Five degrees of methylation

species but it took a defect in only a single gene to convert the plump flowers of the toadflax, Linaria vulgaris (main picture, on the left), into the spikes of Peloria, on the right. Peloria (Greek for 'monster') was named by Linnaeus 250 years ago, when he realized it must have arisen from the normal toadflax. This naturally occurring mutant has five identical petals, whereas wild-type flowers have distinctly different upper and lower petals. The difference between the two turns out to be in the gene Lcyc, a homologue of the cycloidea gene which controls dorsoventral asymmetry in Antirrhinum. But it's not a permanent modification of the DNA that generates this monster. Lcyc is transcriptionally silenced in the peloric mutant because of extensive methylation, and occasionally during somatic development the mutant reverts (see inset images) because of demethylation of Lcyc, which restores gene expression.