Connectivity, genetic diversity and adaptive potential of an endangered beetle: genomic evidence for veteran tree management

Habitat fragmentation and destruction in old-growth forests represents a primary threat to specialist species, frequently resulting in the erosion of genetic diversity. The Hermit Beetle, Osmoderma barnabita, is a protected relic of primeval forests and an obligate saproxylic specialist whose larval development depends exclusively on the wood-mould of hollow trees. This study provides the first high-resolution, genome-wide assessment of O. barnabita across seven Polish forest complexes using double-digestion Restriction-site Associated DNA sequencing data from 49 individuals. Following rigorous quality control and reference bias assessment, we evaluate contemporary population structure. To distinguish between evolutionary processes, we isolated 2,920 putatively adaptive outliers from the neutral background, evaluating both neutral and adaptive genomic signals. The analyses revealed four distinct genetic clusters, confirming that the Polish metapopulation is severely subdivided. Crucially, the westernmost population in the Lower Silesian Forest emerged as the most divergent evolutionary unit (FST up to 0.360), suggesting potential hybrid origin or long-term isolation. We observed a striking linear correlation (R2=0.986) between neutral diversity and adaptive potential, indicating that genetic drift in small and isolated forest fragments is actively depleting the variation required for long-term persistence of this species. Our findings demonstrate significant gene flow restriction across all sites, suggesting that current conservation frameworks fail to encompass these highly differentiated lineages. To mitigate this genetic vulnerability, we recommend reclassifying these clusters as independent Management Units and Evolutionary Significant Units. Maintaining forest continuity and protecting local micro-habitats are essential to prevent further isolation and preserve the genomic integrity of this saproxylic flagship species.