Comparison of complete mitochondrial genome sequences in the Aporrectodea caliginosa species group (Annelida, Crassiclitellata, Lumbricidae)

We present for the first time the complete mitochondrial genomes (mt genomes) of the earthworms Aporrectodea caliginosa and Ap. trapezoides (Clitellata, Megadrili) collected in Hungary and Korea, respectively. The complete mt genome of Ap. trapezoides comprised 15,014 base pairs. Lengths of the three complete Ap. caliginosa mt genomes varied between 15,090 and 15,123 bp. All four mt genomes contained 13 protein-coding genes (PCGs), two rRNA genes, 22 tRNA genes, and one major non-coding control region. These mt genome arrangements are identical to those observed in the mt genomes of most earthworms, and all the 37 genes are transcribed from the same directional strand. All 13 PCGs had the same ATG start codon. Most of the PCGs end with TAA or TAG, whereas the remaining end with an incomplete stop codon, T. Stop codons were consistent in the PCGs throughout the mt genomes, except Ap. caliginosa 5, which contains a TAG stop codon in ND5 instead of the TAA found in the other samples. Both species' genomes showed biased base composition, with 63.5% AT and 36.4% GC content in Ap. trapezoides and 62.8% and 37.2% in Ap. caliginosa. Phylogenetic analysis of the mt genomes corroborated the monophyly of the family Lumbricidae and the close relationship between Ap. trapezoides and Ap. caliginosa species pairs. The available Ap. tuberculata sequences were embedded between the Ap. caliginosa samples, thereby supporting the synonymy of the two names.