Polygenic adaptation and negative selection across traits, years and environments in a long-lived plant species ( Pinus pinaster Ait.)

Results from a decade of association studies in different organisms suggest that most complex traits are polygenic, that is, their genetic architectures are determined by numerous causal loci distributed across the genome each with small effect-size. Thus, determining the degree of polygenicity is a central goal to understand the genetic basis of phenotypic variation. Recently, multi-loci methods able to detect variants associated with a phenotype of interest despite the subtle allele frequency changes between populations usually observed have been developed. In this study, we applied two multi-loci methods to estimate the degree of polygenicity of fitness-related traits in a long-lived plant species (maritime pine) and to analyze how polygenicity changes across years and environments. For this purpose, we evaluated five categories of fitness related traits, such as, height, survival, phenology-related, biotic-stress resistance and functional traits in a clonal common garden network planted in contrasted environments. Most of the analyzed traits showed evidences of local adaptation. We observed a remarkably stable degree of polygenicity (average 6%) across traits, environments and years. Additionally, some of the measured traits showed evidences of negative selection that could explain the observed degree of polygenicity, as previously suggested in humans. The observed genetic architecture of fitness-related traits in maritime pine is in accordance with the polygenic adaptation model. Because polygenic adaptation can take place rapidly, our results can contribute to improve the predictions about the capacity of natural populations of forest trees to adapt to new environments, which is of special relevance in the current context of climate change.