Parasites: can they really be that bad?
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Parasite! The very word is an insult. But is that entirely fair to parasites? Have parasites just had a bad press? It’s not a new idea that some parasitic side-effects can be beneficial. Now work by evolutionary geneticist Andrew Weeks at the University of Melbourne in Australia demonstrates some truth in the hypothesis that parasitism may just be a temporary aberration on the journey towards a mutually beneficial symbiosis.
Weeks and colleagues studied the interaction between wild Drosphila fruit flies in California (shown above) and a bacterium called Wolbachia. Wolbachia is a bacterium that lives in the cells of various arthropods, and is passed down from the mother to her offspring. The bacterium is generally not a nice one for the host – causing reproductive disruptions ranging from a problem called ‘cytoplasmic incompatibility‘ – causing the death of embryos – to a bewildering diversity of additional effects, such as turning males to females, causing infected females to reproduce without males and triggering vicious cycles of increasing female promiscuity and male sexual exhaustion.
Weeks proposed that the Wolbachia (above) could not be all bad, otherwise it would selectively drive infected Drosophila towards extinction, which would not benefit the parasite:
We had a very thorough theoretical analysis which suggested that this could and should evolve, but we had no idea of the timeframe that this might take (Livescience)
Scientists have studied the Californian population for twenty years, and so Weeks had some useful baseline data to work on. His findings revealed that beneficial adaptations were indeed occurring, and much faster than had been anticipated. Twenty years ago the parasite reduced the fertility of infected female flies by 15 to 20 percent under laboratory conditions. After what must have been pretty arduous research that involved counting more than 200,000 fruit fly eggs by eye under a microscope, Weeks and colleagues discovered that today, the parasite causes infected females to display an average 10 percent boost in fertility. So not only has the deleterious effect been neutralised, there is now actually an increase in egg production in infected flies. Such a dramatic evolutionary change is traditionally thought to take place over thousands to millions of years, and not in just two decades. Weeks said:
We just didn’t expect it to happen so quickly… it is becoming clearer that evolution does work on such short time scales. (Livescience)
The implications are obvious. In two rapidly adapting species such as Drosophila and Wolbachia this effect is quite noticeable because of the quick reproductive time scale. But there is no reason to think that something similar could not be occurring more slowly in other species, even our own. It’s an alluring and intriguing idea to think that in a few centuries parasites which plague us today, such as malaria or trypanosomiasis, might have evolved along with us to be less harmful – or even beneficial.
