Adaptive introgression and standing genetic variation, two facilitators of adaptation to high latitudes in European aspen (Populus tremulaL.)

Abstract

Understanding local adaptation in plants from a genomic perspective has become a key research area given the ongoing climate challenge and the concomitant requirement to conserve genetic resources. Perennial plants, such as forest trees, are good models to study local adaptation given their wide geographic distribution, largely outcrossing mating systems and demographic histories. We evaluated signatures of local adaptation in European aspen (Populus tremula) across Europe by means of whole genome re-sequencing of a collection of 411 individual trees. We dissected admixture patterns between aspen lineages and observed a strong genomic mosaicism in Scandinavian trees, evidencing different colonization trajectories into the peninsula from Russia, Central and Western Europe. As a consequence of the secondary contacts between populations after the last glacial maximum (LGM), we detected an adaptive introgression event in a genome region of ~500kb in chromosome 10, harboring a large-effect locus that has previously been shown to contribute to adaptation to the short growing seasons characteristic of northern Scandinavia. Demographic simulations and ancestry inference suggest an Eastern origin - probably Russian - of the adaptive Nordic allele which nowadays is present in a homozygous state at the north of Scandinavia. The strength of introgression and positive selection signatures in this region is a unique feature in the genome. Furthermore, we detected signals of balancing selection, shared across regional populations, that highlight the importance of standing variation as a primary source of alleles that facilitate local adaptation. Our results therefore emphasize the importance of migration-selection balance underlying the genetic architecture of key adaptive quantitative traits.

Local adaptation of a native herbivore to a lethal invasive plant.

“…For example, in spatially and temporally varying habitats, gene flow augments standing genetic variation and thus increases local adaptation (Blanquart et al, 2012(Blanquart et al, , 2013. Local adaptation can also occur in the presence of gene flow through adaptive introgression and strong selection against the immigrant alleles (Griffiths et al, 2021;Leroy et al, 2020;Rendón-Anaya et al, 2021).…”