Petrov, 2001), yet the underlying processes for this variability are not yet fully understood (Elliott & Gregory, 2015). To understand and study the mechanisms of genome size variation, such as proliferation of repetitive elements (Blommaert et al., 2019), effective population size (Lefébure et al., 2017;Lynch & Conery, 2003) or correlation to other traits (Gardner et al., 2020;Prokopowich et al., 2003), reliable estimates for the taxon under scrutiny are therefore mandatory. This is all the more important as substantial changes in genome size may even occur among closely related sister species, that is over relatively short evolutionary timescales (Agudo et al., 2019;Keyl, 1965;Vitales et al., 2020). An accurate estimation of genome size is also important for genomic projects. For example, in the assembly of genomes, the proportion of the true genome size covered by a given assembly draft is a quality criterion and limits the maximum size of the draft. In addition, resequencing projects requiring a certain sequencing depth (e.g., for genotyping) profit from a reliable genome size estimate (Fountain et al., 2016).Flow cytometry is generally deemed to yield reliable estimates of genome size (Doležel & Greilhuber, 2010;Johnston et al., 2019). Yet, this method is not without caveats (Wang et al., 2015) and requires specialized laboratory skills and availability of the relatively expensive equipment. Moreover, the method depends on the availability of fresh or frozen tissue with largely intact cells, which narrows the range of taxa for which such analyses are practically feasible (Johnston et al., 2019). Bioinformatical analysis of next generation sequencing (NGS) dataprovides an alternative for estimating genome size (Vurture et al., 2017). Besides the widely used k-mer-based methods (Li & Waterman, 2003;Lipovský et al., 2017), Schell et al. (2017 introduced a very simple method for genome size estimation, relying on mapping statistics

In the course of global climate change, central Europe is experiencing more frequent and prolonged periods of drought. The drought years 2018 and 2019 affected European beeches (Fagus sylvatica L.) differently: even in the same stand, drought damaged trees neighboured healthy trees, suggesting that the genotype rather than the environment was responsible for this conspicuous pattern. We used this natural experiment to study the genomic basis of drought resistance with Pool-GWAS. Contrasting the extreme phenotypes identified 106 significantly associated SNPs throughout the genome. Most annotated genes with associated SNPs (>70%) were previously implicated in the drought reaction of plants. Non-synonymous substitutions led either to a functional amino acid exchange or premature termination. A SNP-assay with 70 loci allowed predicting drought phenotype in 98.6% of a validation sample of 92 trees. Drought resistance in European beech is a moderately polygenic trait that should respond well to natural selection, selective management, and breeding.

In the course of global climate change, central Europe is experiencing more frequent and prolonged periods of drought. These drought events have severe and detrimental impacts on the forest ecosystem. The drought years 2018 and 2019 affected European beeches (Fagus sylvatica L.) in noticeably different ways: even in the same local stand, badly drought damaged trees immediately neighboured apparently healthy trees. This led to the hypothesis that the genotype rather than the environment was responsible for this conspicuous pattern. We used this natural experiment to study the genomic basis of drought resistance in a Pool-GWAS approach. Contrasting the extreme phenotypes, we identified 106 significantly associated SNPs throughout the genome. The majority of affected genes was previously implicated in drought reaction in other plant species. Most observed non-synonymous changes led either to a substantial functional amino acid exchange or a stop-codon. A SNP-assay with 70 informative loci allowed the successful prediction of drought phenotype from the multilocus genotype in 98.6% in a validation sample of 92 trees with Linear Discriminant Analysis. Drought resistance in European beech appeared to be a moderately polygenic trait that should nevertheless respond well to natural selection, selective management, and breeding. The widespread distribution of drought resistance across natural beech populations represents an important argument for maintaining genetic diversity in dynamic forest ecosystems. The results from this study could therefore contribute to harness beech wood forests against ongoing climate change.