Adaptive introgression as a driver of local adaptation to climate in European white oaks

TLDR

Latitudinal and elevational gradients, combined with long‑term phenological data from common garden experiments on sessile oak, provide a natural laboratory to study how global warming may affect forest tree species. The study aims to investigate the evolutionary processes shaping genetic variation across these gradients by sequencing 18 sessile oak populations and integrating data from three related European white oak species. The authors sequenced 18 populations, incorporated data from the related species, and performed genome‑wide scans of differentiation and genotype‑environment/phenotype associations using common garden phenotypes and climatic data. The analyses reveal that interspecific gene flow from *Q.

Abstract

Latitudinal and elevational gradients provide valuable experimental settings for studies of the potential impact of global warming on forest tree species. The availability of long-term phenological surveys in common garden experiments for traits associated with climate, such as bud flushing for sessile oaks (Quercus petraea), provide an ideal opportunity to investigate this impact. We sequenced 18 sessile oak populations and used available sequencing data for three other closely related European white oak species (Quercus pyrenaica, Quercus pubescens, and Quercus robur) to explore the evolutionary processes responsible for shaping the genetic variation across latitudinal and elevational gradients in extant sessile oaks. We used phenotypic surveys in common garden experiments and climatic data for the population of origin to perform genome-wide scans for population differentiation and genotype-environment and genotype-phenotype associations. The inferred historical relationships between Q. petraea populations suggest that interspecific gene flow occurred between Q. robur and Q. petraea populations from cooler or wetter areas. A genome-wide scan of differentiation between Q. petraea populations identified single nucleotide polymorphisms (SNPs) displaying strong interspecific relative divergence between these two species. These SNPs followed genetic clines along climatic or phenotypic gradients, providing further support for the likely contribution of introgression to the adaptive divergence of Q. petraea populations. Overall, the results indicate that outliers and associated SNPs are Q. robur ancestry-informative. We discuss the results of this study in the framework of the postglacial colonization scenario, in which introgression and diversifying selection have been proposed as essential drivers of Q. petraea microevolution.