Genetic consequences of contrasting restoration strategies: bottleneck effects versus lineage admixture in Eurasian beavers (Castor fiber) in Western Europe

The recovery of the Eurasian beaver ( Castor fiber ) in Western Europe provides a prime model to study the genetic consequences of contrasting restoration paradigms. We conducted a large-scale genetic assessment across France, Belgium, and Luxembourg using mitochondrial DNA and 14 microsatellites to evaluate population structure and diversity. Our results identify a profound genetic dichotomy: a geographically expansive but genetically homogenous "French core" versus a diverse "northeastern contact zone." The French population, predominantly of the relict galliae lineage, exhibits significantly reduced allelic richness and heterozygosity, likely reflecting a severe historical bottleneck and subsequent founder effects. In contrast, northeastern populations show extensive admixture between multiple lineages ( albicus , fiber , galliae , lineage A), resulting in enhanced regional diversity. We also confirmed the absence of the invasive Castor canadensis , highlighting the efficacy of current transboundary biosecurity. High inbreeding risks in the French core support the hypothesis that long-term isolation may constrain adaptive potential. We argue that maintaining rigorous monitoring and fostering natural connectivity are critical to facilitate "genetic rescue" through admixture. These findings emphasize the importance of managing functional connectivity to ensure the evolutionary resilience of restored keystone species in fragmented landscapes.