Evolutionary History And Adaptive Significance Of Heterospore In Selaginellaceae

Selaginellaceae is a group with the largest number of species of lycophytes,and it is widely distributed.It is distributed all over the world except Antarctica,but it is mainly distributed in tropical,subtropical and temperate regions.As a special group with heterospory in ferns and lycophytes,the spore morphology(size,ornamentation,etc.)between species within Selaginella is very different.Microspores and megaspores are often an important basis for classification of subgenus,sections and species of Selaginella.Previous studies have observed and analyzed the morphology of spores of a large number of Selaginella,and discussed the taxonomic significance of the spores.With the deepening of research in recent years,more and more studies have shown that the obvious difference in the spore of Selaginella is not only of great significance in the classification of Selaginella,but also in the phylogeny and ecology of Selaginella.This study used the specimen materials of major domestic herbariums(CDBI,KUN,PE,PYU),combined with the collection of selaginella specimens in the four major foreign herbariums(MO,NY,UC,SING),as well as field studies in Hainan,Yunnan,Guizhou,Sichuan,Tibet and other places(collecting 90% of the national species)collected specimen materials.Scanning electron microscope and dissecting microscope were used to observe,measure and record the spore size and ornamentation of Selaginella genus,and obtain the data of spore size of each species of Selaginella,combined with phylogeny,species latitude distribution pattern,habitat,etc.Research on the evolutionary history and adaptive significance of the heterospory of Selaginella at the level of subgenus,sections and species.This research is carried out from five aspects,as follows:1.Molecular phylogenyThis study is based on 1 cp DNA sequences(rbc L)and 5 n DNA sequences(18S,26 S,ITS,pgi C,SQD1),using maximum likelihood method(ML)and Bayesian method(BI)to construct the topological structure of the phylogenetic tree,the topological structure of the ML tree is basically the same as the BI tree.According to the phylogenetic results,the genus is divided into 7 main branches: Selaginella.subg.Selaginella,S.subg.Boreoselaginella,S.subg.Ericetorum,S.subg.Pulviniella,S.subg.Heterostachys,S.subg.Stachygynandrum and a newly added branch of Sinensis.In the basis of previous studies,this study adds S.sinensis and their close relatives.Through molecular phylogenetic research,the systematic position of S.Sinensis branch has been better resolved,and further studies on molecular systematics and taxonomy will be carried out in the future.2.Spore size and ornamentationObservation of Selaginella spores through SEM shows that megaspores are generally spherical or hemispherical,with three prominent laesurae,occasionally 1 laesurae or 4 laesurae,and the laesurae extend 1/3–1/2 of the distance to the equator,the size variation range is 150 to1100?m;the spore ornamentation of different species are not the same,and the spore ornamentation can be divided into 7 main types,of which reticulate ornamentation(169 species)is the main type of megaspores,Followed by tuberculate ornamentation(85 species),and the other five types,scabrate(9 species),psilate(3 species),verrucate(1 specie),papillate(1specie),and lophate ornamentation(1 species)is less.The microspores of Selaginella are often hemispherical or bowl-shaped,and sometimes exist in the form of tetrads.The microspores often have three laesurae,occasionally one laesurae or no laesurae,and the laesurae often extend to the equator,and the size variation range is 15 to 65?m.The ornamentation can be divided into 11 types,of which the tuberculate ornamentation(72 species)is the main type,followed by the baculate ornamentation(21 species),the echinate ornamentation(20 species)and the scabrate ornamentation(20 species).And the reticulate(3 species),verrucate(11species),papillate(3 species),flamellate(10 species),lophate(2 species),circinate(3 species),and rugate ornamentation(1 specie)is less.3.Phylogenetic signalsThe combined analysis of the phylogeny and spore size of Selaginella showed that the phylogenetic signal of megaspore under the genus is not significant(Pagel' ?= 0.78,?<1,P=1.67063e-22);The phylogenetic signal of microspore under the genus is weak(Blomberg' K= 0.002,K<1,P=0.004),but the results of the analysis of the phylogenetic signal of the subgenus indicate that most of the megaspores and microspores are significant,such as the microspores of the subgenus Ericetorum(?=0.8,P=9.30301e-05),the microspores of the subgenus Heterostachys(?=0.95,P=5.43603e-05),the megaspores of the subgenus Stachygynandrum(?=0.85,P=1.37389e-05).4.Ancestral character reconstructionThe study on the size evolution history of the entire Selaginellaceae plants shows that in the entire evolution history of Selaginella,the earliest differentiated groups often have relatively larger megaspores,such as subgenus Selaginella(536-588?m)and subgenus Boreoselaginella(313-432?m);while the recently differentiated groups have smaller megaspores,such as the subgenus Heterostachys and the subgenus Stachygynandrum(more than 60% of the species have a spore size less than 300?m).The size evolution history of microspores is the same as megaspores.Early differentiated groups also have relatively larger microspores,such as subgenus Selaginella(35-41?m)and subgenus Boreoselaginella(31-52?m),and later differentiated groups only the subgenus Stachygynandrum showed a tendency to have smaller microspores.5.Relationship between megaspore and microspore and latitudeThrough the analysis of the relationship between the size of megaspore and microspore and the latitude of species distribution,the results show that there is a complicated relationship between the megaspores and latitude.Larger megaspores(Size > 500?m)tend to gradually decrease in size with increasing latitude,while smaller megaspores(Size < 500?m)often increase in size with increasing latitude;The size of microspores shows a strong correlation with latitude.As the latitude increases,the size of microspores tends to increase gradually.Through the analysis of the combination of ornamentation and species distribution latitude,the ornamentation of megaspores at low latitudes are often reticulate and tuberculate,and reticulate ornamentation is predominant.For microspores,species with low latitudes are mainly tuberculate and echinate ornamentation,while at high latitudes are mainly tuberculate ornamentation.