Study On Oogenesis And Fertilization In Two Ferns With Different Evolutionary Statuses

Gametogenesis and fertilization are two principal events during the sexual reproduction process of ferns. Our lab finds a new mating structure, i.e. the fertilization pore, in the mature egg cell of Ceratopteris thalictroides. It is a gateway through which the sperm enters the egg cell. However, it is still not clear whether the fertilization pore exists universally in ferns or whether oogenesis of ferns with different evolutionary statuses is the same in sexual reproduction. Two ferns with different evolutionary statuses are chosen, they are Phymatosorus hainanensis (Polypodiaceae) and Osmunda japonica (Osmundaceae). The studies show a great difference on oogenesis in the two kinds of ferns. Besides, it conducts a primary study into the fertilization of the fern Osmunda japonica. This study has some scientific significance in explaining the oogenesis features and fertilization mechanisms of the ferns with different evolutionary statuses. Moreover, it has some important value in revealing the evolution of the fern sexual reproduction.The major contents and results of this study are as follows:1. The oogenesis process of Phymatosorus hainanensis and osmunda japonica undergoes stages of initial cell, primary cell, central cell and egg cell development, among which the egg development is the key stage in oogenesis.2. During oogenesis of Phymatosorus hainanensis, the ventral canal cell and the egg cell are separated, which results in the formation of a separation cavity. The two cells are, however, connected by a pore region which is located in the side of the upper surface of the egg. The diameter of the pore region is about 2.5um, where plasmodesmata developed well. The ventral cell may experience a process of autophagy. It may absorb materials in the egg just behind the pore region into itself or the separation cavity, which results in no deposition of the egg membrane on the pore region. With the help of the ventral cell, a fertilization pore is formed in the pore region. In the mature egg, plastids are degenerated and the mitochondria become well developed. The nucleus produces numerous nuclear evaginations.3. In the early stage of the archegonia development of osmunda japonica, the osmiophilic materials occur in the vacuoles of the jacket cells of the archegonium, and these materials increase with the development of the egg. During oogenisis, a separation cavity is formed between the egg and the ventral canal cell. A thin extra egg membrane is formed outside the plasmalemma when the egg matures. At the side of the upper surface of the mature egg, an indentation of the egg membrane is often seen. The indentation may be formed by the breakdown of the egg membrane. However, a certain fertilization pore is not discovered in the mature egg of osmunda japonica. The plastids inside the mature egg become well developed and filled with starch grains. The nucleus is ellipsoidal and no nuclear evaginations formed.4. The egg of osmunda japonica shrinks obviously at fertilization. Most of the organelles of the spermatozoid enter the egg cell. A large vesicle is formed at the joint region of the spermatozoid and the egg. The vesicle can prevent other sperms enter the egg, and avoid polyspermy.