Alleles of genes can be heritably expressed at different levels despite sharing the same sequence: they are called epialleles (epigenetic alleles). Occasionally, epialleles communicate between themselves and the silent epiallele converts the active one into another silent epiallele. This contagious silencing is called paramutation, and despite being discovered in 1915 very few examples are known today, limiting our understanding of this process.
Armed with modern high-throughput sequencing technologies, Quentin Gouil and David Baulcombe from the Baulcombe laboratory, in collaboration with Ondřej Novák from Palacký University in Czech Republic, revisited the SULFUREA paramutation in tomato, described 58 years ago. Silencing of both epialleles results in a striking yellowing of the leaves (sulfurous color). The researchers were able to identify the gene responsible for the sulfurea phenotype (SLTAB2). Its role in the translation of photosystem subunits explains the lack of pigmentation of the paramutated plants. The epigenetic marks associated with its silencing, such as small RNAs and DNA methylation, support a small RNA-mediated model of paramutation between epialleles.
The characterisation of the SULFUREA gene and its mode of silencing opens the door to further investigation of the mechanism of paramutation and its frequency in crosses. Paramutation-like interactions between epialleles may be much more common than previously thought, and their consequences for crop breeding and evolution have yet to be defined.
Gouil Q, Novák O, and Baulcombe DC. (2016) SLTAB2 is the paramutated SULFUREA locus in tomato. Journal of Experimental Botany.