Morphology, Molecular Characterization, and Phylogeny of Travassosius rufus Khalil, 1922 (Strongylidea: Trichostrongylidae), a Parasite from Endangered Sino-Mongolian Beaver (Castor fiber birulai) in Xinjiang, China

The genus <i>Travassosius</i> Khalil, 1922, the smallest genus in the subfamily Trichostrongylinae (family Trichostrongylidae), primarily infects the only two extant beaver species worldwide and can be lethal in severe infections. However, the mitochondrial genome evolution of <i>Travassosius</i> remains poorly understood, and its phylogenetic placement within Trichostrongylinae is still unresolved. In this study, we applied both morphological techniques (differential interference contrast microscopy) and molecular tools (nuclear ITS2 and mitochondrial genome) to examine <i>T. rufus</i> Khalil, 1922. Specimens were collected from the Sino-Mongolian beaver, a subspecies of the Eurasian beaver native to the Ulungur River Basin in northern Xinjiang, China. This work presents the first complete mitochondrial genome sequence and annotation of <i>T. rufus</i>, and it is also the first mitochondrial genome reported for the genus <i>Travassosius</i>. The mitochondrial genome of <i>T. rufus</i> measures 13,646 bp and contains 36 genes, including 12 protein-coding genes (PCGs) (excluding <i>atp</i>8), 22 transfer RNA genes, and 2 ribosomal RNA genes. Phylogenetic analysis based on amino acid sequences of 12 mitochondrial PCGs strongly supports the distinctiveness of the genus <i>Travassosius</i>. Additionally, <i>T. rufus</i> appears to be closely related to <i>Nematodirus</i> within Trichostrongylinae. This study also addresses the possible consequences of parasitic infection for the Sino-Mongolian beaver and offers a scientific foundation for conserving this endangered subspecies and managing parasitic diseases in its population.