The Complete Mitochondrial Genomes Sequence Analysis And Phylogenetic Study Of 4 Genera 13 Species Of The Family Neritidae

Neritid is a relatively primitive marine gastropod,its fossil record extends back at the Ordovician ca.500 Mya（million years ago,Mya）.The members of the family live on the tropical and subtropical coasts,usually inhabit the middle to upper intertidal rocky shores,and prefer to be gregarious.It is widely distributed in the coastal area of Zhejiang Province and south to it.They are euryhaline,which is one of the few organisms in the intertidal zone that can live in fresh water and seawater at the same time.It is a dominant species in the intertidal zone.They are mostly herbivorous,and often grazing on the algae on rock surfaces.They are important microalgal filter-feeders and can be used in the study of environmental pollution as bio-indicators.At present,there are many problems to be solved in the study of classification of the family Neritidae,and there is a lack of phylogeny related research.This study is devoted to research of phylogeny and genetic diversity of Neritidae in coastal areas.The genetic characteristics and phylogeny of Neritidae can be understood by comparing mitochondrial genome.The phylogenetic tree was constructed based on the sequence of complete mitochondrial genome,and their evolutionary relationships were inferred and evaluated.To provide more references for snail breeding and evolution,the taxonomic classification of each genus in Gastropoda was discussed,and its origin and phylogenetic model were elucidated.At present,the common genera of the family Neritidae in China are Nerita,Clithon and Neritina.In the present study,the complete mitogenome of 13 species belonging to four genera of Neritidae（including Nerita:Nerita albicilla,Nerita yoldii,Nerita balteata,Nerita undata,Nerita japonica,Nerita chamaeleon;Clithon:Clithon corona,Clithon lentiginosum,Clithon squarrosum,Clithon sowerbianum,Clithon oualaniense;Neritina:Neritina iris and Septaria:Septaria lineata）were initially sequenced,assembled and annotated.The length of complete mitochondrial genome ranged from 15,314bp to 15,975bp.The sequences of these species were compared with those reported in Gen Bank.The results showed that the structural characteristics of each genus were conservative.All of them contains 37 typical genes,including 13 protein coding genes,22 t RNA genes and two r RNA genes,a short control region between COIII and t RNA^Gly.The mean length of PCGs,t RNA and r RNA genes were 11,177bp,1477bp and 2193bp,respectively.The content of AT is higher than GC,except to r RNA gene.The start and stop codons of PCGs are usually ATN and TAN.The ratio of Ka/Ks for all PCGs was below one,indicating they were evolving under purifying selection.Phylogenetic analysis of Neritimorpha according to the reported Neritidae and the 13species of this study.The phylogenetic tree was reconstructed based on the sequence of Vetigastropoda,Caenogastropoda,Heterobranchia,Neomphaliones and Patellogastropoda.The 13 protein coding genes were used to construct phylogenetic tree by Bayesian and maximum likelihood method,and the consistent topological structure was obtained.The results show that the subfamily Neritimorpha is an independent branch of Gastropoda.It is between the Caenogastropoda and the Patellogastropoda.In the Neritimorpha,the family Helicinidae is located at the root of the whole subfamily.In addition,Neritina and Septaria are most closely related,which are sister groups,followed by the Clithon.Nerita is located in the outermost branch of the whole family Neritidae.The phylogeny of the complete mitochondrial genome of the subfamily Neritimorpha has been highly bootstraps and posterior probabilities.The results improve the resolution of the phylogenetic tree and provide a more taxonomic analysis of the family Neritidae.In addition,we used 13 protein coding genes to estimate the divergence time between the main lineages of the subfamily Neritimorpha by the existing fossil record under a relaxed molecular clock.The results show that Pleuropoma jana originated from the Triassic period more than 200 Mya,and it is the earliest species among the existing Neritids.The species divergence of Neritidae is mainly concentrated in the Cenozoic Era（From 65 Mya）,the latest geological age in the history of the earth.Compared with other genera,the Nerita melanotragus of the genus Nerita originated from late Cretaceous Period.The species with the latest divergence time in this study are in Neogene of Cenozoic,including C.corona and C.lentiginosum,N.japonica and N.yoldii.This geographical isolation produced by the geological movement provides the environmental conditions for the divergence of Neritidae,and the marine sediments provide the food source for the Neritidae growth.