October 2, 2010
Parasitism vs egotism
“I can’t believe it’s a vestigial organ”
Now just hang on a minute there.
I was in the middle of my viva voce, a pre-graduation interview in which an external examiner ensures you’ve not blagged your way this far (as a serial blagger, this was actually quite worrisome). We were discussing my dissertation, and in particular a feature of the parasite I studied, the function of which was still uncertain. A couple of references had suggested it was vestigial, but the examiner was unconvinced. I took this as my cue to spring to the defence of parasites.
In his excellent book Parasite Rex, Carl Zimmer describes how these organisms have been much maligned, even by biologists, over the years. He quotes 19th century zoologist Ray Lankester’s view that they were little more than degenerates, abandoning the adaptations natural selection had crafted in their ancestors and seizing the chance to become featureless nutrient-suckers: “Let the parasite life once be secured, and away go legs, jaws, eyes and ears; the active, highly gifted crab may become a mere sac, absorbing nourishment and laying eggs”. Lankester’s essay on degeneration led another writer to describe parasitism as “one of the gravest crimes in nature”, which, if nothing else, gave a whole slew of sexual activities a brief break from moralistic denunciation.
This attitude continued well into the 20th century. Zimmer quotes parasitologist Horace Stunkard; “the dependent animal is proverbially looking for the easy way” and Konrad Lorenz; “If one judges the adapted forms of the parasites according to the amount of retrogressed information, one finds a loss of information that coincides with and completely confirms the low estimation we have of them.” to highlight the ongoing view of parasites as lazy, degenerate and feeble. The rest of his book then goes on to show how utterly wrong this viewpoint is, that parasites “are complex, highly adapted creatures that are at the heart of the story of life”. If nothing else, we probably have them to thank for the existence of sex – were we somehow able to pin pizza and beer on them too, I’m pretty sure I could get a parade going in their honour.
So, anyway. For my dissertation I studied the effects of host starvation on the parasite Schistocephalus solidus, a tapeworm that has rejected the simple life in favour of a rather more complicated state of affairs, growing in two different hosts (a freshwater copepod, then a stickleback) before reproducing in a third (a fish-eating bird). As with the other parasites described by Zimmer, Schistocephalus is no passive drain on its host’s hard-won resources. There is, for example, evidence of what may be active manipulation of host behaviour to increase the chances of transmission to the next host in the cycle: infected copepods show a preference for fish odour – risky behaviour, to say the least – and infected sticklebacks are less alarmed by, and recover more quickly from, simulated bird attacks.
I studied Schistocephalus in the copepod host (pictured in situ here). When the worm has matured and is capable of infecting a stickleback, it has an obvious circular structure at one end, called a cercomer. It’s so strongly associated with the ability to infect the next host that it’s generally thought to help in establishing the parasite in the stickleback’s body, but exactly how is unknown. Indeed, a recent paper has shown that the cercomer is actually discarded by the parasite early in the infection process, and it has been suggested by JD Smyth – a man who, frankly, knows his cestodes – that it might in fact be vestigial.
The external examiner in my viva wasn’t sure about this. And so I talked about how complex these creatures actually were, so well-adapted to multiple hosts and multiple environments, surviving passage through mouths and stomachs and able fight off the immune systems of three different animals. About how it wasn’t too long ago that some biologists could barely bring themselves to acknowledge that parasites had even the most rudimentary adaptations, something that turned out to be completely wrong – why, then, could they not have vestigial organs, too? In my head, I may even have heard some stirring music begin to swell as I fought the good fight for the dignity of parasites everywhere. I know, I know, just be glad you’re not me.
As a postscript to this tale, I downloaded the Timetree app the other day and, being a massive nerd, decided to see how closely related we are to Schistocephalus solidus. According to Timetree it’s about 650 million years since we shared a common ancestor. Next I checked the copepod, and was somewhat startled to see that almost 1 billion years separated humanity and the humble crustacean. I’m not sure this is actually accurate – a quick bout of googling suggests both are equally related to us – but the point is that it felt instinctively wrong. How could it be that we were nearer to the tiny, hermaphroditic, boring worm that hung around doing nothing but suck up nutrients all day, rather than the independent, sexually reproducing copepod; the mobile, active hunter? It was, of course, simple human egotism, the sort of thing that encourages us to see kinship in the majestic likes of tigers and sharks and not so much in dung beetles. We want to be associated only with what we perceive to be admirable, and not with what we believe to represent laziness and degeneracy. Even I, arch-parasite defender, was not immune. Perhaps there’s hope for me yet.
