When the leaves they depend on turn a deathly yellow in the fall, leaf miner moths (Phyllonorycter blancardella) perform CPR. Even as the rest of the leaf wilts, the patch surrounding a leaf miner larva stays a bright and photosynthetically active green. Now, a new study shows that these green islands spark to life thanks to bacteria living within the grubs themselves.
Like mammals, many insects host internal microbes called endosymbionts that help them digest meals, often passing these friends from generation to generation. Insect endosymbionts have also proven themselves keen inventors, developing new defenses for their hosts. "The more we look at endosymbionts in insects, the more people find interesting and new functions," says ecologist David Giron of the Université François Rabelais in Tours, France.
It seemed possible to Giron that bacteria like those in the genus Wolbachia, which dwell in leaf miners, could also rewire plant metabolism. Many microbes, including Wolbachia, carry a gene also found in plants that spurs some plant cells to make hormones called cytokinins, he says. Cytokinins, which delay death in plant cells, can spur green islands on their own and are plentiful in leaf-miner islands.
To determine whether the bacteria nurtured the green islands, Giron and colleagues gave some female leaf miners oral doses of insect-safe antibiotics to kill their microbial partners. The researchers allowed the insects to lay eggs as usual on apple-tree leaves. Larvae from untreated mothers were able to form green islands, but larvae from dosed moms were not. Without bacteria "you don't have green islands, and if you don't have green islands, you die," Giron says. The team reported the results online in the Proceedings of the Royal Society B.
The researchers don't yet know if Wolbachia bacteria are making cytokinins on their own or if they're triggering plant life support through other means. In a previous study, scientists discovered that birch tree–infecting leaf miners could churn out cytokinins, but they didn't investigate whether these hormones came from the insects or their endosymbionts. Regardless of how they work, the bacteria are valuable compatriots. In dying leaves, green islands buy the moths as much as an extra month to reproduce and grow, the researchers suspect. With that additional time, leaf miners may be able to sneak in an extra generation of grubs before winter.
"It's a beautiful example of how an organism can solve a major problem by getting into a [mutually beneficial] relationship with a microbe," says chemical ecologist Ian Baldwin of the Max Planck Institute for Chemical Ecology in Jena, Germany. He says there are a lot of unanswered questions, such as whether Wolbachia living outside of insect bodies can still manage this breath of life. But the study shows just how much an insect's success is the work of its inner bugs, he says. "One can safely say that most insects ... are just Trojan horses."