Comparative mitogenomics of the zoonotic parasite 'Echinostoma revolutum' resolves taxonomic relationships within the 'E. revolutum' species group and the Echinostomata (Platyhelminthes: Digenea)

The complete mitochondrial sequence of 17,030 bp was obtained from Echinostoma revolutum and characterized with those of previously reported members of the superfamily Echinostomatoidea, ie., 6 echinostomatids, one echinochasmid, 5 fasciolids, one himasthlid and two cyclocoelids. Relationship within suborders and between superfamilies, such as Echinostomata, Pronocephalata, Troglotremata, Opisthorchiata, and Xiphiditata, are also considered. It contained 12 protein-coding, two ribosomal RNA, 22 transfer RNA genes, and a tandem repetitive consisting non-coding region (NCR). The gene order, one way-positive transcription, the absence of atp8 and the overlapped region by 40 bp between nad4L and nad4 genes were similar as in common trematodes. The NCR located between tRNAGlu (trnE) and cox3, contained 11, long (LRUs) and short repeat units (SRUs) (7 LRUs of 317 bp, 4 SRUs of 207 bp each), and an internal spacer sequence between LRU7 and SRU4) specifying high level-polymorphism. Special DHU-arm missing tRNAs for Serine were found for both, tRNAS1(AGN) and tRNAS2(UCN). E. revolutum indicated the lowest divergence rate to E. miyagawai and the highest to Tracheophilus cymbius and Echinochasmus japonicus.The usage of ATG/GTG start and TAG/TAA stop codons, the AT composition bias, the negative AT-skewness, and the most for Phe/Leu/Val and the least for Arg/Asn/Asp codons were noted. Topology indicated the monophyletic position of E. revolutum to E. miyagawai. Monophyly of Echinostomatidae and Fasciolidae was clearly solved with respect to Echinochasmidae, Himasthlidae and Cyclocoelidae which were rendered paraphyletic in the suborder Echinostomata