Analysis of the phylogenetic position of Pseudexorchis Yamaguti, 1938 (Digenea: Opisthorchioidea) suggests new complexity in the relationships of the Cryptogonimidae

Abstract

Pseudexorchis Yamaguti, 1938 is a genus of opisthorchioid digeneans, which is currently classified as a member of the Heterophyidae. Only one species is recognised in this genus, Pseudexorchis major (Hasegawa, 1935), which parasitises freshwater silurid fishes as adults. The aim of this study is to evaluate the family affiliation of Pseudexorchis based on phylogenetic data. We obtained the sequences of several genetic markers from a single adult specimen of P. major ex Silurus asotus Linnaeus, 1758 from Japan, which allowed us to identify it as a member of a phylogenetic lineage previously named by Miura and Takisawa (2024) as Type B. The lineage Type B appeared as a sister to P. major Type A of Miura and Takisawa (2024). The degree of genetic differentiation between Type B and Type A, mainly based on the cox1 gene partial sequence, allows us to hypothesise the species status of these lineages. The trees based on 28S rRNA, 28S rRNA + ITS2 , and ITS1–5.8S–ITS2 datasets were most informative for phylogenetic inferences. In our analyses, Pseudexorchis was clustered with cryptogonimids Acanthostomum yahuarcaquense Cajiao-Mora and Bullard in Cajiao-Mora, Brule, Dutton, Caicedo-Portilla, and Bullard, 2025, Neocladocystis spp., Tanganyikatrema spp., and the nonugens KC489791 and KC489792 ( 28S rRNA-tree ), or only A. yahuarcaquense (28S rRNA + ITS2-tree), or Acanthostomum cf. americanum (Pérez-Vigueras, 1956) (ITS1–5.8S–ITS2-tree); the clade of these species was closer to the clade formed by opisthorchids and heterophyids than to other cryptogonimids. Thus, the presented data do not support the hypothesis that Pseudexorchis belongs to the Heterophyidae and demonstrate paraphyly of the Cryptogonimidae.