Genomic differentiation of British oaks

Abstract

The morphological continuum between Britains two native oak species helped Darwin formulate his "one long argument" in The Origin of Species. Here we comprehensively analyse the genome-wide differentiation of 418 individuals from 60 British oak populations. Using 7 million biallelic SNPs, we find evidence for two species with islands of variation containing 8,230 SNPs of which 2,005 are within gene annotations. We find extensive hybridisation and back-crossing between the species, and this is biased in favour of introgression from Quercus robur into Q. petraea. Chloroplasts are shared between the species, mainly of haplotypes derived from Iberian glacial refugia. Genomic allocation of species and hybrids allows analysis of niche differentiation, showing that a larger Q. robur component is disproportionately found in warmer and more thermally variable environments with alkaline soils, while a larger Q. petraea component is found in more topographically complex terrain with high rainfall. This leads us to emphasise the balance between the opposing forces of hybridisation and niche differentiation in explaining the continuum between the two species. Using long-term field data, we show that stem growth over three decades was higher in Q. robur than in Q. petraea or hybrids, but this difference could be explained by environmental factors. In contrast, the five triploid trees in our sample grew significantly faster than diploids even after accounting for environmental effects, suggesting that triploids could be a valuable economic resource.