The role of epigenetics in hybrid vigour

•Hybrids with similar genome sequences can show significant heterosis.•The final level of heterosis, either in biomass or grain yield, is dependent on changes that occur throughout development.•Hybrids exhibit increased levels of transcription of chloroplast-targeted genes and increases in the size and/or number of photosynthetic cells in cotyledons and early true leaves.•Hybrids show changed 24-nucleotide small interfering RNA levels in localized regions of the genome and altered DNA methylation patterns, which may account for differences in gene expression.

Hybrid vigour, or heterosis, refers to the increased yield and biomass of hybrid offspring relative to the parents. Although this has been exploited in plants for agriculture and horticulture, the molecular and cellular mechanisms underlying hybrid vigour are largely unknown. Genetic analyses show that there are a large number of quantitative trait loci (QTLs) that contribute to the heterotic phenotype, indicating that it is a complex phenomenon. Gene expression in hybrids is regulated by the interactions of the two parental epigenetic systems and the underlying genomes. Increasing understanding of the interplay of small RNA (sRNA) molecules, DNA methylation, and histone marks provides new opportunities to define the basis of hybrid vigour and to understand why F1 heterosis is not passed on to subsequent generations. We discuss recent findings that suggest the existence of several pathways that alter DNA methylation patterns, which may lead to transcriptional changes resulting in the heterotic phenotype.