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

Results from a decade of associationstudies in different organisms suggestthat most complex28traitsare polygenic, that is,theirgenetic architecturesaredetermined by numerous causal loci 29distributed across the genomeeach with small effect-size. Thus, determining the degree of 30polygenicity isa central goalto understand the genetic basis of phenotypic variation. 31Recently, multi-loci methods able to detect variants associated with a phenotype of interest 32despite thesubtle allele frequency changes between populations usually observedhave been33developed. In this study, we applied two multi-locimethods to estimatethe degree of 34polygenicity of fitness-related traits in a long-lived plant species(maritime pine) and to 35analyze how polygenicitychangesacross years and environments. For this purpose, we 36evaluated five categories of fitness related traits, such as, height, survival, phenology-related, 37biotic-stress resistanceand functional traits in a clonal common garden network planted in 38contrasted environments.Most of the analyzed traits showed evidences of local adaptation.39We observed a remarkably stable degree of polygenicity(average6%) across traits, 40environments and years. Additionally, some of the measuredtraits showed evidences of 41negativeselectionthat could explainthe observed degree of polygenicity, as previously 42suggested in humans. The observed genetic architecture of fitness-related traits in maritime 43pine isin accordance with the polygenic adaptation model. Because polygenic adaptation can 44take place rapidly, our results can contribute to improve the predictionsaboutthe capacity of 45natural populations of forest trees to adapt to new environments, which is of special relevance 46in the current context of climatechange.