Field of Science

Stereochemistry by natural selection

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Back in India when I was a school kid, I was obsessed with entomology, and used to spend much of my spare time catching insects and studying them in jars borrowed from our kitchen, to the consternation of my mother. One of my favourite insects was the 'walking stick' pictured above. The pleasure of catching this little critter used to be compounded because of the difficulty of detecting him in the grass; slender and brown, the walking stick is notorious for being able to almost perfectly camouflage himself to match the colour of wood or grass. But catch many walking sticks I did, and they made fascinating creatures for study.

In any case, I never met a walking stick who sprayed me with a noxious spray from his underbelly. Apparently, you do get such walking sticks of the obnoxious kind in the US. And so did grad students and a professor in Florida catch a few sticks and analyse the irritating secretion by NMR. I am sure this process has been done before for other insects, but I doubt whether any of those researchers found such an interesting fact. It turns out that every walking stick individual has a unique mixture and ratio of the stereoisomers of the 10 carbon cyclopentane derivative that packs the punch in their secretion. There's 3 chiral centers, so 8 stereoisomers, which can be mixed together in myriad proportions. It's a fascinating find.

I am guessing that the reason for the evolution of such varying stereochemical cocktails could be the survival of the fittest. What if your predator gradually got used to and immune to your special chemical warfare compound? If I think about it, it seems easier to modify this compound by changing the mixture of isomers in it, rather than manufacture a totally new compound to ward off the predator, a process which would take too much time and energy. Also, this difference could also help the insects keep each other at bay during courtship, mating, or aggression. Again, if everyone had the same composition, they could become immune to it, but with each individual having his own secret formula, his adversary can never guess what is going to emerge out of his underside. What I am interested in knowing is if this composition of stereoisomers changes during the individual's own lifetime. Now that would be evolution at its best. Let me check out the original article.

Update: The paper with the cute name is from Chemical Biology- Single-Insect NMR: A New Tool To Probe Chemical Biodiversity. Interestingly, the authors are chemists to the core. They don't seem to have speculated about the evolutionary or biological significance of their findings.

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