Way before people started inhaling nicotine in cigarette smoke, plants were using the chemical to defend themselves from animals. Nicotine is a poison, and an exceptionally deadly one. It targets proteins that tell our muscles to fire when they receive signals from our nerves. At high enough doses of nicotine, these proteins force muscles to constantly contract, leading to paralysis and death. And since the same proteins are found in every animal with muscles, nicotine can kill cows and caterpillars alike.
The tobacco hornworm is an exception. As a caterpillar, this moth specialises in eating tobacco leaves, because it can cope with doses of nicotine that would kill other species. It gets rid of most of the poison in its waste but, adding insult to injury, it also co-opts a small fraction for its own protection.
Pavan Kumar and colleagues from the Max Planck Institute for Chemical Ecology in Germany have shown that it exhales the poison through pores in its skin, creating a toxic miasma that deters hungry spiders. They call it a “defensive halitosis”.
In 2010, Kumar’s team, led by Ian Baldwin, raised tobacco hornworm caterpillars on genetically modified tobacco that doesn’t make much nicotine. They found that a gene called CYP6B46 was less active than usual in the guts of these insects, suggesting that it’s usually involved in resisting the effects of nicotine.
To test this idea, the team engineered tobacco plants that could deactivate the gene in any caterpillars that fed upon them, and planted them at a private ranch in Utah’s Great Basin Desert. They waited, and watched.
Soon, they noticed that hornworm caterpillars were more likely to die during the night if they ate the modified plants. A few nocturnal surveys revealed the cause of their deaths—wolf spiders. These powerful, fast-running hunters usually pose no threat to hornworms that eat nicotine-rich meals. However, they readily killed any caterpillars that ate the modified tobacco and had inactivated CYP6B46 genes. Why?
The answer seemed obvious at first. CYP6B46 is part of a large family of metabolic genes, which animals frequently use to detoxify the chemicals in the plants they eat. The team assumed that CYP6B46 was neutralising nicotine by breaking it down into safer substances. But, to their surprise, they couldn’t find any traces of these by-products in the caterpillars’ bodies or faeces.
Instead, they showed that CYP6B46 redirects a tiny amount of nicotine from the caterpillars’ guts to their haemolymph—the liquid that fills their bodies and acts as their bloodstream. From there, the caterpillars can vent the nicotine into the outside world by opening their spiracles—small breathing holes in their flanks, which allow air to enter and leave their bodies.
The caterpillars send just 0.65 percent of the nicotine they eat into their haemolymph. But even this tiny amount is enough to quadruple the concentration of nicotine in the air around them, creating an effective anti-spider spray.
When a wolf spider attacks, it first inspects its prey with chemically sensitive appendages. Here’s what happens when it approaches a caterpillar with a nicotine cloud.
And here’s what happens when it approaches a caterpillar with an inactivated CYP6B46 gene. The caterpillar can’t shunt nicotine from its gut to its haemolymph and can’t exhale the poison into the surrounding air. It pays the price for it.
The hornworm’s nicotine cloud probably works against other predators too. In earlier studies, when caterpillars are reared on tobacco, ants are less likely to attack them. Parasitic wasp larvae are also less likely to survive inside the caterpillars’ bodies, presumably because they are directly poisoned by the nicotine in their haemolymph. But the defence isn’t fool-proof. Kumar’s team showed that two predators—big-eyed bugs and antlions—will kill hornworms despite their halitosis. No one knows why.
Chemical theft is fairly common in the animal world, and many caterpillars store defensive poisons from the plants they eat. For example, the eastern tent caterpillar munches on plants that are loaded with hydrogen cyanide, which it then vomits onto marauding ants. But nicotine is too deadly to store. Instead, the tobacco hornworm has evolved a way of getting rid of it, which also doubles as a potent defence.
Reference: Kumar, Sagar, Pandit, Steppuhn & Baldwin. 2013. Natural history-driven, plant-mediated RNAi-based study reveals CYP6B46’s role in a nicotine-mediated antipredator herbivore defense. PNAS http://www.pnas.org/cgi/doi/10.1073/pnas.1314848111