The first mitochondrial genome of the relic Acanthobdella peledina (Annelida, Acanthobdellida).

In this study, we assembled a complete mitochondrial genome of the Acanthobdella peledina sample from the Pitea River, Sweden. Thirty-six genes of the mitogenome sequence were identified, including 13 protein-coding genes (PCGs), 2 ribosomal genes (12S and 16S), 21 transport RNA genes, and 1 control region. The complete mitogenome is 14,640 bp long and A + T biased (70.59%).

Acanthobdellida is a unique order of leech-like parasites characterized by the presence of the traits typical of both oligochaetes and leeches. This fact suggests that Acanthobdellida is an evolutionary link between the Oligochaeta and Hirudinea subclasses based on purely morphological features (Livanow 1905). Earliest molecular works on the phylogeny of acanthobdellids were based on short contaminant sequences starting from a pioneer work of Siddall and Burreson (1998), as was later acknowledged (Tessler et al. 2018). Although later discoveries have yielded similar results, there are still no long genomic sequences of A. peledina. However, a complete mitochondrial genome may serve as a foundation for more thorough evolutionary investigations. The study of genomic sequences of intermediate forms may shed light on nuances of molecular evolution mechanisms as well as conditions of relic species survival.

The samples of A. peledina were taken from grayling (Thymalus thymallus) in the upper part of the Pitea River, Sweden: 66°26′59.2”N 18°02′23.2”E. Biological material is deposited in Limnological Institute, Russia (voucher no. B28). The total DNA was extracted with the DiaGene Cell Culture kit. The reads were obtained through Illumina NextSeq 550. The complete mitogenome assembly was performed in MIRA5 (Chevreux et al. 1999) with an additional confirmation by MitoZ v2.4 (Meng et al. 2019).

The complete mitogenome of A. peledina is 14,640 bp long with the AT content of 70.59%. A total of 36 genes were identified on the mitogenome sequence, including 13 PCGs, 2 ribosomal genes (12S and 16S), 21 transport RNA genes, and 1 putative control region. The genes order is largely the same as for the closest GenBank relative, Placobdella parasitica (LT159850), except for double trnD between the cox2 and atp8 genes as well as trrR after the atp6 gene.

Phylogenetic position of the first obtained mitogenome sequence of A. peledina relative to 19 evolutionarily close taxa (Oligochaeta, Hirudinea and Polychaeta) available in the NCBI database was inferred using BEAST v2.6.0 (Bouckaert et al. 2019) under optimal substitution model (GTR + G + I) selected by jModelTest v2.1.10 (Darriba et al. 2012). The resulting tree (Figure 1) showed that all samples were grouped into distinct clades according to their taxonomy. The A. peledina sequence was positioned at the root of the Hirudinea clade, which supports the hypothesis about Acanthobdellida as an ancestral form for Hirudinea (true leeches).

Figure 1.

Bayesian phylogenetic tree with posterior probability values. The scale axis below represents time in the proportion of substitutions.