Published online 9 September 1999 | Nature | doi:10.1038/news990909-10


Shape shifting

The plant yellow toadflax (Linaria vulgaris) is common European weed that is now blighting many parts of America. It is sometimes known as ‘butter-and-eggs’ because of its striking colouring. The plant has tube-like yellow flowers with deep orange centres, five petals and five stamens of varying length. Usually these flowers are symmetrical in both shape and colour in only one plane - through the tip of one petal, through the barrel and out between two other petals.

But there is a naturally occurring mutant toadflax whose flowers have radial symmetry - through every petal and stamen, that is. Now, new work published in the 9 September issue of Nature, reveals that the difference between these two plants is not genetic but ‘epigenetic’.

The term ‘epigenetic’ refers to non-genetic forces that control which genes are active and when. In other words both forms of toadflax have exactly the same genes, but in the so-called ‘epimutant’ form, one of the genes is switched off - or ‘silenced’, to give the state its technical term. The gene in question is called ‘Lcyc’ and it is inactive in the mutant plant because it is - for some unknown reason - bristling with carbon-hydrogen ‘methyl’ molecules (CH3). These bulky add-ons make Lcyc inaccessible to the cellular machinery that usually translates genes into proteins.

Moreover, Enrico Coen of the John Innes Centre, Norwich, UK and colleagues, who made this discovery, found that there are varying degrees of epimutation. In flowers that appear to be semi-natural or semi-mutant, the Lcyc gene, they announce, is bonded to fewer methyl groups. From this, and from their studies of wild-type toadflax, Coen’s group deduce that the Lcyc gene plays an important role in establishing symmetry during the earliest stages of the development of toadflax flowers, when stamens and petals are being formed.

Most surprising of all, the researchers also found that the carbon-hydrogen crowding - formally known as ‘methylation’ - of the Lcyc gene is heritable. That is to say, the offspring of a toadflax plant with radially symmetrical flowers and copious methylation usually has the same symmetry, and the same degree of methylation. Occasionally, however, the flowers do revert to wild-type when, so the researchers found, the methyl groups are stripped from Lcyc; why this happens is not yet clear.

“This indicates,” Coen’s group conclude, “that epigenetic mutations may play a more significant role in evolution than has hitherto been suspected, [especially as] methylated DNA is more prone to conventional genetic mutation.”