Complete mitochondrial genome sequences of two parasitic/commensal nemerteans, Gononemertes parasita and Nemertopsis tetraclitophila (Nemertea: Hoplonemertea).

Wen-Yan Sun,Per Sundberg,Wei Shi,Dong-Li Xu,Shi-Chun Sun,Malin Strand,Hai-Xia Chen

doi:10.1186/1756-3305-7-273

Abstract

BackgroundMost 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.MethodsThe 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.ResultsComplete 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.ConclusionGene 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.

Highlights

IntroductionMost nemerteans (phylum Nemertea) are free-living, but about 50 species are known to be firmly associated with other marine invertebrates
Most nemerteans are free-living, but about 50 species are known to be firmly associated with other marine invertebrates
The A + T content of the coding strands in G. parasita and N. tetraclitophila is 68.8% and 71.2% respectively, which falls within the range of the previously sequenced nemertean mitogenomes (Table 2)

Summary

Introduction

Most nemerteans (phylum Nemertea) are free-living, but about 50 species are known to be firmly associated with other marine invertebrates. We determined the first complete mitogenome sequences of two parasitic/commensal nemerteans, Gononemertes parasita Bergendal, 1900 and Nemertopsis tetraclitophila Gibson, 1990, which taxonomically belong to Monostilifera (a group that contains most known symbiotic nemerteans). G. parasita lives in the branchial chamber of some ascidians in European waters [10], whereas N. tetraclitophila has been recorded from the mantle cavity of the balanomorph barnacle Tetraclita squamosa (Bruguiére, 1789) in Hong Kong, China [11] Worms of both species seem to be firmly associated with a host, and possess some adaptive features that might be related to none free-living lifestyle, e.g., the greater number of gonads than most freeliving monostiliferans; the absence of a proboscis apparatus (G. parasita) [11,12]. The ecology, the feeding biology, of G. parasita and N. tetraclitophila has not been well understood

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