The latitudinal trend in genetic diversity and distinctiveness of Quercus robur rear edge forest remnants calls for new conservation priorities

Knowledge of the spatial distribution of intraspecific genetic variation is essential for conserving biodiversity, designing rational networks of protected areas, and informing translocation strategies. Although the Italian peninsula likely harbours unique genetic variation as a legacy of Quaternary migrations towards southern Europe, few genetic data are available for many Italian forest tree species and populations. Here, we present the first, comprehensive characterisation of the nuclear genetic variation of Italian populations of pedunculate oak (Quercus robur L.), an iconic broadleaved species of lowland forests. A total of 745 individuals from 25 populations were sampled across the local range of the species and genotyped with 16 microsatellite markers. The genetic layout of Q. robur populations was assessed through various metrics of diversity and distinctiveness, as well as by Bayesian clustering and multivariate methods. Based on the inferred genetic structure, the demographic history of Q. robur gene pools was evaluated through Approximate Bayesian Computation (ABC) analysis. We found a clear spatial trend in the estimates of genetic diversity and distinctiveness of Italian Q. robur populations. The genetic distinctiveness and private allelic richness values showed a linear decline with increasing latitude, while allelic richness reached its peak in central Italy. We also observed a south-to-north trend in the complexity of genetic structure, with peninsular Italy being characterised by a mosaic of gene pools, in contrast to the homogeneity exhibited by northern Italian populations. Demographic inference indicated that the southern gene pool has had separate dynamics for the last 135,000 years and, therefore, populations in central Italy did not originate from a northward migration route after the Last Glacial Maximum (LGM). Therefore, they were not only a "melting pot" of multiple recolonization routes, but most likely a mosaic of small refugia where pedunculate oak persisted during the LGM. Our results shed new light on conservation priorities for highly fragmented Q. robur populations, and call for consideration of whether treating the southern and central Italian populations as independent conservation units, or intervening to increase their genetic connectivity, is the best strategy for their long-term conservation. ### Competing Interest Statement The authors have declared no competing interest.