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June 25, 2009

Sex: A Solution to Microbial Invasion?

We're halfway through the Origins series of essays in honor of Charles Darwin's 200th birthday, and I'd wager that the other writers who have contributed to it will agree that it's a guaranteed recipe for glorious failure. The origin of life in 2000 words? That's just enough room to give a taste of the wide range of research going on these days but hardly enough to set up a proper banquet. The same goes for my latest essay, on the origin of sex. There, I focused on the intriguing question of why eukaryotes (animals, plants, fungi, and protozoans) have so much sex when it seems to come at a high cost compared with just cloning yourself. But there's an equally intriguing question that I didn't have room to address: Do bacteria have sex, too?

If you define sex as the way we reproduce, then the answer is no. Bacteria (left) aren't born as males and females, and they don't make sperm and eggs. And if you define sex as meiosis—the shuffling of two genomes to produce a new one—-again, the answer is no. But if you define sex as the combining of DNA from two individuals, they've definitely got it.

Viruses can move DNA from one bacterial host to another. Many bacteria carry little extra ringlets of DNA called plasmids that can cause bacteria to join together so that copies of the plasmids can be transferred. Sometimes the plasmids even drag along some of the DNA from the main chromosome. Some species of bacteria will even secrete DNA into their surroundings and slurp up naked DNA they encounter.

This foreign genetic material can be smoothly integrated into a bacterium's own genome. In some cases (known as homologous recombination), the microbe takes up a different version of a gene it already has. It swaps the new version for the old one. In other cases (nonhomologous recombination), it acquires a gene it never had before.

Like eukaryotic sex, bacterial sex has some evolutionary disadvantages. It takes energy to secrete DNA into the environment, for example, and it also takes energy to pump it in and incorporate it into a genome. The energy bacteria put into having sex could be used to grow faster and make more offspring. So, once again, the question arises: Why sex?

In a review in this month's issue of Trends in Microbiology, Michiel Vos of the Netherlands Institute of Ecology takes a look at the potential answers. A lot of them echo the answers that have been offered for the evolution of our own brand of sex. Sex can speed up the evolution of adaptations, for example, by combining beneficial mutations from different bacteria. Sex can bring about entirely new adaptations (such as antibiotic resistance) with the importing of entirely new genes. Sex can add more variation to a population of bacteria, allowing them to adapt to an ever-changing environment, instead of getting stuck in an evolutionary dead end. Sex may help some bacteria do a better job of making us sick by generating new variants that our immune system may not recognize very well.

It's possible, however, that these long-term benefits of sex do not account for their origin through the short-term, generation-by-generation process of evolution. In fact, sex may actually be more of a side effect—what Stephen Jay Gould and Richard Lewontin termed a spandrel. Taking in loose DNA can have an immediate benefit to bacteria that has nothing to do with sex: It's good eating. Some strains of bacteria can live on DNA alone. The fact that sometimes some of the genes they devour end up inserted into their genome does not necessarily mean that the bacteria have evolved a full-blown sexual system. The proteins that swap in new versions of genes during homologous recombination spend most of their time repairing damaged DNA. They may plug new genes in purely by accident.

It's also possible that the adaptation for sex resides not in the bacteria but in their parasites. Plasmids and viruses may evolve increasingly sophisticated ways to move their own DNA from host to host. If they bring genes that benefit their new bacterial host, they benefit as well.

Vos's paper makes the evolution of sex in eukaryotes all the more remarkable. Sex in eukaryotes is a far more complex process, and it's at the core of our biology. Whereas bacteria occasionally swap a gene, eukaryotes blend their genomes every time they reproduce. Biochemist Nick Lane, author of the new book Life Ascending, argues that eukaryotes became different because of a landmark event in their evolution: A microbe took up residence in the eukaryote cell, becoming mitochondria, which we depend on to generate energy. Now the eukaryotic genome was under constant invasion from foreign DNA, coming from close quarters. Worst of all, this foreign DNA included viruslike segments that could make copies of themselves, swamping our own genes. True sex—complete with meiosis—became our best defense. If Lane is right, then it's bacteria we have to thank for not having sex like bacteria.

Carl Zimmer

Credit: Anlace, English Wikipedia Project

1 Comments

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