Introgression as an Important Driver of Geographic Genetic Differentiation within European White Oaks

The genetic composition of 5797 white oaks assigned in forest inventories as Quercus robur (3342), Quercus petraea (2090), Quercus pubescens (170), or as unspecified Quercus. spp. (195) sampled all over Europe were genotyped at 355 nuclear SNPs and 28 maternally inherited SNPs of the chloroplast and mitochondria. The sampling had a focus on Central and Eastern Europe, as well as the Black Sea and Caucasus region. Using a sparse nonnegative matrix factorization (snmf) algorithm, the nuclear genetic information was best represented by K = 4 different genetic clusters, whereas a principal component analysis visualized three different groups. The snmf run with K = 3 corresponded, for most individuals with the assignment in the forest inventories, to the three different species. The majority of the samples (88%) had an admixture coefficient q > 0.8 for one of the three species clusters, underlining the species integrity with a minor level of admixture. In contrast to Q. petraea, Q. robur and Q. pubescens showed a clear geographic genetic substructure. These large-scale within-species genetic structures were correlated to regionally variable levels of introgression between the species. For Q. petraea, introgression from Q. robur and Q. pubescens was less focused to particular regions, and this widespread inter-specific gene flow reduced the geographic genetic differentiation. The genetic variation at the maternally inherited SNPs led to 12 different haplotypes with a clear cross-species geographic pattern, further supporting the observation of significant hybridization and introgression among the species.