EMBO J 31 2, 257–266 (2012); published online December162011
Hybrid seeds have been a key component of greatly increasing the yield of many important crops, foremost of maize. If the parents are properly chosen, non-additive interactions between diverged genomes can lead to strongly superior performance of the F1 progeny, known as heterosis. While many different explanations have been advanced, a consensus for the causes of genome-wide positive epistasis in hybrids has not emerged. In this issue of The EMBO Journal, Shivaprasad and colleagues describe a new mechanism that can account for heterosis often being a genome-wide phenomenon. These authors show that small RNA (sRNA) loci of tomato can exhibit transgressive activity, which can in turn lead to epigenetic and gene expression changes within hybrid progeny. This is particularly exciting because many sRNAs are produced from non-coding regions or transposable elements (TEs), which diverge more quickly than protein-coding genes and thus provide more opportunity for unexpected genetic interactions.