Since plants generally can’t move around, they have to rely on other strategies to cope with animals eager to turn them into a meal. Chemical weapons are a significant part of plants’ defensive arsenal. For example, thousands of plant species produce precursors of the deadly poison hydrogen cyanide; when an animal eats the plant, the precursors get converted into cyanide, which kills the offending animal.
Of course, herbivores have developed their own counter-strategies in response — after all, they’ve got to eat! Some insects have worked out how to safely store the cyanide precursors from their food, which they then use to defend themselves against predators! In fact, cyanide plays such an important role in these insects’ lives that some, like the burnet moth have even evolved to synthesize it themselves. Other insects have come up with a much more straightforward solution to the ‘poisonous food’ problem; they’ve evolved chemical machinery to detoxify their meals by converting hydrogen cyanide into a harmless molecule.
While we’ve long known that insects can detoxify cyanide, the identity of the molecular machinery with which they accomplish the feat has remained a mystery. In a paper published earlier this year in eLife, a team of researchers identified the enzyme responsible for the detoxification and the gene which encodes it. To do this, they transferred red spider mites that had been feeding on string bean plants (which don’t produce cyanide) to a strain of lima bean plants which defend themselves with cyanide. The team gave the mites plenty of time to adapt to their new chow — 35 generations! — and then they used modern sequencing techniques to identify which genes were more or less active on lima beans. From there, it was just a bit of sleuth-work to identify the detoxification gene.
It turns out that the really interesting bit isn’t which gene converts cyanide into a harmless form; it’s where that gene came from. The team surveyed the published genomes of mites and ticks for sister copies of the gene, but they only found it in two closely-related species. Casting a wider net, they searched arthropod genomes; arthropods include mites, ticks, and spiders (arachnids) as well as insects, crustaceans, and centi-/millipedes. This turned up a collection of sister genes in a handful of butterfly and moth species. How did this gene end up in such distantly related groups — mites and moths — but not in other insects or arachnids?
According to the paper, the two groups got it from the same place: they picked up the gene from bacteria. The bacteria might have been living on a plant that was munched on by a mite (or a butterfly), or they may have been symbiotic bacteria living inside the critter, but at some point a few hundred million years ago, the detoxification gene hopped over from the bacterial genome into the herbivore’s. The researchers can’t tell exactly how the transfer happened; the gene may have been transferred from bacteria to just one group of arthropods, which then passed it on to the other, or there may have been two independent transfers. Either way, it’s a pretty remarkable finding! ‘Horizontal gene transfers’ like this are much more common than people realize, and they can have a significant ecological impact, as the study shows. In this case, as the article’s summary eloquently points out, “the lateral gene transfer from bacteria to animals is a remarkable coalition of two kingdoms of life against another”.
Wybouw N, Dermauw W, Tirry L, Stevens C, Grbić M, Feyereisen R, & Van Leeuwen T (2014). A gene horizontally transferred from bacteria protects arthropods from host plant cyanide poisoning. eLife, 3 PMID: 24843024