Latitudinal trends in genetic diversity and distinctiveness of Quercus robur rear edge forest remnants call for new conservation priorities

Knowledge of the spatial distribution of intraspecific genetic variation
is essential for planning conservation actions, designing networks of
protected areas, and informing possible assisted migration strategies.
Although the Italian peninsula harbours unique genetic variation as a
legacy of Quaternary migrations, only sporadic genetic information is
available for forest tree species. Here, we present the first
geographically comprehensive genetic characterisation of Quercus
robur, an iconic broadleaved species of European floodplain forests, in an
area which acted as a primary glacial refugium for the species. 745
individuals from 25 populations were sampled and genotyped with 16
microsatellite markers. Their genetic structure was assessed through
various metrics of diversity and distinctiveness, as well as by Bayesian
clustering and multivariate methods. The demographic history of inferred
gene pools was evaluated through Approximate Bayesian Computation
analysis. Genetic distinctiveness showed a decline with increasing
latitude, while allelic richness reached its peak in central Italy. A
south-to-north trend in the complexity of the genetic structure was
observed, with peninsular Italy being characterised by intermingled gene
pools in contrast to the relative homogeneity exhibited by northern
populations. Demographic inference indicated that the southern gene pool
has been genetically isolated since the penultimate interglacial, whereas
populations from central Italy persisted locally in a mosaic of small
refugia during the last glaciation. Our results brought out the
complexity of the genetic structure of forest trees’ populations in
southern Europe. At least three Q. robur refugia contributed to the
genetic layout of extant populations after the last glacial period, but
refugial areas were probably even more numerous in central Italy. Such a
detailed characterization sheds new light on the priorities to be
established for the conservation of highly fragmented Q. robur populations
in an area rich in diversified genetic lineages.