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出典(authority):フリー百科事典『ウィキペディア（Wikipedia）』「2012/10/16 18:43:47」(JST)

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Paramutation was first observed by the effect it had on the color of corn kernels in maize plants

In epigenetics, a paramutation is an interaction between two alleles at a single locus, whereby one allele induces a heritable change in the other allele. The change may be in the pattern of DNA methylation, for example. The allele inducing the change is said to be paramutagenic, whilst the allele that has been epigenetically altered is termed paramutant (or paramutated). This change is meiotically inheritable, and therefore paramutation violates Mendel’s first law.

A paramutant allele may have altered levels of gene expression, which may continue in offspring which inherit the allele even though the paramutating allele may no longer be present. Through proper breeding, paramutation can result in isogenic sibling plants with drastically different phenotypes.

Interestingly, paramutation can result in a single allele of a gene controlling a spectrum of phenotypes. At r1 in maize, for example, the weaker expression state adopted by a paramutant allele can range from completely colorless to nearly fully colored kernels. This is an exception to the general observation that continuous variation is controlled by many genes.

History

Paramutation was first discovered and studied in maize (Zea mays) by R.A. Brink at the University of Wisconsin–Madison in the 1950s. Brink noticed that specific weakly expressed alleles of the red1 (r1) locus in maize, which encodes a transcription factor that confers red pigment to corn kernels, can heritably change specific strongly expressed alleles to a weaker expression state. The weaker expression state adopted by the changed allele is heritable and can, in turn, change the expression state of other active alleles in a process termed secondary paramutation. Brink showed that the influence of the paramutagenic allele could persist for many generations.

Mechanism

The molecular basis of paramutation is being unravelled almost exclusively in maize. Paramutation may share common mechanisms with other epigenetic phenomena, such as gene silencing and genomic imprinting. In maize, paramutation seems to share many traits with the well understood RNA-directed DNA-methylation pathway in Arabidopsis thaliana, even though it has never been observed in that species. In maize:

"paramutation is RNA-directed. Stability of the chromatin states associated with paramutation and transposon silencing requires the mop1 gene, which encodes an RNA-dependent RNA polymerase."^[1]

Exactly how the RNA produced by this polymerase causes paramutation in maize is not yet understood, but like other epigenetic changes, it involves the covalent modification of DNA and/or the DNA-bound histone proteins without changing the DNA sequence itself.

Occurrence

Allelic interactions similar to paramutation have since been reported in other organisms, including tomatos, peas, and mice.

References

^ Alleman M, Sidorenko L, McGinnis K, et al. (July 2006). "An RNA-dependent RNA polymerase is required for paramutation in maize". Nature 442 (7100): 295–8. doi:10.1038/nature04884. PMID 16855589.

BBC Science News - An easy to understand article involving paramutation
Hollick JB, Dorweiler JE, Chandler VL (August 1997). "Paramutation and related allelic interactions" (Review). Trends Genet. 13 (8): 302–8. doi:10.1016/S0168-9525(97)01184-0. PMID 9260515. http://linkinghub.elsevier.com/retrieve/pii/S0168952597011840.
Rassoulzadegan M, Grandjean V, Gounon P, Vincent S, Gillot I, Cuzin F (May 2006). "RNA-mediated non-mendelian inheritance of an epigenetic change in the mouse". Nature 441 (7092): 469–74. doi:10.1038/nature04674. PMID 16724059.
Chandler VL, Stam M (July 2004). "Chromatin conversations: mechanisms and implications of paramutation" (Review). Nat. Rev. Genet. 5 (7): 532–44. doi:10.1038/nrg1378. PMID 15211355.
Stam M, Mittelsten Scheid O (June 2005). "Paramutation: an encounter leaving a lasting impression". Trends Plant Sci. 10 (6): 283–90. doi:10.1016/j.tplants.2005.04.009. PMID 15949762. http://linkinghub.elsevier.com/retrieve/pii/S1360-1385(05)00104-4.

English Journal

Messing with mendel.

Birchler JA.SourceDivision of Biological Sciences, University of Missouri, Columbia, MO 65211, USA. Electronic address: birchlerj@missouri.edu.
Developmental cell.Dev Cell.2012 Oct 16;23(4):678-9. doi: 10.1016/j.devcel.2012.10.004.
Paramutation, a phenomenon of epigenetic switching that violates Mendel's Law of Segregation, was first discovered in maize and later observed in other plants. In a recent report in Nature, de Vanssay and colleagues (2012) describe in Drosophila an operationally analogous phenomenon to paramutation
PMID 23079594

Paramutation in Drosophila linked to emergence of a piRNA-producing locus.

de Vanssay A, Bougé AL, Boivin A, Hermant C, Teysset L, Delmarre V, Antoniewski C, Ronsseray S.SourceLaboratoire Biologie du Développement, UMR7622, CNRS-Université Pierre et Marie Curie, 9 quai Saint Bernard, 75005 Paris, France.
Nature.Nature.2012 Oct 4;490(7418):112-5. doi: 10.1038/nature11416. Epub 2012 Aug 26.
A paramutation is an epigenetic interaction between two alleles of a locus, through which one allele induces a heritable modification in the other allele without modifying the DNA sequence. The paramutated allele itself becomes paramutagenic, that is, capable of epigenetically converting a new param
PMID 22922650

Japanese Journal

RNA Silencing Systems and Their Relevance to Allele-Specific DNA Methylation in Plants

SHIBA Hiroshi,TAKAYAMA Seiji
Bioscience, biotechnology, and biochemistry 71(11), 2632-2646, 2007-11-23
… Some examples of epigenetic phenomena mediated by allele-specific DNA methylation in plants found to date include genomic imprinting, nucleolar dominance, and paramutation. …
NAID 10027520559