| Taraxacum (Asteraceae), which about70species have been identified in China. According to the statistic,11species of Taraxacum distributed in Northeast China. Taraxacum is used to treat various ailments, such as dyspepsia, spleen and liver problems, heartburn, hepatitis, and anorexia. In traditional Chinese medicine, the dry whole herbage of the dandelion has been used for centuries to cure the common cold, reduce swelling, and facilitate urination. The research location in Ex-situ Conservation, where in shenyang agricultural university. Five Taraxacum species, T. ohwianum Kitag. T. antungense Kitag. T.junpeianum. T. coreanum Nakai. T. variegatum Kitag. were collected from Northeast China In this paper, we studied the flowers syndrome, breeding systems, pollen mother cell meiosis behavior, embryology, pollination mechanism, the dynamic and reproductive allocation of biomass distribution, ploidy level and seed germination characteristics of these species. The main conclusions were as follow:(1) The number of chromosomes in dandelions were calculated by root tip squashing method. The ploidy level was analyzed by karyotyping. Results showed that the number of chromosomes in T. ohwianum was16with satellites. The pollen mother cell(PMC) meiotic was normal in T. ohwianum, The pollen germination rate in vitro was above70%; the number of chromosomes in T. antungense and T. junpeianum was24, which were allotriploid.; the number of chromosomes in T.coreanum and T. variegatum was32, which were allotetraploid. while abnormal meiosis of PMCs were common in polyploidy dandelions. We observed some abnormal events including lagged chromosomes, chromosome bridge and fragment. At tetrad stage, dyad, triad, tetrad with micronucleus and polyad appeares. The meiosis in Taraxacum undergoes simultaneous cytokinesis, and tetrads were tetrahedral,decussate was lesser. Polyploidy pollen activity were lower than30%. Mature pollen grain was3-cell type, existed abnormal generative cell, abnormal pollen mother cell meiosis and unusual development of generative nucleus were the main reasons for pollen abortion of polyploidy dandelion.(2)Fluorescence microscope observation showed that the pollen of T. ohwianum can germination only under the condition of cross-pollination, which mean strict self-incompatibility. Polyploidy pollen can also germination on the stigma by the cross-pollination, but the pollen tubes grow slowly, and observed anomalies deformation and fracture of pollen tube before reaching ovule. The results showed that apomixis embryo and endosperm could form autonomously without fertilization.(3) Anther wall includes four layer. The development of anther wall is primitive type which is composed of epidermis, endothecium and tapetum.apetum cell in T. coreanum is Glandular tapetum, the others apetum cell is of amoeboid tapetum. The megaspore mother cells are developed directly by a hypodermal archesporical cell. Which undergoes meitotic division and forms a linear tetrad. And the megaspore near the chalaza continued to develop into a functional megaspore that would develop into a mononucleate embryo sac, whereas the other three megaspores near the micropyle gradually degenerated. The development of embryo sacs belonged to polygonum type. Apomixis embryo sac megaspore formated dyad, one degradation, the other one which undergoes three tmies mitosis and develops into a mature embryo sac with7cells and7nucle, then egg Continue to develop to forme proembryo, central cell autonomous development later, and the formation of the endosperm, which belongs to the cellular type. Egg formed proembryo befor the Inflorescence open,So pollination or not can not influence the development of apomixis dandelion.(4) The aim was to develop a simple and efficient screening method that identifies simultaneously different reproductive pathways based on the proportional DNA content of embryo and endosperm nuclei, notwithstanding the real ploidy level. Seed samples of three species were investigated by flow cytometry to reveal the pathway of reproduction. Three different pathways of seed formation could be reconstructed considering whether the female gametes were reduced or unreduced, the embryos arose via the zygotic or parthenogenetic route and the endosperm via the pseudogamous or autonomous route. Flow cytometry showed that the T. ohwianum, T. antungense and T. coreanum ratio of seed cell ploidy level are2c:3c,2c:4c and2c:3c:4c respectively, determine Their reproduction modes were sexual, obligate apomixis and facultative apomixis. The screen of mature seeds by flow cytometry yielded more information about the reproductive behavior of individual plants than any other available test. In conclusion, diploid T. ohwianum for obligate outcrossing, triploid T. antungense for obligate apomixis and tetraploid T. coreanum as apomixis reproductive type.(5) Several kinds of insects, like hymenoptera, lepidoptera, diptera classified as pollination insects for Taraxacum, so Taraxacum belonging to the generalization pollination. the Main pollination insects were Apis mellifera l,and Apis mellifera1., hoverfly and butterfly, etc. Insect pollination activity and pollination behavior were affected by the weather, the number of insects for visit the flower was higher in sunny day than in rainy or cloudy day, the visit insects. The visit flower peak were different by photoperiod and greatly influenced by temperature.at the beginning of may, around13:00had the strongest activity peak, temperature was the most appropriate.with the warming of weather, peak activity ahead constantly too, in the middle May, at about11:00to form obvious peak activity. On the different time of the same day, pollination frequency was differnet.(6) In order to understand the seed germination characteristics, we take apomixes T. variegatum (tetraploid) as materials, Surveyed the effect of seed germination by pollination, pappus, seed soaking and temperature. Results indicate that pollination or not has significantly influence on seed germination potential and germination rate. The germination percentage of pollination processing seeds is40%higher than processing without pollination. These two processing (with or without pollination) own different germination strategies:pollination seeds belongs to the’opportunism’germination strategies of rapid germination, but unpollination seeds belongs to’bet hedging’germination strategies of low germination. The germination for seeds lacking a pappus is16%lower than control seeds (with an intact pappus) and that the seeds lacking a pappus require more time to germinate. seed soaking has different effect on germination rate, seed soaking12h has the highest germination rate, more than48h, germination rate, germination potential are both reduced. But for unpollination processing seed, seed soaking can promoting germination. Seed germination of T. variegatum were suppressed at low temperatures(<5℃) and high temperatures(>30℃),10-20℃is the optimum temperature, and has the highest seed germination(>60%).(7) Male reproductive output has been shown to constitute a substantial cost for many organisms. In parthenogenetic hermaphrodites, selection is therefore expected to reduce the allocation of resources to male reproductive output. However, sustained production of pollen has been observed in numerous asexual hermaphrodites. We studied the widespread production of pollen by triploid asexual dandelions, Taraxacum junpeianum, comparing with rare male sterile individuals with pollen producing asexuals. We found that individuals can show plasticity in the production of pollen, but that it is nevertheless possible to distinguish between male sterile asexuals and male fertile asexuals. Male sterile lineages did not produce more seeds per flower head, heavier seeds or seeds that were more viable. However, male sterile plants did produce more seed heads and hence more seeds than pollen producing ones, indicating that they were able to reallocate resources toward seed production. Considering the difference in seed production, it remains puzzling that not more asexual dandelions are male sterile. Germination characteristics of infertility and sterility of T. junpeianum have been studied at different temperatures. Results from seed-germination experiments indicated that temperature of final percentage germination was qualitatively similar for both Infertility and sterility-germination was suppressed at high and low temperatures.(8) Gynomonoecy is the sexual system in which individual plants bear both female and bisexual flowers. Little attention has been paid to the adaptive significance of this sexual system, which is particularly prevalent in the Asteraceae. Nevertheless, the sexual system of some dandelions currently remains vaguely characterized as having "bisexual and female flowers." The present study investigates the significance of female flowers in Taraxacum antungense Kitage. It’s confirmed that T. antungense is gynomonoecy in field and greenhouse. In addition, sexual dimorphism is found in T. antungense, namely a red syngenesious stamen in female compared to a yellow one in bisexual flowers. A position effect on floral sex was presented in T. antungense. At the inflorescence level, central florets were predominantly female while lateral florets were mainly bisexual, which were opposite to other reports of gynomonoecious in Asteraceae. The observation can improve our knowledge of sexual systems in plants and document relevant evolutionary patterns in sexual dimorphism and position effect of floral sex. Nutrient stress produced fewer bisexual flowers and a higher ratio of female flowers in dandelion capitulums. T. antungense has a phenotypically plastic in floret morphologic. While the floral-sex ratio responses of gynomonoecious plants may be more complicated than other breeding systems of plants, and they offer the potential to test and refine the already rich body of sex-allocation theory.(9) To obtain dynamic information on endoreplication as well as a division of the pollen mother cells (PMC) in apomicts T. antungense, the entire course of pollen development was observed by the clearing-staining-squash method. Endoreduplication was observed before the PMC. The PMC was formed after endoreduplicational division. The sporogenous cell, which owns24chromosomes and chromosomal DNA replication without intervening mitosis. If the chromosomal replicates successively, the ploidy level would increase exponentially. In this process, the volume of cell increases gradually till the ceasing of chromosomal replication. Consequently, after the division of endoreduplication cell, chromosome number of the PMC becomes normal. There are three kinds of divisions:chromosome division, nuclei division and mix division, meanwhile those processes can produce different sizes of cells. During this period, what we observed are lagging chromosomes and unequal size cells, which suggest that the pollen sterility is so low. The meaning of the occurrence of endoreduplication process and its regulation mechanism need to be further researches. |
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