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

## TL;DR

This study reveals that the Castor fiber birulai beaver in China has low genetic diversity and is genetically distinct from other beaver populations, suggesting it needs special conservation efforts.

## Contribution

The study provides the first detailed genetic analysis of the rare Castor fiber birulai subspecies using mitochondrial DNA markers.

## Key findings

- The Chinese Castor fiber birulai population shows low genetic diversity and distinct genetic differentiation from other Eurasian populations.
- Human activities like dam construction and fencing have likely disrupted gene flow between Chinese and Mongolian beaver populations.
- The subspecies may represent a distinct conservation unit requiring cross-border cooperation for protection.

## Abstract

Castor fiber birulai is one of the rarest lineages of the Eurasian beaver and is found only in the Buergen River Basin of Xinjiang, China. Compared with beaver populations in Europe and Russia, its numbers are small, and it has been relatively understudied. To better understand the genetic status of this endangered population, we analyzed genetic samples from 19 individuals. Our results showed that the genetic diversity of the beavers in China is low, with close relationships between individuals. Compared to other Eurasian populations, they exhibited clear genetic differences, suggesting that this group may represent a distinct conservation unit. Although geographically close to beavers in Mongolia, gene flow between the two populations appears to have been disrupted by human activities such as dam construction and border fencing. Based on these findings, we recommend strengthening cross-border cooperation between China and Mongolia to protect the habitat of this rare species. We also suggest including nuclear DNA in future studies to fully assess its conservation value. This research provides important scientific support for ecological protection and biodiversity conservation in border regions.

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 Buergen 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 > 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—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 C. f. birulai population meets the criteria for Evolutionarily Significant Unit classification, and strengthening cross-border protection and cooperation between China and Mongolia.