Description of a new species, Cryptocotyle lata sp. nov., and discussion of the phylogenetic relationships in Opisthorchioidea.

Adult Cryptocotyle lata sp. nov. worms were obtained from experimental studies. In the Russian southern Far East, the life cycle of this parasite is carried out using freshwater snails (Boreoelona ussuriensis), freshwater fish, and birds as the first intermediate, second intermediate, and definitive hosts, respectively. The morphological indices of C. lata sp. nov. are closest to Cryptocotyle concava; however, these two species differ in terms of their sizes of body, oral and ventral suckers, eggs, and the shape of their testes and ovaries. Analysis of the life cycles of the Cryptocotyle representatives suggested that C. concava were at least two cryptic species, one of which circulates using brackish water Hydrobia snails, and the other using freshwater Amnicola snails as the first intermediate hosts. Molecular data (i.e., the 28S gene and the second internal transcribed spacer (ITS2) of rDNA) were used to analyze the phylogenetic relationships of C. lata sp. nov. and other representatives of Opisthorchioidea. The long repeats and secondary structure of the ITS1 region were studied. Representatives of the Opisthorchiidae and several species from Heterophyidae (including the genus Cryptocotyle) were found to have molecular features that suggested that these species belonged to Opisthorchiidae. At the same time, the genetic relatedness of worms, which are united in common clusters on phylogenetic trees, is consistent with the use of the first intermediate hosts from different taxonomic groups in their life cycles; namely, snails of the Truncatelloidea are hosts of trematodes from a cluster with Opisthorchiidae and a number species of the family Heterophyidae, while snails of the Cerithioidea are hosts of worms from a cluster that includes only the Heterophyidae. In addition, the results of genetic studies indicate that Clonorchis sinensis, Metorchis ussuriensis, Metorchis bilis, Metorchis xanthosomus, and Metorchis orientalis should be included in the genus Opisthorchis.