Why is there sex? To fight the parasite army

By Carl Zimmer | July 7, 2011 2:02 pm

For several decades now, biologists have been puzzling over sex. In some ways, it seems like a huge waste of effort.

Sexual reproduction requires splitting a species into two sexes, only one of which will be able to produce offspring. There are some species of animals that do without males; the females simply trigger their eggs to develop into embryos without any need for sperm. All the offspring of an asexual animal can produce offspring of their own, instead of just half. So it would make sense that genes that gave rise to asexual reproduction would win out in the evolutionary race.

Clearly that hasn’t happened. The world is rife with sex. Animals do it. Plants do it. Even mushrooms do it. So evolutionary biologists have carried out a number of studies to get an answer to the question, “Why sex?”

In 2009, I wrote an essay for Science about this research. If I had been writing that essay today, I’d have focused some attention on an elegant experiment on the sex life of a humble worm. It gives a big boost to the long-floated idea that evolution favors sex because it lets hosts fight better against parasites.

Allow me to explain by self-plagiarizing:

In the 1970s, several researchers built mathematical models of how parasites influenced the evolution of their hosts and vice versa. Their research suggested that both partners go through cycles of boom and bust. Natural selection favors parasites that can infect the most common strain of host. But as they kill off those hosts, another host strain rises to dominate the population. Then a new parasite strain better adapted to the new host strain begins to thrive, leaving the old parasite strain in the dust.

This model of host-parasite coevolution came to be known as the Red Queen hypothesis, after the Red Queen in Lewis Carroll’s book Through the Looking Glass, who takes Alice on a run that never seems to go anywhere. “Now here, you see, it takes all the running you can do to keep in the same place,” the Red Queen explains.

The Red Queen conundrum, some researchers have argued, may give an evolutionary edge to sex. Asexual strains can never beat out sexual strains, because whenever they get too successful, parasites build up and devastate the strain. Sexual organisms, meanwhile, can avoid these dramatic booms and busts because they can shuffle their genes into new combinations that are harder for parasites to adapt to.

Red Queen models for sexual reproduction are very elegant and compelling. But testing them in nature is fiendishly hard, because biologists need asexual and sexual organisms that share the same environment and parasites.

Scientists have found some mixed populations in the wild where they’ve made some important discoveries. But it’s also possible to test the Red Queen in laboratories. It’s not easy, because scientists need to bring together a host that can reproduce sexually and asexually with a parasite, and then they both have to be able to evolve in response to each other. But that’s what a team of scientists at Indiana University managed to do recently.

As they describe in a paper published today in Science, they reared populations of a tiny worm called Caenorhabditis elegans. C. elegans are born either as males or hermaphrodites. A hermaphrodite worm can fertilize its eggs with its own sperm, or it can seek out a male. The worms typically don’t have a lot of sex, and the rate at which they do is partly programmed into their genes. The Indiana team of scientists were able to engineer the worms so that they could have no sex at all, or could only reproduce through sex.

For their parasite, they chose a species of soil bacteria called Serratia marcescens. Soil bacteria are the regular prey of C. elegans, but if they slurp up S. marcescens by accident, they get sick and can die in under 24 hours. Previous studies had shown that the worm can evolve stronger resistance to the germ, and the germ can evolve to be deadlier for the worm. So the Indiana researchers set about combining their evolution into one big experiment.

They mixed together worms and germs in several different arrangements and let them duke it out for 30 worm generations. In each trial, the worms were either sexual or asexual.  In some trials, the bacteria coexisted with the worms for the whole experiment, so that they could evolve along with the worms. In other trials, the worms were repeatedly presented with the same, fixed strain of S. marcescens. In other words, the bacteria could not evolve. And in control experiments, the worms enjoyed a Serratia-free life.

As this graph to the left shows, the asexual worms that faced co-evolving germs were annihilated in just 20 generations. (“Obligate selfing” means no sex.) If the germs couldn’t evolve, however, the asexual worms did fine. The scientists also tested the bacteria for deadliness after the experiments were over. They found that the bacteria that were allowed to co-evolve with the asexual became much deadlier. The co-evolving sexual worms, on the other hand, suffered far lower mortality rates from their germs.

In another experiment, the scientists started out with ordinary worms, which only had sex about 20 percent of the time they reproduced. Again, they exposed the worms to unchanging bacteria, or co-evolving ones, or no bacteria at all. The graph to the right says it all. The worms not exposed to the bacteria went on having infrequent sex. The worms that could evolve but faced fixed bacteria had more sex for a while, but eventually crashed back down to their original levels. The coevolving worms, on the other hand, became mostly sexual.

In each of these results, the Red Queen has left her mark. Far from being a waste of time, sex may save organisms from a swift oblivion.

[Images: turtles via Creative Commons from man of mud/Flickr. C. elegans via Creative Commons licence from AJ Cann/Flickr.]

(Update: paper link fixed)

Comments (31)

  1. A very elegant study on the function of sex. Thank you for summarizing it so well.

  2. Have you read this paper arguing that sex reduces genetic variation? It showed up in my RSS feed via a news release right after your blog here, and the juxtaposition struck me. I understand the Red Queen hypothesis and what you’ve written makes sense. I’m not sure that the Evolution paper necessarily contradicts the argument, but I don’t have access to read the full thing either and even the summary doesn’t make a lot of sense to me.

  3. I concur with Patrick – this is an excellent summation of what sounds like a truly beautiful study. It’s rare in science to see such a big question tackled in a way that’s so immediately understandable and gives such clean results. I’m surprised no one tried to do an experiment like this before, but then again, isn’t that what people say about every great scientific result?

  4. zackoz

    This is an enchanting result, and as usual, so well described; I wonder how you could replicate it with other partly asexual species.

    If I recall my reading correctly, it is W D Hamilton who is usually associated with the parasite theory of sex, is it not? He would have liked this experiment.

    The next step has to be to explain the apparently completely asexual bdelloid rotifers; a much harder task, I gather.

    I loved the phrase “obligate selfing”. It reminds me unconfortably of me as a teenager (the “no sex” bit, not the reproduction).

  5. Domingo Zungri

    As far as I know, the human species is the only one I know about that has sexually transmitted diseases. Does this mean that we are having too much sex for our own good?

    [CZ: Many other species have sexually transmitted diseases, too. Presumably the sex itself helps all of them fight of STD's too.]

  6. This particular example relates sex to the presence of co-evolving parasites, but can you elaborate on what is special about parasites here – as opposed to any other co-evolving predator, or competitor (or any other source of changing environmental pressure for that matter)?

    [CZ: Other species can pose a more intense challenge than an environmental condition like climate. A parasite can evolve in response to a new host defense, while the climate does not. Parasite pressure is particularly intense for several reasons. For example, a single virus can produce billions of new viruses in a matter of days, speeding up their rate of evolution. ]

  7. barnyfife

    Nothing evolved you morons. God created everything and created it the way it is today. Only brainwashed fools believe in the myth of evolution.

  8. AuntBea

    LMAO at barnyfife.

  9. NoAstronomer

    “Sexual reproduction requires splitting a species into two sexes, only one of which will be able to produce offspring.”

    Err is that really true? Many plants for example have both male and female parts.

  10. MarshelindaI

    I found this article very fascinating. Sex is very important to nature. What would happen if sex never existed..well none of us would be here. Sex causing generation after generation to be created then eventually died and more keep coming

  11. Tomek

    Domingo, just a heads up, “As far as I know, the human species is the only one I know about that has sexually transmitted diseases. Does this mean that we are having too much sex for our own good?”

    Other species have them. The only immediate one I can think of is “SIV,” Simmian Immunodeficiency Virus. The thing is, it’s not sex that makes diseases transmittable, it’s the fact sex involved body fluids. If you’re taking about genital-related parasites, infections, etc, then that’s something different. I don’t really know.

  12. Rachel Kowal

    This report on sex has given me a lot of insite into how scientists work out theories and come to a conclusion. I think this article has not only given me a better understanding of sex in animals it has also taught me how to work out problems and questions in the classroom.

  13. JEQP

    CZ: Other species can pose a more intense challenge than an environmental condition like climate.

    That being said, climate tends to induce a “directional” evolution, and so do predators. Wouldn’t parasites just encourage change? So if the host changes physiologically to inhibit a parasite and the parasite changes to take advantage of the new host, the host changing back to the original configuration can be just as useful in inhibiting the parasite. So parasite would encourage the evolution of the ability to make small but significant changes rapidly. Which I guess is one of the main benefits of sexual selection…I think I painted myself into a corner.

  14. deekmon

    Read What’s Love Got to Do With It” by forensic anthropologist Meredith F. Small
    “Modern men and women are obsessed with the sexual; it is the only realm of the primordial adventure still left to most of us.”

  15. tarflux

    Marshelinda and Rachel – You have to put your comments on Dr. H’s blog, not here.

  16. Vivek

    A very interesting question this work raises is that why arent there 3 genders. That would ensure a lot of variation in genes. What is so universal about 2 genders ?

  17. Mom w questions

    Survival of the species is important. The next question is- is it love or just hormones at work for our DNA?

  18. erpiu

    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 ! ;)

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The Loom

A blog about life, past and future. Written by DISCOVER contributing editor and columnist Carl Zimmer.

About Carl Zimmer

Carl Zimmer writes about science regularly for The New York Times and magazines such as DISCOVER, which also hosts his blog, The LoomHe is the author of 12 books, the most recent of which is Science Ink: Tattoos of the Science Obsessed.

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