Parasites and the evolution of sex

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What's so great about sex? From an evolutionary perspective, the answer is not as obvious as one might think. An article published in the July issue of the American Naturalist suggests that sex may have evolved in part as a defense against parasites.

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Say a thank you to the pathogens and parasites of the world: Without them, there might be no such thing as sex. A new study finds that in organisms that can reproduce alone or with a partner, sexual reproduction is the result of a deadly arms race between host and pathogen. Without the presence of pathogens, the host organism sticks with the tried-and-true method of asexual breeding.

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Why is sex ubiquitous when asexual reproduction is much less costly? Sex disrupts coadapted gene complexes; it also causes costs associated with mate finding and the production of males who do not themselves bear offspring. Theory predicts parasites select for host sex, because genetically variable offspring can escape infection from parasites adapted to infect the previous generations.

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The evolution of sexual reproduction describes how sexually reproducing animalsplantsfungi and protists could have evolved from a common ancestor that was a single celled eukaryotic species. The evolution of sex contains two related, yet distinct, themes: its origin and its maintenance. The origin of sexual reproduction in prokaryotes is around 2 billion years ago Gya when bacteria started exchanging genes via the processes of conjugation, transformation, and transduction. Since hypotheses for the origins of sex are difficult to verify experimentally outside of evolutionary computationmost current work has focused on the maintenance of sexual reproduction.

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For over 25 years, many evolutionary ecologists have believed that sexual reproduction occurs because it allows hosts to change genotypes each generation and thereby evade their coevolving parasites. However, recent influential theoretical analyses suggest that, though parasites can select for sex under some conditions, they often select against it. These models assume that encounters between hosts and parasites are completely random.

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Paul Schmid-Hempel, Wondering about sex: W. Sexual reproduction, or sex for short, is an extremely successful breeding strategy. With some exceptions, metazoan organisms use sex, and even among protozoans or bacteria, some forms of sex exist, defined by its consequence of gene exchange Bell,

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Use the link below to share a full-text version of this article with your friends and colleagues. Learn more. The adaptive significance of sexual reproduction remains as an unsolved problem in evolutionary biology.

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Check out his excellent blog on evolution, Thoughtomics. Why do we have sex? If this question keeps you up at night, you either have really loud neighbors, or you have the makings of an evolutionary biologist.

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The Red Queen hypothesis for sex is simple: Sex is needed to fight disease. Diseases specialize in breaking into cells, either to eat them, as fungi and bacteria do, or, like viruses, to subvert their genetic machinery for the purpose of making new viruses. To do that they use protein molecules that bind to other molecules on cell surfaces.

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Biologist Robert Vrijenhoek has been studying the Mexican Poeciliid fish for more than 30 years. Some species of Poeciliopsis reproduce sexually while others reproduce asexually. Vrijenhoek found that the genetic diversity produced by sexual reproduction allowed the sexual fish to survive a parasite more successfully than the asexual fish. Invoking the Red Queen hypothesis, Vrijenhoek suggests the sexual populations are able to keep up or adapt to new selective challenges, while the asexual populations, essentially clonal, are not.

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