Study On The Morphological Structure And Development Of Pistils And Ovules Of Four Genera In Menispermaceae

The gynoecium is a unique innovative trait of angiosperms.The ovule in the ovary is an important female reproductive structure.Ovule morphology shows diversity and may imply a relatively stable evolutionary significance.Menispermaceae is one of the core members of Ranunculales,dioecious,flowers in the family are unisexual with one or 3-6 carpels.There is only one fertile ovule in each ovary(with one aborting),and the morphological characteristics of the ovule show rich diversity in the number and thickness of integument,outer integument morphology and the micropyle and so on.However,there is few information about the gynoecium and ovule development in Menispermaceae.In this study,the gynoecium development,ovule morphogenesis and development,megasporogenesis and development of female gametophyte of 5 species of 4 genera were investigated:Sinomenium acutum,Menispermum dauricum,Cocculus orbiculatus,Stephania cephalantha and Stephania japonica.The results are as follows:1)Gynoecium morphology and developmentCarples were largely plicate of the five plants.They shared similar development process and went through four key stages.The carpel primordium occurred,a groove was first formed;followed by a fold,the ventral surface was healed to form ventral slit;the base was expanded to form an ovary,and the top formed a stigma,with a short style;and mature carpel.Carpels 3 or 6 in S.acutum,M.dauricum,C.orbiculatus,but only 1 in S.cephalantha and S.japonica.The 1 carpel occurred in a single carpel primordium.The stigma belonged to wet stigma with secretion on the surface.The surface of the ovary wall had stomata,the epidermal cell was irregular,and the surface had secretions.During the third stage of carpel development,the receptacle stretched to form a conspicuous gynophore,and the length was about 0.5-1.5mm except in S.j.aponica.The stigma were split or lobed,while entired in C.orbiculatus.2)Ovule morphogenesis and developmentIn the early developmental stage,two ovule primordia arose on both side of the ventral suture of the marginal placenta almost simultaneously.Shortly,two ovules developed at different speeds,one of which curved toward the style side,occupied the whole locule at maturity,while the other near receptacle side stopped developing and aborted and shriveled finally(bigger in S.acutum and M dauricum).Four key stages of fertile ovule from primordium to maturity:Stage 1,Ovule primordium occurred in a flat hemisphere and then developed into a short finger shape.Stage 2,The annular inner integument(single integument)occurred with dermal cells division and became cup-shaped at maturity.And semi-annular outer integument occurred with dermal and subdermal cells division and was also cup-shaped at maturity.Stage 3.The integument developped into micropyle,integument lobes and obvious obturator,the single integument of the S.cephalantha and S.japonica had distinct integument lobes with the number up to 6-8,the inner integument of the S.acutum and the outer integument of the M.dauricum formed lobes respectively,C.orbiculatus without lobes.There was about 4-6 cell layers of obturator in C.orbiculatus and S.acutum,and 9-11 cell layers of S.cephalantha and S.japonica.Stage 4,The mature ovules were all campylotropous,and the difference in size was obvious.The ovule in M.dauricum was about 840 μm in length,about 650 μm in S.japonica,and the smallest was about 460 μm in S.acutum.3)Megasporogenesis and female gametophyte developmentCrassinucellate,the crassinucellar ovules formed by the division of parietal cells and nucellus epidernis in S.acutum and C.orbiculatus except in S.cephalantha and S.japonica which formed by the division of nucellus epidermis only and belonged to"pseudocrassinucellar".The megaspore mother cell underwent meiosis to form a linear megaspore triad in S.acutum and C.orbiculatus,and a linear megaspore tetrad in S.cephalantha and S.japonica.Embryo sac development belonged to polygonum type.Mature embryo sac consisted of two antipodal cells,one egg cell,one central cell and three antipodal cells.4)Sterile ovule developmentIn Menispermaceae,development of sterile ovules were similar to that of fertile ovules at the early developmental stages.In the early stage,both ovules maintained upright growth,when they touched the ovary wall,the positions of the two ovules changed,fertile ovules curved to the side of the style,sterile ovules curved to one side of the receptacle,and the funiculus were thin and short in sterile ovules.When in the stage of 4-nucleus embryo sac(S.acutum and M.dauricum),2-nucleus embryo sac(C.orbiculatus and S.japonica)and megaspore mother cell(S.cephalantha)the sterile ovules stopped development.At the time of flowering,the sterile ovules were different in size,about 180μm in length(S.acutum),and about 400pμm(M.dauricum),which was dried(C.orbiculatus,S.cephalantha and S.japonica).Based on the development characteristics of ovules of the five plants in Menispermaceae,it is found that there was only one integument initiation in Stephania Lour.The different developmental characteristics of the unitegmy in Stephania were reflected in stage 2,and no outer integument was present in unitegmy.The formation of the unitegmy was probably due to the integumentary shifting belonged to the third way of unitegmy.In bitegmy,the origin of the cup-shaped outer integument were formed due to outer integument’s extension to the concave side of funiculus,it was derived from the division of dermal cells in S.acutum,and both dermal and subdermal cells division in C orbiculatus.Compared with other groups of Ranunculales,it is based on the following characteristics:annular initiation of inner integument with dermal cells division;cup-shaped at maturity;polygonum type of embryo sac;linear megaspore tetrad or triad which support that Menispermaceae,Lardizabalaceae and Berberidaceae in relationship are near with each other.