Biology Reference
In-Depth Information
transfer of information through
trans
-interactions between homologous
sequences resulting in distinct states of gene expression that are heritable
through generations (Reviewed in
Arteaga-Vazquez & Chandler, 2010;
Hollick, 2012
). Paramutations are unique among epialleles for their “infec-
tious” behavior: when an allele capable of inducing paramutation (para-
mutagenic) is crossed to a paramutable allele, the latter is changed to the
paramutagenic state. Using the
B
locus in maize as a model, it has been shown
that sequences required for paramutation are direct tandem repeats
100-kb
upstream of the transcription start site (
Stam, Belele, Dorweiler, & Chandler,
2002
). In addition, clonal analyses indicate that paramutated states are
established during reproduction (
Coe, 1966
). Paramutation in plants involves
proteins implicated in RdDM (Reviewed in
Arteaga-Vazquez & Chandler,
2010; Hollick, 2012
). Indeed, siRNAs generated from tandem repeats are nec-
essary for paramutation, suggesting that it might be an “extreme” illustration of
RdDM (
Teixeira & Colot, 2010
). There are, however, unique and distinct
properties of paramutation that cannot be accounted for by RdDM. Impor-
tantly, loci silenced by RdDM are generally not paramutagenic. Also, whereas
siRNAs are necessary for paramutation, they are not sufficient, as all alleles at
b
(paramutagenic, paramutable, or neutral) produce the same amount of siRNA
(
Arteaga-Vazquez et al., 2010
). Yet, paramutations illustrate the possibility that
RNA-induced chromatin changes at repeats, when established during repro-
ductive development, might contribute to TEI. This possibility is strongly
supported by the recent observation that maternal Piwi-interacting RNAs
are necessary and sufficient to induce paramutation-like behavior in
Drosophila
(
de Vanssay et al., 2012
). By extension, it is tempting to speculate that para-
mutation in plants results from an RNAi-induced, indiscriminate repro-
gramming of both the paramutagenic and paramutable alleles during
reproduction. This might imply a phased reprogramming step during repro-
ductionwhen both alleles acquire a “neutral” epigenetic state that is thenmod-
ified by homology-dependent RNAi toward the paramutagenic state. This
would fit with a model in which, at least for some loci, reprogramming
involves successive phases of erasure and reestablishment of information based
on extra chromosomal, siRNA-based inheritance.
6. CONCLUDING REMARKS AND FUTURE OUTLOOK
Genetic, genomic, and biochemical studies in a variety of organisms
have begun to reveal that similar chromatin-based strategies are employed
in plants and metazoans to control in a timely and accurate manner the
