Genetic Erosion and Fine-Scale Genetic Structure in a Mediterranean Oak: Defining Genetically Informed Conservation Strategies for Quercus trojana in Southern Italy

Understanding patterns and processes affecting genetic diversity and structure of fragmented tree populations is essential for defining effective conservation strategies. Here, we investigate the genetic diversity of the Macedonian oak (Quercus trojana Webb 1839) across its entire Italian range by sampling 546 georeferenced trees from 30 forest stands. Using ten polymorphic nuclear microsatellite markers, we assessed within-population genetic variability and population structure, estimated effective population size (Ne), bottleneck signatures, and fine-scale relatedness among individual trees. Our results showed overall moderate genetic diversity, but marked variation emerged among populations and between two subregions. Populations in the North-Western Murge subregion exhibited lower allelic richness (Ar), critically small Ne (<50 in 12 populations), evidence of past bottlenecks, and higher pairwise differentiation, with three populations standing out as genetically distinct. Isolation by distance was significant only within the South-Eastern Murge subregion, suggesting contrasting demographic histories. Fine-scale relatedness analysis revealed higher within-site relatedness in North-Western Murge and a substantial proportion (30%) of related dyads across subregions, indicating large-scale connectivity due to natural or anthropogenic factors. Overall, our results reveal a complex scenario of historical and ongoing genetic erosion in some populations, particularly in the northern part of the range, alongside relatively high gene flow across the study area, especially in the south. We recommend prioritizing conservation actions for populations with critically low Ne and distinct gene pools, while maintaining genetic diversity hotspots and landscape connectivity.