Paramutation: Just a Curiosity or Fine Tuning of Gene Expression in the Next Generation?

Gene silencing is associated with heritable changes in gene expression which occur without changes in DNA sequence. In eukaryotes these phenomena are common and control important processes, such as development, imprinting, viral and transposon sequence silencing, as well as transgene silencing. Among the epigenetic events, paramutation occurs when a silenced allele (named paramutagenic) is able to silence another allele (paramutable) in trans and this change is heritable. The silenced paramutable allele acquires paramutagenic capacity in the next generations. In the 1950s, Alexander Brink described for the first time the phenomenon of paramutation, occurring in maize at the colored1 (r1) gene, a complex locus (encoding myc-homologous transcription factors) that regulates the anthocyanin biosynthetic pathway. Since then, paramutation and paramutation-like interactions have been discovered in other plants and animals, suggesting that they may underlie important mechanisms for gene expression. The molecular bases of these phenomena are unknown. However in some cases, the event of paramutation has been correlated with changes in DNA methylation, chromatin structure and recently several studies suggest that RNA could play a fundamental role. This last consideration is greatly supported by genetic screening for mutants inhibiting paramutation, which allowed the identification of genes involved in RNA-directed transcriptional silencing, although it is possible that proteins are also required for paramutation. The meaning of paramutation in the life cycle and in evolution remains to be determined even though we might conjecture that this phenomenon could be involved in a fast heritability of favourable epigenetic states across generations in a non- Mendelian way.