Study On Oogenesis And Fertilization In The Fern Ceratopteris Thalictroides

Pteridophyte is a special group, which lies in the key position of the phylogenetic revolution in the plant kingdom. The investigation on the sexual reproduction of the pteridophyte posseses an important scientific significance in revealing the mechanism of sexual reproduction and its evolution of the plant. The gametogenesis and fertilization are key stages in sexual reproduction, but the studeis on the gametogenesis and fertilization of the sexual reproduction are weak in fern. So, we use the Ceratopteris thalictroides (L.) Brongn which is the model plant of the fern to study the oogenesis and fertilization mainly by transmission electron microscopy (TEM). The main results are as follow:1. The channel of the spermatozoid entering eggA fertilization pore which a special structure related to fertilization is found for the first time. It is a pore structure that only covered by a plasmalemma in the central region of the upper surface of the egg in Ceratopteris thalictroides, no extra egg membrane covering. The diameter of the fertilization pore about 2.5μm, which is slightly wider than the transverse diameter of the spermatozoid.2. The formation and development of the fertilization poreThe new-formed egg is appressed closely to the ventral canal cell. There are well developed plasmodesmata between them. During maturation, a separation cavity begins to form at the periphery of the upper surface of the egg. The separation cavity expands centripetally and the connection region correspondingly decreases. It finally forms the pore region. After that a layer of extra egg membrane which is generated by the deposition of the sheets of ER forms around the egg. However, the extra egg membrane doesn't deposite at the pore region which generating the fertilization pore.The study proves that the ventral canal egg takes part in the formation of the fertilization pore. The VCC possibly can absorb the material of the egg through the plasmodesmata, which leads to a result is that the ER can not deposit in the pore region and the formation and development of the fertilization pore is affected.3. Prevent polyspermyThere are several spermatozoids entering the cavity above the egg, but only one spermatozoid can enter into the egg through the fertilization pore. The egg shrinks obviously as the spermatozoid penetrating the egg. Then, a lot of vesicles of concentric membrane and a sac containing a large amount opaque material appear, block the fertilization pore, and prevent polyspermy.4. The fate of the sperm organellesThe whole male organelles including the MTr, the MLS, the male mitochondria, the flagella, band of the flagella and so on enter the egg,. During the egg maturing, they are gradually digested. For example, the flagella lose the peripheral membrane at first. Then, they lose the inner structure. At last, the sperm organelles are wrapped by a layer of ER and finally digested in the zygotic cytoplasm.5. KaryogamyWhen the spermatozoid has entered the egg, the MTr separates from the male nucleus. The male chromatin gradually decondenses. The nuclear envelope reappears around the male chromatin before the karyogamy. While the gametic nuclei begin to fuse with each other, the egg nucleus forms many nuclear folds and fuse with the male nucleus actively. Karyogamy marks that the formation of the zytoge.6. The formation and development of the zygoteAt the beginning of the karyogamy, the zygotic nucleus is extremely irregularity. The male chromatin in the zygotic nucleus can be identified by its fibrous or granular structure. From then on, the male chromatin gradually becomes homogenization with that of the egg nucleus. The zygotic nucleus contracts and transforms to spherical shape..The organelles in the zygotic cytoplasm are rearranged. The cell polarity correspondingly changes from the vertical polarity to the horizontal polarity, which is important to the zygotic division for the first time. The zygote rebuilds plasmalemma and cell wall before the first division. So the zygote becomes a cell having division function in the morphology and structure and is ready for the embryo development.The process of the sexual reproduction in the fern is very precise and complicated. This paper solves some questions which are little understood such as the questions of the channel of the spermatozoid entering egg, the formation and development of the fertilization pore, prevent polyspermy and so on. It illuminates the mechanism of the sexual reproduction of the fern in the cytological level.