Mitochondrial and nuclear ribosomal DNA dataset suggests that Hepatiarius sudarikovi Feizullaev, 1961 is a member of the genus Opisthorchis Blanchard, 1895 (Digenea: Opisthorchiidae).

The taxonomy and classification of the family Opisthorchiidae have been revised by several authors with the exclusion or synonymization of some genera. The genus Hepatiarius Feizullaev, 1961 accommodated two species: Hepatiarius sudarikovi Feizullaev, 1961 and H. longissimus Linstow, 1883. Recently, some experts have suppressed Hepatiarius as a junior synonym of Opisthorchis Blanchard, 1895 based on morphological features alone. Prior to the present study, no molecular data either from nuclear or from mitochondrial DNA was available for any species of this genus. In the present study, four specimens of H. sudarikovi Feizullaev, 1961 were recovered from the bile ducts of the little egret, Egretta garzetta. The complete sequences of the internal transcribed spacers (ITS-1 and ITS-2) of ribosomal DNA (rDNA) and the nearly complete mitochondrial genome sequences were determined and the phylogenetic relationship of H. sudarikovi with related taxa was assessed based on the mitochondrial (mt) DNA sequences. The sequence similarity in the ITS rDNA between H. sudarikovi and Opisthorchis felineus was higher (97.62% in ITS-1 and 96.22% in ITS-2) than with other opisthorchiids. Phylogenetic analysis using Bayesian inference (BI) based on the concatenated amino acid sequences of 12 protein-coding genes (PCGs) clustered H. sudarikovi into the clade of opisthorchiids, with O. felineus being the closest related species, which supports the affinity of H. sudarikovi with trematodes in the genus Opisthorchis. This is the first avian liver fluke whose nearly complete mitochondrial genome was sequenced. The mtDNA sequences of H. sudarikovi, in combination with its rDNA sequences, provide novel resources of genetic markers for the identification, species differentiation, and systematic studies of H. sudarikovi with other avian opisthorchiid flukes.