FOREST TREE ECOLOGICAL GENETICS: INTERPLAY OF GENE FLOW AND ENVIRONMENTAL VARIABILITY IN SHAPING LOCAL ADAPTATION AND GENETIC ADAPTIVE POTENTIAL – FLAG

«There will be two experimental phases, addressing questions in population genomics and quantitative genetics, and a modelling phase, that will model the processes underlying the maintenance of genetic diversity and to predict future changes.
Genetic markers will beused to genotype natural populations of each species, growing in sites displaying a gradient of environmental conditions. This will lead to the estimate of the amount of loci under selection along the gradient as well as to the identification of loci and polymorphisms related to stress response / adaptation to the environment.
Classical quantitative genetic tools will be applied to in-situ progeny tests (reciprocal transplant experiment and replicated provenance tests) to validate SNP-trait-environment associations. The intensity of migratory and adaptive processes will be modelled thanks to advanced modelling strategies and will allow us to provide predictions of the response of forests to climate changes. «

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Environmental gradients and patchiness shape the level and the distribution of genetic diversity of adaptive significance. A rich literature describes how gene frequencies vary along gradients and deals with the interaction of local selection and global gene flow. The implicit assumption is that populations evolving according to divergent selective pressures have to be sufficiently isolated for long term selective divergence to take place. Genetic pools differentiating along gradients and between patches store large amounts of genetic variability and thus favour the maintenance of a species’ adaptive potential. Measuring and modelling the amount of divergence among populations under divergent ecological conditions is therefore central to the prediction of how populations may respond to future local and global environmental changes.
The turnover of genetic variants over gradients and habitat patches has traditionally been studied on geographical scales between regional and global. However, it can be argued that, for long-lived organisms, the large amounts of observed within-population genetic diversity may be maintained at least partly by local selection. In some instances, environmental conditions vary on a spatial scale that is comparable to, or even shorter than, average gene flow distances, thus setting the conditions for the interplay between selection and dispersal in the generation of observed patterns of diversity. Under these conditions, local differentiation among sub-populations for adaptive traits and genes may be maintained in spite of, and in some cases thanks to, ongoing gene flow. A combination of landscape and ecological genetic approaches makes it possible to investigate these phenomena, which have been shown to occur only for a limited set of plant species.
Strategies to infer and model the strength of selection at target genes require prior knowledge of the loci under selection; however, current sequencing technology, combined with intensive field sampling of natural populations, can lead to the identification of selectively significant loci by population genomic approaches, even for non-model species. Providing genotypes for thousands of Single Nucleotide Polymorphisms (SNP) for hundreds of individuals has currently become feasible and relatively cheap. Quantitative genetics tools can now be successfully used to validate the association between SNP frequencies, phenotypic values, and environmental gradients. The time is therefore ripe for starting to address these major ecological-genetic questions directly in ecologically relevant species.
The present project aims at investigating the complex genome-wide effects of local adaptation in nine keystone tree species from four major terrestrial ecosystems. Here we address the question of whether and how genetic diversity in forest tree stands, occurring across environmental gradients, is spatially structured by selective forces, and we propose to estimate the proportion of the genome undergoing such processes as well as to identify genes under selection. We propose an original use of classical quantitative genetic tools applied to in-situ progeny tests (reciprocal transplant experiment and replicated provenance tests) to validate SNP-trait-environment associations. The intensity of migratory and adaptive processes will be modelled thanks to advanced modelling strategies and will allow us to provide predictions of the response of forests to climate changes. We focus here on water availability gradients in particularly sensitive forest areas, such as the Guiana shield, the Mediterranean basin, Sub-Saharan Africa and the Brazilian Cerrado; all these regions suffer broad seasonal changes in soil water availability and are expected to undergo abrupt rainfall changes in the near future. Therefore, studying how forest tree populations cope with ecological gradients is at the fore among tools to predict the impact of expected future environmental changes.