Genetic Diversity and Phylogenetic Relationships of Castor fiber birulai in Xinjiang, China, Revealed by Mitochondrial Cytb and D-loop Sequence Analyses

<i>Castor fiber birulai</i> is a subspecies of the Eurasian beaver that has a relatively small population size compared to other <i>Castor</i> subspecies. There is limited genetic research on this subspecies. In this study, mitochondrial cytochrome b (<i>Cytb</i>) and D-loop sequences were analysed in genetic samples obtained from 19 individuals residing in the Buergen River Basin, Xinjiang, China. The <i>Cytb</i> region presented a single haplotype, whereas three haplotypes were identified in the D-loop region. The genetic diversity within the Chinese population was low (D-loop Hd = 0.444; Pi = 0.0043), markedly lower than that observed in other geographical populations of <i>C. fiber</i>. Phylogenetic reconstructions and haplotype network analyses revealed substantial genetic differentiation between <i>C. f. birulai</i> and other Eurasian lineages (Fst > 0.95), supporting the status of <i>C. f. birulai</i> as a distinct evolutionary lineage. Although the genetic distance between the Chinese and Mongolian populations was relatively small (distance = 0.00269), significant genetic differentiation was detected (Fst = 0.67055), indicating that anthropogenic disturbances-such as hydraulic infrastructure and fencing along the cross-border Bulgan River-may have impeded gene flow and dispersal. Demographic analyses provided no evidence of recent population expansion (Fu's Fs = 0.19152), suggesting a demographically stable population. In subsequent studies, we recommend increasing nuclear gene data to verify whether the <i>C. f. birulai</i> population meets the criteria for Evolutionarily Significant Unit classification, and strengthening cross-border protection and cooperation between China and Mongolia.