Whole genome resequencing of 51 Populus nigra (L.) individuals from across Western Europe was performed using Illumina platforms. A total number of 1 878 727 SNPs distributed along the P. nigra reference sequence were identified. The SNP calling accuracy was validated with Sanger sequencing. SNPs were selected within 14 previously identified QTL regions, 2916 expressional candidate genes related to rust resistance, wood properties, water-use efficiency and bud phenology and 1732 genes randomly spread across the genome. Over 10 000 SNPs were selected for the construction of a 12k Infinium Bead-Chip array dedicated to association mapping. The SNP genotyping assay was performed with 888 P. nigra individuals. The genotyping success rate was 91%. Our high success rate was due to the discovery panel design and the stringent parameters applied for SNP calling and selection. In the same set of P. nigra genotypes, linkage disequilibrium throughout the genome decayed on average within 5-7 kb to half of its maximum value. As an application test, ADMIXTURE analysis was performed with a selection of 600 SNPs spread throughout the genome and 706 individuals collected along 12 river basins. The admixture pattern was consistent with genetic diversity revealed by neutral markers and the geographical distribution of the populations. These newly developed SNP resources and genotyping array provide a valuable tool for population genetic studies and identification of QTLs through natural-population based genetic association studies in P. nigra.

& Key message Despite variable dynamics of genetic diversification at the different altitudinal levels, strong gene flow tends to standardize larch genetic diversity: the larch forest distributed along the altitudinal gradient can be regarded as a single population. & Context While in forest tree species many studies focus on the structure of the genetic diversity at the natural range and at the forest stand levels, few studies have worked at intermediate levels like the landscape level. & Aims We tried to determine to what degree altitude variation can affect the genetic diversity and the local structure of the genetic diversity of European larch (Larix decidua Miller) at the landscape level. & Methods Using microsatellite markers, we determined the between-and within-plot genetic structure and the spatial genetic structure (SGS) of four altitudinal plots distributed between 1,350 and 2,300 m a.s.l. in a European larch forest located in the French Alps. & Results A homogenous neutral genetic structure was detected along this gradient. The intensity of the SGS was found to be stronger at 2,300 m and decreased at the 2,000-m plot. It was low or non-existent at the 1,700-and 1,350-m altitudinal levels. & Conclusion Our results suggest that the genetic structure observed at the landscape level in this European larch forest was only slightly affected by climatic variation, human activities, or historical events. However, the variation of intensity of the SGS within the altitudinal plots indicates the existence Contribution of the co-authors Maxime Nardin is the PhD student in charge of the study. Dr. Brigitte Musch supervised the writing of the article, then reviewed and commented on successive drafts of the paper. Dr. Yves Rousselle reviewed and commented on successive drafts of the paper. Vanina Guerin participated in cambium sampling and development of the genetic markers, supervised and organized the genotyping, and commented on successive drafts of the paper. Dr. Leopoldo Sanchez participated in the project and trial design and establishment, cambium sampling, and project coordination. Dr. Jean-Pierre Rossi participated in cambium sampling, designed and helped in performing the spatial genetic analysis (SGS), and reviewed and commented on successive drafts of the paper. Dr. Sophie Gerber supervised the development of the genetic markers and participated in cambium sampling, trial design and establishment, and project coordination. Sara Marin participated in the trial design and establishment, increment core and cambium sampling, and microdensity preparation and analysis process. Dr. Luc E. Pâques participated in the trial design and establishment and coordination of the research project, reviewed and commented on successive drafts of the paper, and co-supervised Maxime Nardin. Dr. Philippe Rozenberg led the project and trial design and establishment, participated in cambium sampling, reviewed and commented on successive drafts of the paper, coordinated the research project, and co-supervised Maxime Nar...

Microgeographical adaptation occurs when the effects of directional selection persist despite gene flow. Traits and genetic loci under selection can then show adaptive divergence, against the backdrop of little differentiation at other traits or loci. How common such events are and how strong the selection is that underlies them remain open questions. Here, we discovered and analysed microgeographical patterns of genomic divergence in four European and Mediterranean conifers with widely differing life-history traits and ecological requirements (Abies alba MIll., Cedrus atlantica [Endl.] Manetti, Pinus halepensis Mill. and Pinus pinaster Aiton) by screening pairs from geographically close forest stands sampled along steep ecological gradients. We inferred patterns of genomic divergence by applying a combination of divergence outlier detection methods, demographic modelling, Approximate Bayesian Computationinferences and genomic annotation to genomic data. Surprisingly for such small geographical scales, we showed that selection is strong in all species but generally affects different loci in each. A clear signature of selection was systematically detected on a fraction of the genome, of the order of 0.1%-1% of the loci depending on the species. The novel modelling method we designed for estimating selection coefficients showed that the microgeographical selection coefficient scaled by population size (Ns) was 2-30. Our results convincingly suggest that selection maintains withinpopulation diversity at microgeographical scales in spatially heterogeneous environments. Such genetic diversity is likely to be a major reservoir of adaptive potential, helping populations to adapt under fluctuating environmental conditions.

Summary• R US is a major dominant gene controlling quantitative resistance, inherited from Populus trichocarpa, whereas R 1 is a gene governing qualitative resistance, inherited from P. deltoides.• Here, we report a reiterative process of concomitant fine-scale genetic and physical mapping guided by the P. trichocarpa genome sequence. The highresolution linkage maps were developed using a P. deltoides · P. trichocarpa progeny of 1415 individuals. R US and R 1 were mapped in a peritelomeric region of chromosome 19. Markers closely linked to R US were used to screen a bacterial artificial chromosome (BAC) library constructed from the P. trichocarpa parent, heterozygous at the locus R US .• Two local physical maps were developed, one encompassing the R US allele and the other spanning r US . The alignment of the two haplophysical maps showed structural differences between haplotypes. The genetic and physical maps were anchored to the genome sequence, revealing genome sequence misassembly. Finally, the R US locus was localized within a 0.8-cM interval, whereas R 1 was localized upstream of R US within a 1.1-cM interval.• The alignment of the genetic and physical maps with the local reorder of the chromosome 19 sequence indicated that R US and R 1 belonged to a genomic region rich in nucleotide-binding site leucine-rich repeat (NBS-LRR) and serine threonine kinase (STK) genes.

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