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Particle and Fibre Toxicology Volume 7 ,Issue 1 , ,2014-06-19
Complete mitochondrial genome sequences of two parasitic/commensal nemerteans, Gononemertes parasita and Nemertopsis tetraclitophila (Nemertea: Hoplonemertea)
Wen-Yan Sun 1 Dong-Li Xu 1 Hai-Xia Chen 2 Wei Shi 3 Per Sundberg 2 Malin Strand 4 Shi-Chun Sun 1
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Received 2014-04-20, accepted for publication 2014-06-14, Published 2014-06-14

Background Most nemerteans (phylum Nemertea) are free-living, but about 50 species are known to be firmly associated with other marine invertebrates. For example, Gononemertes parasita is associated with ascidians, and Nemertopsis tetraclitophila with barnacles. There are 12 complete or near-complete mitochondrial genome (mitogenome) sequences of nemerteans available in GenBank, but no mitogenomes of none free-living nemerteans have been determined so far. In the present paper complete mitogenomes of the above two parasitic/commensal nemerteans are reported. Methods The complete mitochondrial genomes (mitogenome) of G. parasita and N. tetraclitophila were amplified by conventional and long PCR. Phylogenetic analyses of maximum likelihood (ML) and Bayesian inference (BI) were performed with both concatenated nucleotide and amino acid sequences. Results Complete mitogenomes of G. parasita and N. tetraclitophila are 14742 bp and 14597 bp in size, respectively, which are within the range of published Hoplonemertea mitogenomes. Their gene orders are identical to that of published Hoplonemertea mitogenomes, but different from those of Palaeo- and Heteronemertea species. All the coding genes, as well as major non-coding regions (mNCRs), are AT rich, which is especially pronounced at the third codon position. The AT/GC skew pattern of the coding strand is the same among nemertean mitogenomes, but is variable in the mNCRs. Some slight differences are found between mitogenomes of the present species and other hoplonemerteans: in G. parasita the mNCR is biased toward T and C (contrary to other hoplonemerteans) and the rrnS gene has a unique 58-bp insertion at the 5′ end; in N. tetraclitophila the nad3 gene starts with the ATT codon (ATG in other hoplonemerteans). Phylogenetic analyses of the nucleotide and amino acid datasets show early divergent positions of G. parasita and N. tetraclitophila within the analyzed Distromatonemertea species, and provide strong support for the close relationship between Hoplonemertea and Heteronemertea. Conclusion Gene order is highly conserved within the order Monostilifera, particularly within the Distromatonemertea, and the special lifestyle of G. parasita and N. tetraclitophila does not bring significant variations to the overall structures of their mitogenomes in comparison with free-living hoplonemerteans.


Phylogeny; Mitochondrial genome; Parasitic/Commensal; Nemertopsis tetraclitophila; Gononemertes parasita;Nemertea


2014 Sun et al.; licensee BioMed Central Ltd.


Figure 1. Map of the mitochondrial genomes of Gononemertes parasita and Nemertopsis tetraclitophila. Genes coded on the coding strand are arranged clockwise; those on the other strand are counter-clockwise. Thirteen protein-coding genes are shown in blue and two ribosomal RNA genes in pink. Transfer RNA genes are labeled by their single letter of corresponding amino acids. Major non-coding regions (mNCR) are represented in grey.

Figure 2. Scatter plot of AT- and GC-skews in 14 nemertean species. Values were calculated for the coding strand of the overall mitogenome sequences (▲) and the major non-coding region (Cephalothrix sp. not included because the major non-coding region of this species is incomplete) (●). AT-skew = (A-T)/(A + T); GC-skew = (G-C)/(G + C). Af = Amphiporus formidabilis, Ch = Cephalothrix hongkongiensis, Csp = Cephalothrix sp., Eg = Emplectonema gracile, Ip = Iwatanemertes piperata, Gp = Gononemertes parasita, Lv = Lineus viridis, La = Lineus alborostratus, Nt = Nemertopsis tetraclitophila, Np = Nipponnemertes punctatula, Nm = Nectonemertes cf. mirabilis, Ps = Prosadenoporus spectaculum, Pp = Paranemertes cf. peregrina, Zr = Zygeupolia rubens.

Figure 3. Length comparisons of protein-coding genes (A) and ribosomal RNA genes (B) among 14 nemertean mitogenomes. Abbreviations of species names see Figure 2.

Figure 4. Phylogenetic trees resulting from maximum likelihood and Bayesian inference. A. Nucleotide sequences (3rd codon position removed)/amino acid sequences of 13 protein-coding genes (same tree topology obtained from the both datasets). B. Nucleotide sequences (3rd codon position removed) of protein-coding genes, rRNA and tRNA sequences. Numbers at the nodes correspond to posterior probabilities (left) and bootstrap proportions (right) (in tree A, the upper values are those of the nucleotide tree and the lower ones are those of the amino acid tree). Capital letters (A to K) in tree B correspond to the nodes for which Bremer support values were calculated (see Table 3).

Table 1.

Table 2.

Table 3.


Shi-Chun Sun. Institute of Evolution & Marine Biodiversity, Ocean University of China, 5 Yushan Road, Qingdao 266003, China .sunsc@ouc.edu.cn


Wen-Yan Sun,Dong-Li Xu,Hai-Xia Chen,Wei Shi,Per Sundberg,Malin Strand,Shi-Chun Sun. Complete mitochondrial genome sequences of two parasitic/commensal nemerteans, Gononemertes parasita and Nemertopsis tetraclitophila (Nemertea: Hoplonemertea). Particle and Fibre Toxicology ,Vol.7, Issue 1(2014) : .



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