Paramutation is an epigenetic interaction between two alleles of a gene that leads to an inheritance that is incompatible with Mendel's rules . The phenomenon was discovered in 1956 by the Canadian geneticist RA Brink in maize ( Zea mays ).
Paramutation of the b1 gene in maize
The b1 gene in maize codes for a protein that activates the anthocyanidin signaling pathway through which purple pigments are synthesized. Plants that are homozygous for the BI allele have high b1 expression and a purple color; Plants that are homozygous for the B ' allele, on the other hand, are light because b1 is only weakly transcribed. Crosses between these two plants result in heterozygous plants that are also light in color. This apparently means that BI is recessive to B ' . If, however, the heterozygous plants are crossed with one another, all plants of the following and future generations are bright. This violates Mendel's cleavage rule, according to which some of the plants must be homozygous for BI and therefore purple.
The nucleotide sequence of BI and B ' is identical, but the alleles have a different epigenetic state; H. the chromatin structure is different. Through an interaction between the two alleles, the epigenetic state of BI is reprogrammed by B ' , so that in subsequent generations no more plants appear that are homozygous with regard to BI . The exact molecular mechanism of the interaction has not yet been clarified, but non-coding RNA such as siRNA seems to play a major role in communication between the alleles.
Other cases of paramutation
Cases of paramutation have also been found in other genes in maize and in other organisms, including Arabidopsis thaliana and mice . There are less clear cases of paramutation known in animals than in plants. The example of the b1 gene locus in maize can be analyzed particularly well because the B ' allele is very stable and has 100% penetrance . This is not the case with most other cases of paramutation, often the expression of a gene can change gradually and the effect can subside in the following generations.
- Brink RA: A genetic change associated with the R locus in maize which is directed and potentially reversible . Genetics 41, pp. 872-879. (1956)
- Griffiths et al .: Modern Genetic Analysis
- Alleman et al .: An RNA-dependent RNA polymerase is required for paramutation in maize. Nature 442: 295-298 (2006)
- Chandler: Paramutation: From Maize to Mice . Cell 128, p. 641 (2007)
- Chandler, Stam: Chromatin conversation: Mechanisms and implications of paramutation . Nature 532 (2004)
- Cuzin, Grandjean, Rassoulzadegan: Inherited variation at the epigenetic level: para mutation from the planning to the mouse . Genetics and Development 18, pp. 193-196 (2008).