Sunday, 28 October 2007

Everyone loves parasitoids

The latest post by Bioephemera kick-started my sluggish brain ...

I've talked about parasitoids to the first years but they don't seem keen on them. I'm not sure why. Animals that grow inside their hosts, eating out the viscera, keeping them alive until the point where they no longer have a use, then bursting out to leave a dying husk ... I mean, what's not to love?

I even mentioned that the Alien's life cycle was loosely based on that of the ichneumon wasp. But I'd forgotten that the Alien sequence started in 1979 and concluded in 1997 and that only the first two (Alien, Aliens) paid much attention to the biology. Unless they were movie buffs, most of the first years didn't have any idea what I was talking about.

Ichneumon wasps and their relatives lay eggs on or in a host — usually a caterpillar or other insect larva. When the eggs hatch, the baby wasps dine on the plat de jour until they are ready to pupate. The Alien life cycle was slightly more complicated — the adult laid an egg, out of which hatched a 'face-hugger' that implanted the next stage into the body of a host. This is where the real and fictional life cycles run parallel (for a short while). After the Alien larva emerged ... well ... burst out ... it scuttled off somewhere to grow into a carnivorous adult.

There are a few problems with this. By the time the larval critter was done with John Hurt, it had grown to quite a size. How could it have hung around in his viscera without causing discomfort? Maybe it was feeding on his liver? I dunno. I'm sure there are entire sites on the Intaweb that discuss this very point but I'm not that sad concerned about it.

The other point is that it metamorphosed into an adult of quite a different size. When it played peek-a-boo on the operating table it was only a couple of hands long. But when it reappeared in the gloomy, dank bowels of the Nostromo it was ten times the length it had reached in the gloomy, dank bowels of its host. That's a hell of a growth rate. When ichneumon pupate, there's a lot of rearranging and reconstructing but no major size difference between larva and adult.

Now, these could be dismissed as the ramblings of a too literal mind. But my point — and there is one, because I've just thought of it — is that there are more interesting life cycles than that of the ichneumons. And by interesting, I mean rife with dramatic possibilities.

All those blood-soaked, gore-globbed eruptions are passé — the weirdest involve brain-manipulation. (Okay, and a little gore.)

Some of those fiendish wasps have it down to a fine art, creating zombie hosts that are compliant in their grisly fate. Presumably, the cockroaches don't know what awaits them, but what if the wasps controlled something sentient?

Horsehair worms (Nematomorpha) are powerful persuaders too. When a host accidentally ingests the egg, the larva grows inside until it's ready to emerge. No surprises there ... except it's aquatic, so must encourage the normally-water avoiding host to go for a swim. How does it do that? Of course! It messes with the poor sap's brain. Once the host has plunged into the drink, the adult worm bursts forth. What might it feel like if you were compelled to do something that would almost certainly kill you ... but just as there was some hope that you would survive out pops a wriggling, cuticle-covered surprise. Just a thought.

Even parasites, which don't usually kill their definitive host (the species they occupy as adults), often act as brain-manipulating parasitoids in the intermediate host. In many cases, the parasite arrives in its definitive host through the digestive tract, so there is a huge advantage in increasing the opportunities for consumption. Making the intermediate host bleedin' obvious is the way to go.

Once the fluke Dicrocoelium gets into its ant host, it influences the ant to climb up grass stems and stay there. It won't come as a shock to you that Dicrocoelium's definitive host is a grazer. Similarly, woodlice infected with acanthocephalan (thorny-headed worm) larvae hang about in the open, just asking to be eaten. When the starlings oblige, the worms can finish their development.

But brain-manipulation is not confined to animals. Fungi and protists practice it as well. Organisms with no brain or nervous system can control those of others. The diverse fungus genus Cordyceps infects insects by settling on the exoskeleton and then invading through the spiracles (openings of the respiratory systems). Once inside, it spreads through the body, eventually, taking over the whole animal. By that stage, it is far to late for the host but the fungus is having the time of its life. (Not that all Cordyceps are fun-loving fungi — some species infect truffles. No, I don't get it either.) Somehow, Cordyceps draws nourishment from the host's tissues but saves the brain until last. On the host's death, the fungus sends out a spore-bearing fruiting-body that looks for all the world like a battle standard.

So what's the moral of this tale? Parasitoids are cool? Don't forget to wash your hands? Not bad, but how about this — nature really is weirder than fiction.


Shorty CreeKI said...

A top post, snail.

Snail said...

It's true! Everybody does love parasitoids.


Reading this made me thirsty. I am dying for a beer!

sarala said...

Now human parasites. Those are cool. They don't explode like in Alien and don't make human zombies but still. . . . I almost majored in Medical Microbiology in college. Got to take useful classes like Epizootiology. Try saying that one fast three times.