Epigenetic marks are thought to have a role in local adaptation and short-term evolution. However, information about their effect in animal models is scarce, and it remains a topic of debate. W e use the European sea bass , a valuable species for fisheries and aquaculture , to investigate DNA methylation levels and gene expression patterns from three genetically distinct populations: Atlantic (AT), Western (WM) , and Eastern (EM) Mediterranean (Figure 1A). The objective is to reveal genomic regions showing differential methylation between populations exposed to the same environment, elucidate their associations with changes in gene expression and the organism’s ability to allocate energy towards growth. First, we are generating an atlas combining epigenomic and expression data, providing a reference database for each population in three key tissues: liver, muscle and fin . Second, we are investigating which of three tissues is better fit to investigate differences between populations in regards to robustness. Third, we aim to identify bio markers of growth within and between populations (Figure 1B) . When possible, we will identify metastable epialleles, i.e., environmentally responsive loci showing similar trends in epigenetic changes across tissues of the same individual.
To accomplish this , we use fish produced from AT, WM and EM broodstock which were reared under a seasonal thermal regime repre sentative of the Ea stern Mediterranean. Once the fish reached 10 grams , individuals from the three populations were mixed and individually tagged. At one year of age, we sampled 48 individuals (8 fish x 3 populations x 2 sexes = 48 fish; 48 x 3 tissues = 144 samples) and performed DNA and RNA extractions. B y creating pools of four individuals, we are processing up to 12 pools per tissue for both gene expression (RNA-seq, 30 M reads per sample) and DNA methylation (whole genome Enzymatic Methyl-seq, at 15 x coverage) , resulting in a total of 72 libraries . This study aims to identify robustness markers crucial for assessing population vulnerability and sustainable aquaculture. Additionally, it contributes to the understanding of genotype-environment interactions, local adaptation, and the impact of global warming on sea bass populations. This study is supported by the ANR FishNess project and CRECHE (P120-0001-01-DF).