Molecular Phylogeny Of Cephalothricidae And Complete Mitochondrial Genomes Of Three Nemerteans

Cephalothricidae is one of the classes within the order Palaeonemertea of thephylum Nemertea. Taxonomy among this group remains confused due to their fewmorphological characters and the complication of morphology methods based on thehistological sectioning. So far, few reports have been published on the phylogeny ofCephalothricidae, especially at the molecular level. In the present study, we tried totest the potential use of statistical parsimony network analysis based on COI sequenceas a tool to delimit species in Cephalothricidae species, and to estimate thephylogenetic relationship among this group based on CO1, 16SrDNA and 28SrDNA.In addition, we used gene arrangement and the protein-coding genes to assess thephylogenetie position of Nemertea within the lophotrochozoans.The main work and conclusion of the present study were as the following:(1) To test the potential use of statistical parsimony network analysis as a tool todelimit species in Cephalothricidae species. COI gene sequences from 165Cephalothricidae specimens were sequenced and performed a statistical parsimonyanalysis in the program TCS. Genetic distance was calculated in MEGA3.0 software,and phylogenetic trees were constructed with Phyml and MrBayes software. COI genesequences from single species typically stick together in a single haplotype network.In class Cephalothricidae distance within species is obviously lower than that betweenspecies. And in the phylogenetic trees, all individuals of each species formed a clade.From these results, we can draw a conclusion that statistical parsimony networkanalysis based on the mitochondrial CO1 gene can effectively support the speciesdelimitation in Cephalothricidae.(2) Three combined genes (COI, 16SrDNA and 28SrDNA) were employed toestimate the relationship of the Cephalothricidae. Two hoplonemerteans Amphiporus lactifloreus and Amphiporus angulatus were chosen as outgroup. Maximum likelihood (ML)and Bayesian inference (BI) methods were adopted to construct the phylogenetic trees.All these analyses consistently showed that the family Cephalothricidae is notmonophyletic, which is not consistent with the results from morphology study.(3) The mitochondrial genomes from Cephalothrix simula, Paranemetes peregrineand Cephalothrix sp. were sequenced, which is first report of complete mitochondrialgenome in phylum Nemertea. The mitochondrial genomes of them is 16,296 bp,14,560 bp, and more than 15,800 bp in length (partial non-coding region was notdetermined), respectively.As other metazoans, the genomes of C. simula, P. peregrine and C. sp. contain astandard set of 13 protein-coding genes, 22 transfer RNA genes, two ribosomal RNAgenes. The gene arrangements of C. simula and C. sp. mitochondrial genomes areconsistent. Compared to this two, transloeations of 4 protein-coding genes (nad6,cox1, cox2, atp6) and 7 tRNA (trnY, trnE, trnG, trnF, trnQ, trnW, trnC)were found inP. peregrine.All analyses based on gene rearrangements, nucleotide and amino acid datasupport that the nemertean is close to the coelomate lophotrochozoans, rather than theacoelomate platyhelminths. Both the pattern of gene rearrangements andphylogenomic analyses using concatenated nucleotide sequences of the 13mitochondrial protein-coding genes support the close relationship between nemerteanand the phoronid. While the phylogenomic analyses based on the concatenated aminoacid sequences showed the sister relationship between Nemertea and Mollusca.