Milkweed plant evolves new defences to outwit caterpillars


Washington : Some plants are discarding elaborate defences against predatory caterpillars and evolving more favourable stratagems to ensure survival, according to a study.

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The latest example is the milkweed plant, which, according to genetic analysis, instead of resisting predators, is now putting greater efforts into repairing themselves faster than their foe monarch butterfly caterpillar can eat them.

“An important question with co-evolution is where does it end?” asked Anurag Agrawal, Cornell University associate professor of ecology and evolutionary biology and co-author of the study paper.

“One answer is when it becomes too costly. Some plants seem to have shifted away from resisting plant eating (predators) and have taken that same energy and used it to repair themselves.”

The paper sheds light on key theories of co-evolution, claiming that pressure by foraging insects makes plants diversify as they evolve new defensive strategies and that such diversification follows trends in one direction or another, said Agrawal.

Milkweed species have evolved elaborate resistance strategies to fight off caterpillars that eat their leaves. These include hairs on their leaves, heart poisons called cardenolides in their tissues and milky-white toxic latex that pours from the plants’ tubes.

A caterpillar’s bite into a milkweed leaf leads to a flood of latex that is “like getting a gallon of sticky paint thrown into your face,” said Agrawal.

Some caterpillars, in turn, have adapted by shaving the leaf, cutting a leaf’s veins in a circle and then eating in the middle where the latex doesn’t flow. Also, the monarch caterpillar has become immune to the cardenolides.

Using DNA sequence data to look at relationships between 38 species of milkweed, Agrawal and colleague Mark Fishbein, a Portland State University biologist, found evolutionary declines in milkweed’s three most important resistance traits (hairs, cardenolides and latex) and an escalation in the plant’s ability to regrow.

Agrawal was surprised, he said, to find that the plant became more tolerant rather than more diverse in its defences. The reason, he speculated, could be because as its predators have become so specialized, the plant was better off choosing a new defensive tactic “to tolerate the herbivory damage instead of resisting it.”

These findings were published in the current issue of the Proceedings of the National Academy of Sciences.