Sex And Parasites
    By Gunnar De Winter | July 10th 2011 07:12 AM | 7 comments | Print | E-mail | Track Comments
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    Sex is costly. Yet it is widespread throughout the animal kingdom, so there must be some advantages to it. And still, it seems easier to list disadvantages. Sexual reproduction is complicated, requires more time and uses more energy than its asexual counterpart. Partners have to find each other and coordinate their activities to produce the next generation. Another problem is the so-called ‘cost of meiosis’, meaning that, in sexual reproduction, only half of the genome is passed on. Compared to an asexually producing individual which passes on its entire genome, this is a high cost indeed. Another cost is the production of males that will not all succeed in reproducing, and thus waste resources.

    So, the costs are substantial. How are they offset? There are several ideas about the answer, most of which are focused around the production of genetic variation. One popular possible answer, is the 'red queen hypothesis'. This hypothesis states that, due to the genetic variation that arises through sexual reproduction, the offspring of sexually reproducing organisms will be more able to deal with parasites. These parasites, in turn, will co-evolve with their host organisms, resulting in an ‘evolutionary race’ between both species, which will have to run (evolve) as fast as they can just to stay in the same place. (Hence the name of the hypothesis, after the eponymous character in Lewis Carroll’s Through The Looking Glass, that said: “It takes all the running you can do, to stay in the same place", see figure 1.)

    Figure 1: The red queen and Alice, from Lewis Carroll's Through The Looking Glass.


    Now, a research team at Indiana University has tested this hypothesis with laboratory populations of the popular microscopic roundworm Caenorhabditis elegans (see figure 2) and a pathogenic bacteria, named Serratia macrescens. With these two organisms, the researchers generated a host-parasite co-evolutionary system in a controlled environment. The populations of the roundworm were engineered to exhibit sexual reproduction, asexual reproduction, or a mix of both. These populations were subsequently exposed to the parasite, which was either allowed to co-evolve, or prevented to do so. 

    Figure 2: Caenorhabditis elegans.


    The results show that:

    •  Self-fertilizing (asexually reproducing) populations of C. elegans were rapidly driven to extinction by co-evolving parasites.

    •  Sexually reproducing populations were able to adapt to their co-evolving parasites, avoiding extinction.

    •  The mating system in the mixed populations depended on the evolutionary state of the parasite. If the parasite was not allowed to co-evolve, asexual reproduction arose as the dominant mating system. If the parasite did co-evolve, sexual reproduction was favored.

    This extraordinarily elegant study thus provides some experimental corroboration for the red queen hypothesis.


    Morran, L.T.; Schmidt, O.G.; Gelarden, I.A.; Parrish II, R.C. and Lively, C.M. (2011).Running with the Red Queen: Host-Parasite Coevolution Selects for Biparental Sex. Science. 333(6039), pp. 216 – 218. Doi: 10.1126/science.1206360.


    Danna Staaf
    Oh man, that's a pretty rad study! One thing jumps out at me, though: from the parasite's point of view, it's a really bad idea to drive your host to extinction. It's that optimal virulence thing. So I wonder what's going wrong with S. macrescens in these scenarios?
    Insightful remark, Danna.      
    But I think that the bacteria have a certain virulence, and the asexually reproducing worms are just not able to generate enough genetic variation to cope with it (in Red Queen terms, they just can't run fast enough). It's not as if the bacteria go 'Wait a minute guys, they can't handle it, let's slow down a bit.' There's no long-term thinking in bacteria. They just go about their business, and, in this case, that will result in the extinction of their host. Basically, among asexually reproducing little worms, they might be too virulent to allow their own evolution towards less virulence.
    Danna Staaf
    Splendid point about the lack of foresight in evolution! But another important point about evolution is that almost nothing is immutable--that is, bacteria aren't stuck with a certain level of virulence. I suspect virulence is in fact one of the more fluid aspects of a bacterial phenotype--but I'm not a pathologist. =) Anyway, if the bacteria are truly co-evolving, their virulence ought to be evolving along with them. I.e. if some bacteria are so virulent that their hosts die before they can reproduce, then those bacteria will be outcompeted by less virulent strains that allow their hosts to survive and spawn.
    I suppose your point might be that the initial strains of bacteria were so virulent that there's simply no time for this to happen. Could be.
    Gerhard Adam
    I also suspect with a confined population the virulence would lead to extinction whereas in a natural environment, geographic distribution might play a more significant role.  It's similar to the virulence displayed by the Ebola virus, which may wipe out a village, but doesn't wipe out all the other population centers. 

    On the other hand, we could also make the same argument about why humans (who are capable of thinking) invariably reproduce to the point of starvation.  This also includes most organisms, so in one sense, we could make the same virulence argument that it doesn't really make sense to reproduce to the point of decimating yourself, but apparently it works well enough so that large enough population sizes spread over a geographic area provide a sufficient degree of protection to prevent this from being a regular event.
    Mundus vult decipi
    Screw 'Sustainability' - And Cheer Up About It - Howard Bloom has sage wisdom
    Gerhard Adam
    Actually he's wrong in a fundamental way.  If one recognizes limits, then one can prepare and grow to accommodate those limits.  In those instances you will experience expansion.

    If you pretend that such limits don't exist, then you simply expect that a portion of the population will act as the poor S.O.B.'s that have to absorb the problems and then it becomes a distraction to move towards expansion because there are too many remedial actions that have to be taken because we were unprepared.

    The common mistake here is that it confuses sustainability with remaining static.  Sustainability is "living within your means".  This is the cry of all the Republicans these days ... that government must learn to live within its means ... OK ... well that's sustainability.  Basically it means that if your means increase, then so can your lifestyle.  Anything else will extend you beyond your ability to maintain your lifestyle.  While that might sound cute to say "Screw Sustainability", it's isn't nearly as cute when it becomes time to pay the bills or file for bankruptcy.
    Mundus vult decipi
    yeah. these ingenious experimentalists crippled first the worms with a hefty dose of mutagens so that the selfers became immediately homozygous for all kind of nasty mutations (while the sexuals purged the mutations through segregation). then the bacteria had a great time trashing the crippled selfers that for sure started with compromised innate immunity, but had a much harder time with the outcrossers since these started with strong wildtype innate immunity. chapeau to the authors though, since i too would do anything to get published (and avoid perishing)! and congrats to the hyper-expert reviewers ! ;)