June 20, 2009
First Discovery Of Plant That “˜Fakes’ Illness
German botanists working in the rainforests of Ecuador have discovered a plant that "pretends" to be ill. The plant fakes its illness to prevent attacks by mining moths, which would eat the plants' otherwise healthy leaves.
The discovery is the first known example of a plant that feigns being sick, and might explain a pattern seen on plant leaves known as variegation, which many species of plant exhibit.
For a variety of causes, variegated plants have different colored patterns on the surface of their leaves. One of the most common is when the surface cells in the leaf lose chlorophyll and their ability to photosynthesize, giving them a white appearance. In theory, such plants should be at a disadvantage due to their restricted ability to photosynthesize.
But the random discovery by botanists suggests this may not be the case after all. Rather, some variegated plants may mimic illness to avoid being eaten, which would provide the plants a distinct advantage.
Sigrid Liede-Schumann, Ulf Soltau and Stefan Dotterl of the University of Bayreuth in Germany were examining understory plants in southern Ecuador's forests. They noticed that the plain green leaves of a plant called Caladium steudneriifolium were more frequently damaged by mining moths than the variegated leaves of the same species in close proximity.
Mining moths lay their larvae into the leaves. The caterpillars then chew through the leaf surface, leaving a white trail of destruction behind.
"The similarity of the variegation patterns with the criss-cross munching traces of the larvae led to the idea that maybe they deter the mining moth from laying its eggs," Liede-Schumann told BBC News.
To test the concept, the scientists used white correction fluid to simulate the appearance of variegation on hundreds of healthy leaves.
Three months later, they counted the number of leaves affected by the caterpillars, comparing variegated leaves, green leaves and those painted white to appear variegated.
"The results were the same," Liede-Schumann said.
"Visibly variegated leaves were significantly less frequently damaged by mining moth larvae than plain green ones."
The moths infested nearly 8% of green leaves, but infested just 1.6% of variegated ones and 0.4% of those painted to appear variegated.
"I was quite surprised," said Liede-Schumann.
She speculated that the plant mimics being ill, producing variegated leaves that look like those that have already been damaged by mining moth larvae. That appearance deters the moths from laying additional larvae on the leaves since the moths assume the previous caterpillars have already depleted the leaves' nutrients.
"The fact that there are both plain green and variegated leaves in the population indicates to me that both are useful in the long-term success of the species," she said.
The scientists believe the reduction in a variegated leaf's ability to photosynthesize is more than offset by the benefits of not being eaten. If true, this suggests that variegation survives in wild plants because it provides a selective advantage.
The discovery is published in the journal Evolutionary Ecology.
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