Structure and characterization of the ribosomal transcription units of small liver flukes, Opisthorchis viverrini, O. Felineus and Clonorchis sinensis

Opisthorchiasis is a zoonotic parasitic infection caused by small liver fluke species, Opisthorchis viverrini,O. felineus and Clonorchis sinensis, in the family Opisthorchiidae. Vietnam has both species, of which C.sinensis is distributed in the northern and O. viverrini in the central provinces. In addition to the mitochondrialgenomes, the ribosomal DNA sequences (rDNA) of these species are highly needed to obtain for providingmolecular markers in species identification, classification, phylogeny and evolutionary studies. In this study,the near/complete nucleotide sequences of ribosomal transcription units (rTU) from O. viverrini (Vietnamesesample), O. felineus (Russian sample) and C. sinensis (Vietnamese sample) were analyzed. All rTUs for threespecies were determined, which is 7,839 bp for O. viverrini, 6,948 bp for O. felineus and 7,296 bp for C.sinensis containing structures of 18S, ITS1, 5,8S, ITS2 and 28S. The IGS region was not obtained for all threespecies. In all three species, sequence analysis revealed 2 tandem repetitive elements of 47-48 bp/each in ITS1but not in ITS2. The nucleotide sequences of 18S, ITS1, ITS2 and 28S are valuable ribosomal markers that thisstudy provides for diagnosis, identification, taxonomic classification and population genetics. In conclusion,the rTU sequences for the three species of the family Opisthorchiidae have been identified and providesmolecular markers for the use of phylogenetic analysis for species/family classification in the superfamilyOpisthorchioidea and the class Trematoda.