# 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

## TL;DR

This study examines a parasite infecting the endangered Sino-Mongolian beaver, revealing its unique genetic traits and evolutionary history.

## Contribution

The first complete mitochondrial genome sequence of the genus Travassosius is reported, providing new insights into its phylogeny.

## Key findings

- The mitochondrial genome of T. rufus is 13,646 bp and contains 36 genes.
- Phylogenetic analysis shows T. rufus is closely related to Nematodirus within Trichostrongylinae.
- Significant genetic divergence among T. rufus populations is likely due to geographic isolation.

## Abstract

The endangered Sino-Mongolian beaver (Castor fiber birulai), a key species for sustaining river ecosystems in China and Mongolia, is threatened by parasitic nematodes. This study focuses on Travassosius rufus, a gastric parasite in these beavers, aiming to understand its characteristics and evolutionary background. For the first time, we sequenced the parasite’s mitochondrial genome and compared samples from China with those from Norway and the Czech Republic. Our findings revealed significant genetic divergence among populations, likely due to prolonged geographic isolation. We also found that while T. rufus shares ancestry with other parasitic worms, it shows features that are uniquely adapted to its beaver hosts. These results contribute to our understanding of how parasites and hosts evolve together.

The genus Travassosius 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 Travassosius 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 T. rufus 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 T. rufus, and it is also the first mitochondrial genome reported for the genus Travassosius. The mitochondrial genome of T. rufus measures 13,646 bp and contains 36 genes, including 12 protein-coding genes (PCGs) (excluding atp8), 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 Travassosius. Additionally, T. rufus appears to be closely related to Nematodirus 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.