Every so often, I run across some delightfully hideous tidbit of information that makes me think the best horror filmmakers in Hollywood can't hold a candle to the grotesque goings-on in the natural world. Take the aphid. This small insect lives on plants of all kinds, sucking out the sugar-rich sap and generally wreaking havoc on agriculture worldwide. Fortunately for the farmers, and unfortunately for the aphids, these little insects are prime targets for parasitic wasps. And this is where it goes all Alien. A wasp will swoop down from the sky, land next to an aphid, and, in the blink of an eye, twist its body and stab the aphid. It doesn't sting the aphid. It's much worse than that. In that briefest of seconds, the wasp actually deposits an egg instead of the aphid's body. Now it's only a matter of time before the aphid has its John Hurt moment. The aphid will continue on its merry way for a short while, but then, the egg hatches inside its body, and the wasp larva eats the aphid from the inside out to fuel its own development. Eventually, all that is left of the aphid is a brown husk, and the mature wasp emerges by chewing its way out of the mummified aphid corpse. Want to see this in action? Check out this video from Nat Geo below (at 1:18 there is a particularly great shot of a wasp larva moving inside a mummified aphid corpse)..
Okay, that's all well and good (or hideous and terrifying, depending on your view). But what does this have to do with microbiology? Well, as it turns out, the aphids have a hero in this story. It's a bacterium called Hamiltonella defensa. This bacterium has a symbiotic relationship with some aphids. This means that the bacterium lives within the aphid, and the two benefit each other in different ways. Aphids that have these bacteria living inside of them are protected from the parasitic wasps. The wasp will still inject its egg into the aphid, but the bacterium produces a toxin that kills the developing wasp larva. So, through the action of a bacterial partner, the aphid is saved a terrible fate!
This wasp-aphid-bacterium triangle is a great example of coevolution. Wasps evolved a fascinating and gruesome method of producing offspring, and a bacterial partner evolved to defend the aphid. From an evolutionary biology standpoint, this is a great system to help us learn how some toxin-producing bacteria have harmful impacts (the well-known foodborne pathogen Salmonella comes to mind), whereas others, like the aphids' hero Hamiltonella, have beneficial effects. From an agricultural perspective, aphids are well-recognized agricultural pests, and a better understanding of their defenses against natural predators like the parasitoid wasps will help us design more efficient pest control strategies.
If you're interested in more detail, check out the papers below...
This paper describes the genome of Hamiltonella defensa.
This paper discusses the important role of bacteriophage in the wasp-aphid-Hamiltonella triangle.
The wasps aren't taking this sitting down. Here's how they respond.