Gametophyte Development And Cytological Studies Of Oogenesis In The Fern Lygodium Japonicum

Gametophyte development and the studies of oogenesis are important aspects insexual reproduction of the fern. Archegonium is the reproductive organ which canproduce the egg cell.It has degradated from the mosses, ferns (o gymnosperms untilthe disappearance of the angiosperms. Archegonium and oogenesis are closely relatedto plant evolution. So, oogenesis in the fern is transitional type. The series of studiesin our laboratory demonstrated that the core leptosporangiopsida had obvious eggenvelope and fertilization hole structure. But Osmunda japonica which is the mostoriginal groups of leptosporangiopsida has no egg envelope and fertilization porestructure. Therefore, the evolution of egg envelope and fertilization hole in differentevolutionary status are unknown. L. japorticum which has special evolutionary statusis between O. japonica and core leptosporangiopsida. So, the studies of oogenesis in L.japonicum may have important scientific signiifcance in revealing the formation ofthe egg envelope and fertilization hole. Therefore, we choose L.japonicum as theexperimental material in present thesis and made a research about gametophytedevelopment and oogenesis in many aspects. The present studies have the significancein clarity the cytology mechanism of oogenesis and reveal the evolution of sexualreproduction in fern.The major contents and results of this study are as follows:1. The development of gametophyte of L. japonicum undergoes the stages of thegermination of spores, filament, plates and prothallus. The branches of malegametophyte are strong and many antheridiums are formed at the irregular gametophyte. The matured female gametophyte is cordate. Seeding around50d infemale gametophyte the archegonium The archegonium is produced below in thegrowing point of prothallus base region which between two thin plate-like branchesabout two monthsSOd atfer sowing. The archegonium looks like the cylinder in sideview and bents at base.2. The section observations show that the archegonium of L. japonicum isderived from the initial cell. The initial cell divides into three cells by twicecelldivisions，top cell, primary cell and basal cell. Then the primary cell divides intoan egg, a ventral canal cell and a neck canal mother cell. The three cells are closelyconnected in the younger stages of egg. With development, the ventral canal cell andneck canal cell gradually degradated. Separation cavity formed around the eggs.3.The ultrastructural observations of oogenesis of L. japonicum indicate that the egg,the ventral cana】 cell and the neck canal cell are connected by some plasmodesma inthe younger stages of egg. The mitochondria，golgi bodies and plastid in the eggcyoplasm are well developed? And the starch grains in the plastid are also welldeveloped. The separation cavity formed during the early development of egg, but aconnection area between the center of the upper surface of the egg and the ventralcanal cell is still connected, it is called pore region (pr). With the development, theseparation cavity filled with a lot of uncertainty substances and the starch grains in theplastid are decreased gradually. The shape of nuclei change rfom circular to bowl. Inthe mid stage of maturing egg, a large number of vesicles formed in the upper part ofthe cytoplasm of the egg. And many nuclear evaginations occured in the surface ofthe egg cell nucleus. Despite there are mucilaginous materials deposited in the uppersurface of the egg, but no egg envelop formed in the upper surface of the egg. Whenthe egg nearly mature, the separation cavity still iflled with a lot of uncertaintysubstances, but these substances do not deposite on the surface of the egg. However,the connection area between the egg and the ventral canal cell has been separated atthis time. When the egg completely matured, the mitochondria is well developed anda lot of endoplasmic reticulum occured in the cell The color of egg plasma membranedeepened, a layer of uncertainty materials deposited on the outside of plasma membrane. This structure is different rfom the membrane structure of typicalleptosporangiopsida. In the stage of maturing egg, the ventral canal cell and the neckcanal cell secrete lots of mucilaginous materials when they are degenerating and thereare many of the golgi bodies and vesicles produced in the cells. The starch grains inthe plastid disappeared gradually. The shape of the two cells become irregular andwill completely disappear.4. The histochemistry of oogenesis of L. japonicum.indicates that degeneratedmaterials that the canal cells produced is PAS reaction positive which could prove thatthese material are the polysaccharide. In the early stage of the development of egg, theamyloplasts which contain some small starch grain exist both in the egg and canalcells. With development of egg, the volume of the starch grain becomes smallgradually and finally disappear. The staining of Sudan Black B indicates that there arealso lipid materials that the canal cells produced existing.But there is no typical lipidmaterials in the egg., which are emerge in the middle and later stages of the eggdevelopment and forms clusters when the egg is matured.Our study rfistly clarify the particular process and cytology mechanism of thedevelopment of archegonium and oogenesis. These studies indicate that oogenesis ofL japonicum experienced the stage of young egg, the maturing egg (including theformation of the separation cavity, the pore region and nuclear evaginations andorganelle changes) and the mature egg and other processes. But there is no eggenvelop formed in the upper surface of the egg. It reveals that L japonicum isbetween O. japonica and core leptosporangiopsida. It can provides a convenientcondition to reveal the mechanism of the fertilization and sexual reproduction of Ljaponicum. And It can also lay a foundation for evolution of sexual reproduction offerns.