Morphological Characteristics And Phylogeny Of Some Groups Of Freshwater Naviculoid Diatoms

The genus Navicula Bory belongs to Bacillariophyta,Pennatae,Naviculales,Naviculaceae.It is one of the most abundant groups in the biraphid diatom.more than9,800 taxa of Navicula have been reported worldwide.At present,there are still many taxonomical and systemical problems within the Navicula group,made the classification for this group very difficult.In this study,monoclonal strains of some Navicula species collected from the field were cultured,and morphological characteristics of the cultivated species were observed and identified by light microscope(LM)and scanning electron microscope(SEM).The sequencing analysis of 18 S,rbcL,and 28 S genes of isolated strains were carried out in combination with molecular phylogenetic studies.By constructing its bayesian evolutionary tree and maximum likelihood evolutionary tree,the positions of each species in the phylogenetic tree were determined.The results were as follows:1.The morphological study of the genus Navicula and its neighbouring genus successfully separated and cultured,and a total of 10 taxa of Navicula,1 taxa of Sellaphora and 2 taxa of Pinnularia were identified.The morphological description and collection habitat of each species were provided.2.Using ribosome genes(18S,28S)and chloroplast genes(rbcL)to construct the phylogenetic tree of Navicula,the results showed that:(1)The results of the evolutionary tree constructed based on each gene sequence are basically consistent with the conclusion of morphological taxonomy.(2)Based on the phylogenetic tree constructed with genes of 18 S,rbcL and 28 S,we found that Navicula and Sellaphora always gather in a large branch,so it was speculated that Navicula and Sellaphora have close relationship.Navicula tridentula,originally belonging to the genus Navicula,did not cluster with the genus Navicula,but with the genus Sellaphora.Combining with the morphological results and the evidence of molecular phylogeny,Our results support the conclusion of Wetzel et al.(2015)that N.tridentula was transferred to the genus Sellaphora.(3)Phylogenetic trees constructed based on the 18 S and 28 S genes have strongsupport and good discrimination between Navicula and its adjacent species,and can be used as a good molecular marker for diatom molecular systematics.3.Molecular method were used for the cultured strains,the rbcL gene were sequencing and analysed,compared with other gene of biraphid diatoms which downloaded from website of NCBI,the Baye-sian Inference tree and maximum likelihood evolutionary tree were bulid,determine the position of all species in the phylogenetic trees,analysis results show that in the genus level,our results support the current classification system of biraphid diatom;at the family level,our results support Cox's(2015)classification system.Specific conclusions are as follows:(1)In the phylogenetic tree constructed with Cyclotella meneghiniana as the outer group,the species of biraphid diatoms are clustered on a large branch,indicating that the biraphid diatoms are generally a large group with close relationship.(2)According to the analysis results of the evolutionary tree,Navicula,Seminavis and Gyrosigma are far from other genera,so it supports the conclusion that the three genera are divided into one family in Cox(2015)classification system.Sellaphora and Fallacia are separated from other genera,which also support the idea that these two genera belong to the same family in Cox(2015)and Round(1990)'s taxonomical system.The clustering of Pinnularia and Caloneis indicates a close relationship between the two genera,which to some extent supports the idea that they should be classified as one genus in the Round(1990)'s classification system.In addition,Stauroneis is grouped with Craticula,Neidium is grouped with Neidiomorpha,and Amphora is grouped with Frustulia Together,Gomphonema and Gomphoneis come together,and these molecular systematic evidences support the current classification system to a certain extent.(3)In the phylogenetic tree,the Cymbelloid and Gomphonemoid diatoms are clustered together,indicating that the two groups have a closer relationship.Among them,the species of Cymbella and Encyonema are confused.Although the two genera have obvious morphological differences,in the classification system of Patrick &Reimer(1975),Encyonema originally belonged to the genus Cymbella,therefore,the evolutionary relationship between the two genera needs to be further studied.Reimeria and Gomphonemoid diatoms are clustered together,indicating that they are closely related,which is different from Round's taxonomic view,but supports Cox's taxonomic view.