White begins, black wins?

Plant black mustard puts cabbage white in check in evolutionary chess game.
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Over the centuries, butterflies and plants have played an evolutionary chess game. Now the black mustard plant has said ‘check’ to the cabbage white by killing its eggs, thereby protecting the plant from the caterpillar’s jaws. Entomology PhD candidate Eddie Griese discovered this defence strategy only works for a single group of related butterflies.

The large cabbage white lays eggs on the leaves of its favourite food: brassicas. Brassicas, which include the model plant black mustard, defend themselves with toxic substances. But the cabbage white has its own antidote. Now the brassica plants have a new line of attack. The plant lets the cells under the eggs die off, making the eggs dehydrate. ‘The plant doesn’t use this defence strategy against all butterflies,’ says Griese. He discovered this when he placed the eggs of various butterfly species on the black mustard. At first, the plant only seemed to kill the eggs of species that eat brassicas, such as the cabbage white.

Then Griese looked at how the plant defended itself against butterfly species that are less closely related to the cabbage white. The plant didn’t respond to distant relatives, even though some of them live on a diet of brassicas. This means that the black mustard recognizes a specific property of the cabbage white and its close relatives. That substance triggers the existing programme for killing the cells. The entomologists are still investigating which particular substance the plant recognizes. ‘It may be something in the adhesive that sticks the butterfly’s eggs to the leaf,’ says Griese. One thing is certain: the substance that the plant recognizes is crucial for the egg, as evolution would otherwise have led it to disappearing.

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