Epigenetic divergence is sufficient to trigger heterosis in Arabidopsis thaliana

Despite the importance and wide exploitation of heterosis in commercial crop breeding, the molecular mechanisms behind this phenomenon are not well understood. Interestingly, there is growing evidence that beside genetic also epigenetic factors contribute to heterosis. Here we used near-isogenic but epigenetically divergent parents to create epigenetic F1 hybrids (epiHybrids) in Arabidopsis, allowing us to quantify the contribution of epigenetics to heterosis. We measured traits such as leaf area (LA), growth rate (GR), flowering time (FT), main stem branching (MSB), rosette branching (RB) and final plant height (HT) and observed several strong positive and negative heterotic phenotypes among the epiHybrids. For LA and HT mainly positive heterosis was observed, while FT and MSB mostly displayed negative heterosis. Heterosis for FT, LA and HT could be associated with several heritable, differentially methylated regions (DMRs) in the parental genomes. These DMRs contain 35 (FT and LA) and 14 (HT) genes, which may underlie the heterotic phenotypes observed. In conclusion, our study indicates that epigenetic divergence can be sufficient to cause heterosis.\n\nAuthor SummaryCrossing two genetically distinct parents generates hybrid offspring. Sometimes hybrids are performing better than their parents in particular traits and this is referred to as heterosis. Hybridization and heterosis are naturally occurring processes and crop breeders intentionally cross genetically different parental lines in order to generate hybrids with maximized traits such as yield or stress tolerance. So far, the mechanisms behind heterosis are not well understood. In this study we focused on the effect of epigenetic variation onto heterosis in hybrids, and for this purpose we created epigenetic hybrids (epiHybrids) by crossing wildtype plants with a selection of genetically very similar but epigenetically divergent lines. An extensive phenotypic analysis of the epiHybrids and their parental lines showed that epigenetic divergence between parental genomes can be a major determinant of heterosis. Importantly, multiple heterotic phenotypes could be associated with meiotically heritable differentially methylated regions (DMRs) in the parental genomes, allowing us to map epigenetic quantitative trait loci (QTLs) for heterosis. Our results indicate that epigenetic variation can contribute to heterosis and suggests that heritable epigenetic variation could be exploited for the improvement of crop traits.