One of the most curious features of evolutionary biology is the cui bono principle.
Cui bono? is Latin for "who benefits?" and is an idea that found its first expression in courts of law. If a crime is committed, look for who benefitted from it. In evolutionary biology, it's adjuring the researcher to look for an evolutionary explanation for seemingly odd, even self-harming behavior. Somebody, the principle claims, must benefit from it.
A while back, I did a post on one of the strangest and most complex examples of cui bono; the pathogen Toxoplasma gondii, a protist that primarily infects humans, cats, rats, and mice. In each, it triggers changes in behavior, but different ones. It turns rats and mice fearless, and in fact, makes them attracted to the smell of cat urine. Infected cats are more gregarious and needing of physical contact (either with other cats or with humans). Humans are more likely to be neurotic and anxious, impelling them to seek comfort from others... including, of course, their pets. Each of these behaviors increases the likelihood of the pathogen jumping to another host.
That this behavioral engineering is successful can be gauged by the fact that by some estimates three billion people are Toxoplasma-positive. Yes, that's "billion" with a "b." As in, one third of the human population. I can pretty much guarantee that if you've ever owned a cat, you are Toxoplasma-positive.
What effects that has had on the collective behavior of humanity, I'll leave you to ponder.
I just ran into another cool example of cui bono a couple of days ago -- well, cool if you're not a tomato grower. This is another one for which the answer to "who benefits?" turns out to be a pathogen, this time a virus called tomato yellow leaf curl virus, which has the obvious effect on infected plants.
The researchers, led by Peng Liang of the Chinese Academy of Agricultural Sciences, noticed a strange pattern; there's a pest of tomato plants (and many other crops) called the silverwing whitefly (Bemisia tabaci) that shows a distinct preference for tomato plants depending on who is infected with what. If the whitefly is uninfected with the virus, it's preferentially attracted to infected tomato plants; if the whitefly is already infected, it shows a preference for uninfected plants.
So cui bono? The virus, of course. Infected whiteflies pass the virus along to uninfected plants, and uninfected whiteflies pick the virus up from infected plants. Clever. Insidious, but damn clever.
Liang et al. found that the virus accomplishes this by meddling with a chemical signal from tomato plants called β-myrcene. The virus actually up-regulates the β-myrcene gene -- essentially, turning the volume up to eleven on β-myrcene's production -- which attracts uninfected whiteflies. Once the virus gets into the whiteflies, it dials down the sensitivity of the whiteflies' β-myrcene receptors, making them less attracted to it.
No need to be lured in by the infected plants if you're already infected yourself.
So like with Toxoplasma, we have here a microscopic pathogen that is manipulating the behavior of more than one host species. It's fascinating but creepy. You have to wonder what other features of our behavior are being steered by pathogens we might not even be aware of. Recent studies have found that between five and eight percent of our DNA is composed of endogenous retroviruses -- scraps of DNA left behind by viruses in the genomes of our forebears, and which are suspected to have a role in multiple sclerosis and some forms of schizophrenia.
Who knows what else they might be doing?
If you find this whole topic a little shudder-inducing, you're not alone. Science is like that sometimes. If there's one thing I've learned, it's that the universe is under no compulsion to make me feel comfortable. If you agree, sorry I put you through reading this. Go cuddle with your kitty.
I'm sure that'll make you feel better.
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