Genetic Diversity and Phylogenetic Relationships of <em>Castor fiber birulai</em> in Xinjiang, China, Revealed by Mitochondrial <em>Cytb </em>and D-Loop Sequence Analyses

Castor fiber birulai is a subspecies of the Eurasian beaver that has a relatively small population size compared to other Castor subspecies. There is limited genetic research on this subspecies. In this study, mitochondrial cytochrome b (Cytb) and D-loop sequences were analysed in genetic samples obtained from 19 individuals residing in the Ulungur River Basin, Xinjiang, China. The Cytb 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 C. fiber. Phylogenetic reconstructions and haplotype network analyses revealed substantial genetic differentiation between C.f. birulai and other Eurasian lineages (Fst &gt; 0.95), supporting the status of C.f. birulai 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&mdash;such as hydraulic infrastructure and fencing along Cross-border Bulgan River&mdash;may have impeded gene flow and dispersal. Demographic analyses provided no evidence of recent population expansion (Fu&rsquo;s Fs = 0.19152), suggesting a demographically stable population. These findings underscore the importance of the conservation of C.f. birulai as an Evolutionarily Significant Unit and provide a genetic basis for informed Cross-border conservation and management strategies.