Natural variation is widely used as a source of biodiversity to identify alleles controlling various traits. In the Institut Jean Pierre Bourgin (IJPB, INRA, Versailles) several groups are interested in the analysis of Arabidopsis natural variation. Classical approaches involving sets of Recombinant Inbred Lines (RIL) were developed and the genes explaining the QTLs were identified by map-based cloning and/or candidate gene approach. Interestingly, these very classical approaches led to the discovery of natural epialleles such as epiQTLs. The objective of the present project is to use my experience in the analysis of molecular mechanisms underlying naturally varying traits to study one of the very first examples of epiQTL allele isolated from crosses between natural populations of Arabidopsis. This project builds upon the work of Dr. Daniela Vlad, a former PhD student in Dr. Olivier Loudet’s lab, who identified a novel natural epiallele named SG1. The proposed project will focus on a further characterisation of this particular epiallele and of the underlying gene function.The specific aims of this project will be to:• Understand how variation at SG1 results in a dramatic effect on fitness;• Characterize the epigenetic status of the SG1 locus and its impact on the genome to gain a better understanding of the interplay between different epigenetic mechanisms leading to the regulation of plant growth.The results from this study are likely to provide important insights into the regulation of gene expression controlled by DNA methylation, and we expect they will also improve our understanding of the mechanisms by which plants integrate internal cues to adjust growth.