Abstract

Hybrids (progeny from two different species or genera) may display characteristics that are out of the range of the parents. For that reason, hybridisation has been exploited in crop breeding programmes for centuries to obtain plants that produce more fruits or are more vigorous.

Interspecific hybrids may undergo “genomic shock,” as coined by Barbara McClintock, leading to perturbation of gene expression and activation of transposons. Genomic shock can be seen as a source of heritable variation, able to trigger extensive phenotypic effects. However, the molecular mechanisms associated are not well understood.

We predict that small RNAs (sRNAs) may be involved in genomic shock. These 20-24nt long sRNA molecules can mediate gene silencing at transcriptional or posttranscriptional level. In hybrids, due to the variability in the sRNA population between species, sRNAs from one parent may find new targets in the genome of the other parent, modifying gene expression.

To unravel the mechanisms of hybridisation-induced genome shock, we study the F4 generation from a cross of tomato and a wild relative. Our results suggest that hybridisation activates integrated viral genomes of EPR Vs (endogenous pararetroviruses), which are normally latent and found broadly in the plant kingdom. This activation leads to an increased production of sRNAs, as these elements are repressed through post-transcriptional gene silencing.

EPR Vs are a viral reservoir that upon activation can actually cause viral disease. Although why some plants show symptoms while others remain healthy is not known, we are paving the way: we have identified a mechanism that controls the activation of EPRV in hybrids, mediated by Dicer and sRNAs, providing a first molecular handle for EPRV control and the effects of hybridisation in plant genomes.

These findings also implicate that hybridization-induced genome shock leading to EPRV activation and sRNA silencing, as causing changes in gene expression. Such hybridization-induced variation in gene expression could increase the range of traits available for selection in natural evolution or in breeding for agriculture.

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