Saturday, October 9, 2010

The parasite within

I came across this ant one day as it sat still, refusing to be disturbed, on a leaf looking upwards in a remarkable posture and passed it. Returning back two hours later, I found it in the exact same place and posture. It was not dead, for the antennae were mobile and the gaster appeared distended and she probably was a species of Camponotus.

Praying - a parasitized ant ?
Ants are puzzling and captivating. I have no clue what made this ant go into a meditative state, but ants are successful beyond any human definition. And behind every successful organism is a slew of piggy-backing hitchhikers that include parasites that live on individual ants and unwelcome house guests that live off the labours of the colony. Some better known parasites of individual ants are fungi and nematodes. Nobody has yet found the adaptive function of the mind-altering Psilocybin in mushrooms but fungi in the genus Cordyceps and Ophiocordyceps make ants go quite crazy. An ant in a late stage of infection climbs up a twig and sinks its mandibles into the plant, before going into terminal coma after which the fungus emerges to produce its own "fruiting body" - from which arise new spores that will infect another generation of ants. The behaviour of the ant is modified by the fungus to its own benefit. It was thought that "driving" the ant higher up into the vegetation helped the dispersal of the fungal spores but some later research showed that the height was related to the best humidity and temperature conditions for the spores to germinate !
Cordyceps emerging out of a caterpillar

Species of Cordyceps  infect a range of insects. Up in the Himalayas, one species infects caterpillars of an as yet undetermined Hepialid moth (or possibly multiple species of them). The moth caterpillars emerge in the spring on high altitude meadows and some of them get infected by the fungus and a grotesque horn-like fruiting body emerges from the body of the dying caterpillar. Whether the fungus alters the behaviour of the caterpillars is unclear but it makes young men in the mountains across South Asia go mad - Yaartsa Gumbu - as this is known is collected by fit young men who climb up into the high-altitude regions across the region to harvest the caterpillars which are then sold to Tibetan and Chinese traders for an amazing fortune to be used in Chinese medicine. Like parasites, governments, Maoist quasi-governments and mafia in the region attempt to get a share of the enormous money (transactions are said to be made right on the high altitude meadows!) made in the harvest.

Coming back to our ants, their long evolutionary history has allowed them to adopt counter-measures. In some colonies, workers detect behavioural abnormalities among their fellow nest-mates. Any ants that are detected having perhaps a peculiar scent (members of each colony have their own characteristic scent) are quickly sent out of the nest by "bouncers".

Steve Yanoviak and T M Mushtak Ali at UAS, Bangalore
In 2009, some of  us in Bangalore had the good fortune of meeting up and listening to Steve Yanoviak Steve does his research high up in the canopy of the forests of South America and has discovered ants that glide and a bizarre nematode that infects ants. This nematode turns the gaster of an ant from its normal black colour into a large red-cherry like structure. These infected ants walk around with their berry-like hind ends raised and get eaten by  fruit-eating birds. Other ants, foraging for seeds on the forest floor pick up the droppings of these birds and continue the cycle of the nematode.

One of the greatest evolutionary thinkers, W D Hamilton had a knack of thinking about the strategies of organisms by putting himself in their "shoes". A successful parasite for instance should perhaps never kill its host, but manipulate its host to aid its own multiplication and to that end it could even "enhance" the behaviour of its host. In that light, the strategy of a virus like Ebola that produces near 100% fatality has to be rather bizarre - suggesting that humans are not the species with which they evolved. In 2000, some researchers found that rats infected by Toxoplasma acted rather oddly making them more likely to be preyed on by cats, the primary host of the parasite. This got others thinking if human behaviours (particularly "disorders") are in any way manipulated by parasites and infections and some have pointed out that some anti-psychotic drugs actually had the ability to kill parasitic organisms, particularly Toxoplasma and others have found bad drivers to have latent infections !

Now I am not sure what parasite got me to write this, but if you have some "good" viruses of the mind, make sure you spread them.

Further reading

4 comments:

  1. Really interesting Shyamal, the range of control and sophistication of parasites is amazing from DNA modifying bacteria Wolbachia to the bite-inducing Cordyceps! Thanks for this & "Yeh dil mange more".

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  2. Glad you enjoyed. Yes, after a while you wonder what the definition of a parasite should be !

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  3. Thank you, Shyamal, for a VERY informative and interesting write-up! It is like a Discovery channel documentary in text form! I'd like to post this on my Facebook page - I hope that's ok with you. Looking forward to more!

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