Giving the art of mimicry a new meaning

In order to propagate its own species, a parasitic fungus attacks mustard plants and transforms their leaves, making them look like flowers of other species.

Published: Tuesday 15 June 1993

Elongation of the mustard plan PICTURE this: One species invades another to sponge off it and then alters the victim's basic characteristics so it cannot reproduce itself. Finally, the victim adds insult to injury by exploiting the victim's altered biology to perpetuate itself.

Bizarre, yes. But this is precisely what happens when Puccinia monoica -- an airborne parasitic fungus species -- infects mustard plants and takes nature's survival strategy of mimicry to a ruthless extreme (Nature, Vol 362, No 6415).

The fungi cause a disease called rust, which can be seen as dark streaks on the mustard plant's leaves and stem. The disease alters the biology of the plant to such an extent the plant aborts its own flowers and creates instead a cluster of bright yellow, infected leaves that bear a remarkable resemblance to flowers of another species such as buttercup. These mock flowers attract insects, which bring in germ cells from other fungi to fertilise P monoica's own germ cells.

This evolutionary oddity, first noted by B A Roy of the University of California at Davis, is quite unlike the traditionally understood effects of a parasite on its host. The pseudo flowers bear little resemblance to the flowers of the host plant, but their mimicry of flowers such as the buttercup, is so impeccable it fools humans and insects alike. Crab spiders and other predators of pollinating insects sometimes lurk inside the false flowers awaiting their prey and botany students have collected them believing them to be true flowers.

The pseudo flowers bloom at the same time as the true flowers and detain pollinating insects, such as bees, butterflies and flies, to promote their own reproduction. Their bright-yellow, petal-like leaves serve as receptacles for the parasite's spores. Fungi of opposite mating types are often found on the same plant, which makes it easy for the lazier insects and increases chances of fertilisation.

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