A closer look at our distant leafy relatives reveals that plants have deployed a similar means of regulating embryonic growth. Ueli Grossniklaus (University of Zurich, Switzerland) discovered a silent gene in a variety of cress, and has named it after the Greek sorceress Medea. In the mythical tragedy Medea killed her children after being betrayed by Jason of the Argonauts, whom she assisted in his mission to steal the Golden Fleece. The Medea gene in Ueli’s cress plants makes a repressor protein that silences another gene involved in embryonic growth. Ueli christened this gene after one of Medea’s murdered children, Pheres.

The Medea gene is silenced when inherited from the male plant. The active copy from mum serves to keep the Pheres gene quiet in her developing offspring. Without her active Medea gene, seed development goes awry generating small fat embryos reminiscent of mice embryos when Igf2 gene silencing fails. These embryos die when the seeds dry out before dormancy. However, these chubby mutants can be saved by growing them in vitro; adults resulting from such tissue cultures are normal and healthy. The growth moderating effects of the Greek sorceress gene appear to be necessary only during embryonic development. Despite similarities between animal and plant systems in this respect, we don’t know whether DNA methylation keeps Medea quiet.

Why have plants and animals employed this habit of silencing genes? The truth is that no-one really knows, although various explanations have been proposed. The inheritance of silent genes, dubbed genomic imprinting, could be related to reproductive habits. In mammals, imprinting is restricted to species that house their developing young in utero. Egg-laying mammals like the duck-billed platypus, are not known to imprint growth-related genes in this way. For nine months your mother provided all the essential nutrients for your development from her bloodstream through the placenta. Rather than laying an egg and leaving you to fend for yourself on the contents of a yolk sac, she patiently responded to your demands for food during your stay in the womb.