The polygenic basis of adaptation in Arabidopsis lyrata

The global environment changes fast, posing novel challenges for many plant species, which must adapt to these changes. Adaptation requires phenotypic variation that affects fitness, much of which is controlled by polygenic backgrounds. The aim of the thesis is to investigate the polygenic basis of adaptation, using as study model the outcrossing and perennial plant species Arabidopsis lyrata ssp petraea. I focused on two populations originating from the core and edge of its distribution in Europe. During range expansion, edge populations are expected to face increased genetic drift, which in turn can compromise adaptive dynamics. I documented a sharp decline in effective population size in the range-edge population and observed that non-synonymous variants segregate at higher frequencies, suggesting an increase of the genomic burden of deleterious mutations in the range-edge population. However, selective sweeps in this population were not less frequent and thus maintained its ability to adapt. Adaptation at the level of transcript regulation is believed to make a significant contribution to complex trait adaptation. The global gene expression, which we show to have polygenic basis, revealed substantial additive variance that is directly inherited to the next generation and thus contributes to adaptation. Yet, dominance variance, which results from allelic interactions within or between loci, forms the greatest part of the genetic variance in gene expression. Dominance variance is related to gene structural properties, as well as to the degree of gene co-regulation. Additionally, I identified that transcripts with less genetic variance exhibit stronger constraints at the amino-acid level, indicating that purifying selection acts at both amino-acid and transcript levels. By contrast, transcripts with highest additive variance tended to evolve under relaxed selection. Altogether, these studies show that despite a dramatic bottleneck and a mild expansion load, adaptive dynamics are maintained at the range-edge of the species A. lyrata ssp. petraea. However, although we find evidence that gene expression variation contributes to the evolutionary potential of these populations, we also observe that a significant fraction of genetic variation is not available for selection