Gametophyte Development And Cytological Studies Of Oogenesis In The Fern Pronephrium Penangianum

Oogenesis of ferns is an important aspect of the sexual reproduction.Oogenesis of ferns in different evolutionary status is different.Studying oogenesis of ferns is of important significance to clarify their evolutionary status.There are many species in the family of Thelypteridaceae.There are some controversies on its classification of the genera in the family of Thelypteridaceae.Researches on the Oogenesis of in the genus Pronephrium has not been studied yet.Therefore,in this study,Pronephrium penangianum was selected as the material,its gametophyte development and oogenesis were studied by microscopy and ultramicroscopy,which is of great significance to improve the reproductive biology and explore the evolution of ferns.Results of this research are as follows:The gametophyte development of P.penangianum went through the stages of spore germination,filament,plate and prothallus,the spore germination is of Vittaria-type,the development of prothallus is Drynaria-type,and the mature prothallus is heart-shaped.After 46 days the spore sowing,the archegonium and antheridium formed below the growth point of the prothallus.The microscopic structure of the cells taking part in oogenesis development in the fern of P.penangianum indicated that the archegonium originated from the initial cell below the growth point,the initial cell was divided into three cells by twice cell divisions,which were top cell,primary cell and basal cell.The primary cell will divide into a central cell and a mononuclear neck canal cell after an division.With the development of the archegonium,the nucleus of mononuclear neck canal cells was divided into two,the cytoplasm did not divide,forming a binuclear neck canal cell.Subsequently,the central cell undergoes an division to form a ventral canal cell and an egg cell.At the young egg stage,the three cells were closely connected.Subsequently,binuclear neck canal cell and ventral canal cell gradually degenerated,ventral canal cell,neck canal cell and egg were separated gradually.Observation of ultrastructure of P.penangianum showed that the nucleus of binuclear neck canal cell is prior to the formation of ventral canal cell.In the stage of young egg,a binuclear neck canal cell,a ventral canal cell and egg are closely connected,well-developed plasmodesmata among them.With the development of egg,a separate cavity is formed between the ventral canal cell and egg from the cell edge.Finally,only a pore region connected the egg and the ventral canal cell,and there are still plasmodesmata in the pore region.At the same time,the ventral canal cell and the binuclear neck canal cell also gradually degenerated and the plasmodesmata was well-developed between the cells.In the late stage of egg development,the egg is surrounded by egg envelope outside its plasmalemma,the egg envelope is a reticular structure.The fertilization pore is formed in a place that is not covered by the egg envelope,the fertilization spore is located in the off-center position above the egg,and the nucleus forms a large number of nucleus evaginations.The features of oogenesis of P.penangianum make clear that it belongs to an evolutionary pteridophytes.Histochemistry of P.penangianum showed that starch granules are abundant,and then starch granules gradually degenerate and disappear in the stage of mature egg.The amorphous materials of binuclear neck canal cell and ventral canal cell contain more carbohydrate,which indicates that the energy utilization of ferns is different in different evolutionary positions.This study is the first to observe the detailed process and cytological changes of gametophyte development and oogenesis of P.penangianum.As the egg development,the binuclear neck canal cell preceded the formation of ventral canal cell,the appearance of the separation cavity and pore region,the reticulated egg envelope formed by the combination of accumulation of amorphous materials and vesicles containing the osmiophilic materials,and the deviated fertilization pores all indicate that P.penangianum belongs to the evolutionary pteridophyte,which supports the PPG-2016 system for the division of Thelypteridaceae.