The mitochondrial genomes of two nemerteans, Cephalothrix sp. (Nemertea: Palaeonemertea) and Paranemertes cf. peregrina (Nemertea: Hoplonemertea).

The mitochondrial genome sequences were determined for two species of nemerteans, Cephalothrix sp. (15,800 bp sequenced, near-complete) and Paranemertes cf. peregrina (14,558 bp, complete). As seen in most metazoans, the genomes encode 13 protein, 2 ribosomal RNA and 22 transfer RNA genes. The nucleotide composition is strongly biased toward A and T, as is typical for metazoan mtDNAs. There is also a significant strand skew in the distribution of these nucleotides, with the coding strand being richer in T than A and in G than C. All genes are transcribed in the same direction except for trnP and trnT, which is consistent with that reported for Cephalothrix hongkongiensis and Lineus viridis. Gene arrangement of Cephalothrix sp. is identical to that of C. hongkongiensis, while in P. cf. peregrina it is similar to L. viridis, but differs significantly from the three Cephalothrix species in the position of four protein-coding genes and seven tRNAs. Some protein-coding genes have 3' end stem-loop structures, which may allow mRNA processing without flanking tRNAs. The major non-coding regions observed in the two genomes with potential to form stem-loop structures may be involved in the initiation of replication or transcription. The average Ka/Ks values, varying from 0.12 to 0.89, are markedly different among the 13 mitochondrial protein-coding genes, suggesting that there may exist different selective pressure among mitochondrial genes of nemerteans.