Conventional wisdom had it that once a methylated repeat sequence is heavily demethylated, there’s no turning back. Not only don’t they become remethylated, but subsequent generations inherit the epigenetic changes as well.
But a group of European researchers have found that this isn’t always the case and there are at least two distinct classes of methylated repeats — in Arabidopsis — and the CW only applies to the first.
Methylated repeats maintained by methyl transferases can’t be restored once they’ve been compromised.
But a second class can be brought back to its full methylated glory in just a few generations. These, it turns out, are dependent on a fully functional RNAi apparatus – as can be seen in mutation analysis, and by a host of a 24 nt RNAs complementary to the remethylatable sequences but not found for those that aren’t.
The effect is progressive, doesn’t spread beyond the original sequences, and results in the re-silencing of nasty things like transposons. The potential evolutionary implications did not go unnoticed by the researchers. See what they say about it at Science, January 2009