If You Like Sex, Thank a Parasite

The Indiana University News Room informs us: Sex, as we know it, works thanks to ever-evolving host, parasite relationships, IU biologists find. Here are some excerpts, with bold font added by us:

It seems we may have parasites to thank for the existence of sex as we know it. Indiana University biologists have found that, although sexual reproduction between two individuals is costly from an evolutionary perspective, it is favored over self-fertilization in the presence of coevolving parasites. Sex allows parents to produce offspring that are more resistant to the parasites, while self-fertilization dooms populations to extinction at the hands of their biological enemies.

This is good to know. We never cared much for self-fertilization. Let’s read on:

The July 8 report in Science, “Running with the Red Queen: Host-Parasite Coevolution Selects for Biparental Sex,” affirms the Red Queen hypothesis, an evolutionary theory who’s name comes from Lewis Carroll’s Alice in Wonderland text: “It takes all the running you can do, to keep in the same place.” The idea is that sexual reproduction via cross-fertilization keeps host populations one evolutionary step ahead of the parasites, which are coevolving to infect them. It is within this coevolutionary context that both hosts and parasites are running (evolving) as fast as they can just to stay in the same place.

You’ll need a subscription to read more than the abstract, but here’s a link to the paper: Running with the Red Queen: Host-Parasite Coevolution Selects for Biparental Sex. That same issue of Science has another article on the same subject: Sex, Death, and the Red Queen. We continue with the university’s news story:

The widespread existence of sex has been a major problem for evolutionary biology since the time of Charles Darwin,” said lead author Levi T. Morran. Sex does not make evolutionary sense, because it often involves the production of males. [Hey!] This is very inefficient, because males don’t directly produce any offspring. Self-fertilization is a far more efficient means of reproduction, and as such, evolutionary theory predicts that self-fertilization should be widespread in nature and sex should be rare. However, as we all know, this is not the case.

Yes, we know. Here’s more:

“The Red Queen Hypothesis predicts that sex should allow hosts to evade infection from their parasites, whereas self-fertilization may increase the risk of infection,” said co-author Curtis M. Lively.

By combining the DNA of two parents, sex allows parents to produce offspring that are genetically diverse and different from their parents. Parasites that have adapted to infect one generation may have difficulty infecting the next generation. However, offspring produced through self-fertilization inherit the DNA of their single parent, thus any parasites adapted to infect the parent should also be capable of infecting the offspring.

We know you’re interested, so no further excerpts are necessary. Click over to the News Room at Indiana University and read the entire story. And stop complaining about parasites — they made your life far more interesting than it would have been without them.

Copyright © 2011. The Sensuous Curmudgeon. All rights reserved.

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11 responses to “If You Like Sex, Thank a Parasite

  1. I went to Purdue. IU was our rival (the Old Oaken Bucket Game and all that). We were known as a bunch of geeks; they were considered the party school. No surprise that they’d have an article on how sex is so important to biology.

  2. Gary says: “We were known as a bunch of geeks”

    Ah, then you were the self-fertilizers.

  3. SC says:

    Ah, then you were the self-fertilizers.

    To paraphrase Woody Allen, at least it was fertilization with someone we loved.

  4. Sexual reproduction does not _only_ improves parasite resistance, it is an order of magnitude faster for fixing *any* desirable trait, relative to reproduction by cloning or self-fertilization.

    That being said, a significant number of plants practice self-fertilization, and seems to be doing fine.
    For example, take the common dandelion, Taraxacum officinale.
    It practices apomixis, so the pollen fertilizes the flowers, but its genes are thrown away, and the dandelion seeds are clones of the parent plant.
    Now, dandelion is a vey hardy plant, which doesn’t, alas, seem riddled with parasites, as any lawn owner can attest…

  5. From Jared Diamond book “Guns, Germs & Steel”, Chapter 8
    “A third advantage of the Fertile Crescent flora is that it includes a high percentage of hermaphroditic “selfers”—that is, plants that usually polli- nate themselves but that are occasionally cross-pollinated. (…)
    Thus, the high percentage of hermaphroditic selfers in the Fertile Crescent flora aided early farmers, because it meant that a high percentage of the wild flora had a reproductive biology convenient for humans.
    Selfers were also convenient for early farmers in that they occasionally did become cross-pollinated, thereby generating new varieties among which to select. That occasional cross-pollination occurred not only between individuals of the same species, but also between related species to produce interspecific hybrids. One such hybrid among Fertile Crescent selfers, bread wheat, became the most valuable crop in the modern world.
    Of the first eight significant crops to have been domesticated in the Fer- tile Crescent, all were selfers. Of the three selfer cereals among them— einkorn wheat, emmer wheat, and barley—the wheats offered the addi- tional advantage of a high protein content, 8-14 percent. ”

    Hurray for selfers !

  6. Gabriel Hanna

    Sex does not make evolutionary sense, because it often involves the production of males. [Hey!] This is very inefficient, because males don’t directly produce any offspring. Self-fertilization is a far more efficient means of reproduction, and as such, evolutionary theory predicts that self-fertilization should be widespread in nature and sex should be rare.

    I don’t think this guy knows what he’s talking about. Why would efficiency in reproduction be selected for? What about insects and fish that scatter millions of eggs hoping that one survives to breed? Is he saying that natural selection knows what “the good of the species” should be and so would penalize a species that produces males, all else being equal? That is IGNORANT. Evolution doesn’t work for “the good of a species”. He should stick to the topic of his paper and leave the science-mangling sound bites to the journalists.

  7. Gabriel Hanna

    I’ve used this example before, but think of walruses or some other species that has a bull with a harem and the majority of males don’t have mates. Their sex ratios are still 50/50, even though it would be more “efficient” to have a sex ratio of 1 male to however many females–but natural selection doesn’t do that. If there were 1 male for 10 females, say, then any female who gives birth to a higher ratio of males has a huge advantage, as each of her sons will sire many offspring with 10 females but each of her daughters will only have a few, and her descendants will inherit that higher ratio, pushing the population back to 50/50. Of course if it gets too skewed in the male direction then it’s an advantage to produce more females. And this is because of two facts: males can sire more offspring than females can give birth to, and males and females contribute equal amounts of DNA (so the sex ratios might be different for ants or bees).

  8. Gabriel Hanna says: “I don’t think this guy knows what he’s talking about.”

    Awkward, for sure. I don’t know if “efficient” is the word to use in this context. It’s true that sexual reproduction is more complicated than the asexual method, but I don’t know if that makes it less efficient. One might say that a bird is “less efficient” than, say, an amoeba; it certainly uses more energy. But granting that, what is it that’s being said — that a bird should never have evolved?

  9. I always had an itch to read this erotic 19th century classic. Now seems as good a time as any:

  10. Some kind of fish have tiny males that attach to the BIG female just as if they were a parasite.
    It doesn’t seem to give that particular fish any advantage vs the ordinary fishes that have males and females of roughly equal size.

  11. Ants.
    An ant colony may be composed of 1% females (the Queen), 1% males (drones) and 98% neuters / sterile females (the workers).
    According to Professor Moran theory, having few males is good, but having very few fertile females is very bad.
    Oh noes! Ants are on the verge of extinction !!!1!one!