Heterosis and inbreeding depression of epigenetic Arabidopsis hybrids.

We have addressed the possible epigenetic contribution to heterosis using epigenetic inbred lines (epiRILs) with varying levels and distributions of DNA methylation. One line consistently displayed parent-of-origin heterosis for growth-related traits. Genome-wide transcription profiling followed by a candidate gene approach revealed 33 genes with altered regulation in crosses of this line that could contribute to the observed heterosis. Although none of the candidate genes could explain hybrid vigour, we detected intriguing, hybrid-specific transcriptional regulation of the RPP5 gene, encoding a growth suppressor. RPP5 displayed intermediate transcript levels in heterotic hybrids; surprisingly however, with global loss of fitness of their F2 progeny, we observed striking under-representation of the hybrid-like intermediate levels. Thus, in addition to genetic factors contributing to heterosis, our results strongly suggest that epigenetic diversity and epigenetic regulation of transcription play a role in hybrid vigour and inbreeding depression, and also in the absence of parental genetic diversity.