. The population structure and flowering phenology of mixed populations with Sagittaria pygmaea and S. trifoliaWe conducted meticulous observations and survey in natural mixed populations of Lichuan City, Hubei Province. At last, we found that in mixed populations, the number of S. pygmaea is significantly more than the S. trifolia. In the microhabitat occupied by the S. trifolia, at least three, or more than 62 of S. pygmaea distributed around per S. trifolia plant. A variety of visitors visited the two species, but only partly ones were effective pollinators. The most imoprtant is, there were three common flower-visiting insects played the role of the medium to communicate the two species, which included bees belonged to Coleoptera, flies belonged to Syrphine and Sciomyzidae. By counting the numbers of flowers which had been visited and recording the visitors residence time, we firmly believed that the Andrena camellia Wu was the main common pollinators of the both species. Both species had dioecious flowers with white and three bases petals. These morphologically similar flowers had identical period of blooming. The opening flowers derived from two species almost be the same with each other. Therefore, the flowering phenology hardly had marked impact on accessing to pollinating insects, and thus there were no geographical isolation and pollinators isolation between S. trifolia and S. pygmaea under natural conditions.2 The performances of interspecific hybrids derived from S. pygmaea and S. trifoliaBased on the field observation, we did inter-species hybridization experiments between 5. pygmaea and S. trifolia. The designed groups included:S. trifolia (?)×S. pygmeae (?), S. trifolia (?)×S. pygmeae (?) respectively. At last, we found that no matter which one is the female parent, the mature and viable fruits could be obtained. And in both hybrization situation, the external morphological structure of hybrid seeds did not significantly differ with wild-type seeds. However, when the S. trifolia as the female parent, the abortive seeds significantly more than intra-species mating, while in inter-species hybridization which is considered S. pygmaea as female parent, such phenomenon did not exist. In the consequent germination experiments, we found that hybrid seeds can not sprout out when the S. pygmaea as the maternal parent. In contrast, when the S. trifolia as the maternal parent, the hybrid seeds can spout out, but the germination rate are very low. After transplanting to the plastic boxes with growth substrate, only one strain was successful retained and grew into full plant. Although the flow cytometry could confirm the hybrid seeds are diploid, this is the indirect evidence about the real existence of hybridization events in Sagittaria. The reasons for the low rate of seed germination still need further researches to explore. Furthermore, the hybrid seeds can not germinate and then form the completely mature plants possibly resulted in the absence of hybrid in raw natural mixed populations.3 The growth path way of alien pollen tubes in Sagittaria gynoeciumPlants are all polycarpous in Sagittaria. Specialized to our experiment materials, there are more than one hundred carpels in S. pygmaea, while hundreds of carpels in S. trifolia, which spirally arranged on a spherical receptacle. In our researches, the results of the fluorescence microscopy observation showed that the conspecific pollen tubes could go through the basal opening of the carpel, pass into the surface layer of the receptacle tissue, and then enter into neighbour unfertilized ovules which was generally defined as pollen tubes reallocation. In contrast, we found the unique growth path displayed by heterospecific pollen tubes. After placing on the stigma, they directly grew through the surface layer of the receptacle tissue, and then entered into other unfertilized ovules, rather than a priority into its ovule. This may be the case with the unique extracellular matrix pollen tube growth pathway existed in Sagittaria. Even if the time of arrival on the stigma was the same both conspecific pollen, heterospecific pollen tubes need to take a longer path to achieve fertilization. This mechanism, existed in apocarpous plant, is similar to "lengthened style" may play a certain role in the isolation prevented the interspecific hybridization in Sagittaria.