Chronicling a war of beetle vs. leaf.

A rare meshing of family trees may reveal an ancient arms race between plants that deploy poisons and the beeries that deploy metabolic or behavioral countermeasures so they can eat the leaves anyway.

It's been tricky to find evidence that such ploys coevolved in any species of plants and their pests, says Judith X. Becerra of the University of Arizona in Tucson. That's because it's necessary to determine that both sides in a presumed arms race rolled out a weapon and counter-measure at the same time in evolutionary history.

In an upcoming Proceedings of the National Academy of Sciences, Becerra says she's found such evidence. She analyzed DNA to figure out evolutionary lineages for Bursera plants and the Blepharida beetles whose larvae feast on the plants' leaves. Some Burseras defend themselves with chemical cocktails or toxins that squirt out of leaves under attack.

In the family trees of beetles and plants that Becerra constructed, there are species from both the New and Old Worlds. That means that certain lineages diverged at least 95 million years ago, when Africa and South America split. With that timing, and the age of a Bursera fossil, Becerra used models of evolutionary change to date critical branchings within the Bursera and Blepharida lineages.

Dates for the advent of beetles and plants with certain antagonistic traits matched. For example, Becerra found, squirting Bursera lineages arose simultaneously with beeties whose larvae nip the high-pressure leaf channels and defuse them.--S.M.