Abstract
As for most other temperate broadleaved tree species, large-scale genetic inventories of pedunculate oak (Quercus robur L.) have focused on the plastidial genome, which showed the impact of post-glacial recolonization and manmade seed transfer. However, how have pollen mediated gene flow and introgression impacted the large-scale genetic structure? To answer these questions, we did a genetic inventory on 1970 pedunculate oak trees from 197 locations in 13 European countries. All samples were screened with a targeted sequencing approach on a set of 381 polymorphic loci (356 nuclear SNPs, 3 nuclear InDels, 17 chloroplast SNPs, and 5 mitochondrial SNPs). In a former analysis with additional 1763 putative Quercus petraea trees screened for the same gene markers we obtained estimates on the species admixture of all pedunculate oak trees. We identified 13 plastidial haplotypes, which showed a strong spatial pattern with a highly significant autocorrelation up to a range of 1250 km. Significant spatial genetic structure up to 1250 km was also observed at the nuclear loci. However, the differentiation at the nuclear gene markers was much lower compared to the organelle gene markers. The matrix of genetic distances among locations was partially correlated between nuclear and organelle genomes. Bayesian clustering analysis revealed the best fit to the data for a sub-division into two gene pools. One gene pool is dominating the west and the other is the most abundant in the east. The western gene pool was significantly influenced by introgression from Quercus petraea in the past. In Germany, we identified a contact zone of pedunculate oaks with different introgression intensity, likely resulting from different historical levels of introgression in glacial refugia or during postglacial recolonization. The main directions of postglacial recolonization were south to north and south to northwest in West and Central Europe, and for the eastern haplotypes also east to west in Central Europe. By contrast, the pollen mediated gene flow and introgression from Q. petraea modified the large-scale structure at the nuclear gene markers with significant west–east direction.
Highlights
IntroductionThe genetic differentiation of the maternally inherited chloroplast genome of European white oak species has been intensively studied for more than 20 years [1,2,3]
We observed 13 different haplotypes in all 1970 individuals based on the 17 chloroplast SNPs (cp-SNPs) and the five mitochondrial SNPs (mt-SNPs) with a clear spatial pattern (Figure 2a)
The yellow haplotype H07 is dominating the east of Russia and the green haplotype H06 was most frequent in the central and western parts of the species distribution range in Russia including a part of Ukraine
Summary
The genetic differentiation of the maternally inherited chloroplast genome of European white oak species has been intensively studied for more than 20 years [1,2,3]. The screening of chloroplast haplotype variation of sessile (Quercus petraea) and pedunculate oaks (Quercus robur) at the continental level has revealed a clear large-scale structure, which most probably resulted from postglacial recolonization [4]. During the last glacial maximum, pedunculate oaks survived in refugia in southern and south-eastern parts of Europe. The oaks were fixed to different haplotypes in these refugia, and the directions of recolonization can be reconstructed using the large-scale genetic structure of the haplotypes [4]. One lineage from the Iberian Peninsula has colonized many parts of western Europe, a lineage from Italy has migrated north to many parts of Central
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