Friday Science: from Parasite to Mutualist

This is the first in a, hopefully, weekly bi-weekly occasional series of posts in which I will summarize/discuss some new peer-reviewed research that I find interesting. My post on synthetic estrogen and extinction started me thinking that this would be a good addition here. In attempting this, I am in no way suggesting that I am an expert in any of the topics I will be posting, only that I find them interesting and that I hope to be able to present the material in a "chewable" form. I'll also try to restrict my summaries to articles available via open access. Wish me luck.

This week's article that caught my attention is from PLoS Biology and deals with the evolution of parasites into benign or even beneficial organisms. In particular, the researchers show that over approximately 20 years, a particular strain of Wolbachia has evolved in a manner such that it now causes infected female flies (Drosophila simulans) to lay more eggs. Here we have a parasite that used to cause reproductive harm evolving into a "parasite" that helps fly reproduction. Even better, this phenomenon is exactly what evolutionary theory predicts should have happened.

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Wolbachia is a bacterium that infects all sorts of invertebrates, particularly insects. Individuals that are infected with Wolbachia show decreased reproductive success - partly through a process called cytoplasmic incompatibility (CI). CI basically means that the gametes of an infected individual are incapable of (or less successful in) fusing with the gametes of uninfected individuals. Thus, infected females can only breed with infected males.

Another interesting thing about Wolbachia is that unlike most transmissible parasites, it is passed only from mother to offspring via infected eggs. Such "vertical transmission", combined with CI, can cause a rather rapid spread of the parasite within a population. And this is exactly what has been seen in the population of flies in this study. In the mid-1980s Wolbachia-induced CI was discovered in a population of D. simulans in southern California. Since that time, the Wolbachia infestation has spread north through California and is "now pervasive throughout most North American populations of D. simulans."

Notice though that there are two conflicting aspects of Wolbachia infection: 1) Wolbachia is only passed from mother to egg, yet 2) Wolbachia infections cause females to lay fewer eggs. Evolutionaryly speaking, this set-up is not in the bacteria's (nor the fly's) best interest. Under this type of vertically-transmitted infection scenario, evolutionary theory predicts that the parasite will become less virulent harmful over time. At worst, the parasite should evolve so as to have no affect on female fecundity (just hitching a ride on the eggs). At best, theory predicts that the parasite should come around 180° and become a mutualistic partner by actually increasing female reproduction.

Sure enough, 20 years after the initial "outbreak", Wolbachia infection in D. simulans now has a positive effect on female fecundity. Originally, infected females laid 20% fewer eggs compared to uninfected females. Now infected females lay 10% more eggs than uninfected females. The authors also point out that CI levels and transmission rates have stayed constant, so the fecundity effect is not a result of depressed CI or bacterial transmission.

So, is this change in fecundity an effect of an evolving bacterium or an evolving host (or both)? To test this, the authors essentially infected flies with the old strain of the Wolbachia (that had been kept in culture for the past 20 years?) and found that the fecundity advantage disappeared. Thus, the current fecundity effect appears to be mostly a result of the evolution of the bacteria.

Ah, but what if the current Wolbachia strain that is causing the fecundity advantage is actually a more recently introduced strain and not the direct evolutionary lineage of the old strain? To examine this possibility, the authors tested the CI levels between the current strain and the old strain. If they were two different strains, flies infected with one would be reproductively incompatible with flies infected with the other. This was not the case, supporting the idea that the new Wolbachia strain is an "evolved version" of the old strain.

I found this study fascinating for a number of reasons. First, the design and the thoroughness of the experiments seem flawless to me (given, of course, that I am neither a fly biologist nor a bacteriologist) - the authors seem to have covered all the appropriate ground, showing that indeed the effects they witnessed are the result of bacterial evolution. Secondly, the evolution of this strain of Wolbachia is a great example of how quickly natural selection can work given conducive conditions. Thirdly, this paper shows just how testable (and accurate) predictions from evolutionary theory are.

Based on what is known about evolution by natural selection, coupled with a firm understanding of the biology behind a given natural system, we are able to make hypotheses, design experiments (and/or make the necessary observations to test these hypothesis, and come to scientifically tenable conclusions.

What more could you ask for?

Citation: Weeks AR, Turelli M, Harcombe WR, Reynolds KT, Hoffmann AA (2007) From Parasite to Mutualist: Rapid Evolution of Wolbachia in Natural Populations of Drosophila. PLoS Biol 5(5): e114 doi:10.1371/journal.pbio.0050114