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

When I sit down to read a scientific paper, I usually brace myself for the worst. I prepare to slog through esoteric, murky language—to have to dig deep to find the buried beauty of science.

But every now and then, you sit down and read a paper that starts like this:

"Picture a pile of freshly cut weeds at the sunny edge of a tropical forest. Metallic green flies dart and circle over it, chasing one another in short dashes. Your eye is caught when a chase ends as one fly grasps another in midair and the pair immediately lands on the pile of weeds. Their genitalia are already coupled, and the male immediately turns to face away from the female. After a few seconds, paradoxically (because he is already securely attached), he begins to court, rhythmically waving his colorful hind legs and tapping the female's abdomen. The courtship continues for a few minutes as the pair remains coupled, and then the flies separate. The female walks down into the pile where she lays eggs (her larvae will feed on the rotting vegetation), while the male rejoins the frenetic chases above the pile."

Why would a male fly wait to court a female until after he has already achieved his evolutionary objective of copulating with her?

The paper is titled Postcopulatory sexual selection: Darwin's omission and its consequences. It was written by William G. Eberhard, an evolutionary biologist at the Smithsonian Tropical Research Institute in Panama, and was just posted online at the Proceedings of the National Academy of Sciences Web site. It offers some wonderfully bizarre examples of the extremes to which evolution reached once sex emerged a couple billion years ago.

Earlier this month, I wrote in Science about the origin of sex. Despite the disadvantages of reproducing with both males and females, sex dominates the animal kingdom and is common among our fellow eukaryotes (plants, fungi, and protozoans). Studies point to several possible benefits that outweigh the cost of sex. Sex may speed up the evolution of adaptations, cleanse our genomes of harmful mutations, or let us fight against parasites more effectively. However sex evolved, it created a new arena in which the evolutionary process could take place. Now reproductive success was not just a matter of surviving and finding enough food. Now it also depended on whether organisms could find a mate.

Darwin recognized this distinctive kind of selection, which he dubbed sexual selection. In his 1871 book, Descent of Man, he argued that males competed with each other for the opportunity to mate with females, and as a result, males had evolved claws, horns, and other weaponry. Darwin also argued that females were attracted to certain males over others, and this preference drove the evolution of gorgeous courtship displays such as the extravagant plumage on some birds.

Over the past 138 years, scientists have discovered a wealth of evidence demonstrating that sexual selection is indeed a powerful force. But it drives evolution in ways that Darwin did not anticipate. In many species, females actually mate with lots of males. And those multiple matings open up yet another arena for evolution. Along with the courtship and battling that goes on before mating, there's an opportunity for lots of strategies for boosting reproductive success after mating.

The underlying logic of postcopulatory sexual selection is simple: Once a male has inserted his sperm in a female, he has not sealed the deal. A female may have the sperm from several males inside of her. There are many strategies that appear to have evolved because they boost a male's reproductive success, from fast-swimming sperm to male genitals that explode after mating, to prevent other males from adding their own sperm.

One of the most intriguing strategies biologists have discovered is when a male courts after the copulation has started. In the case of the flies that Eberhard describes at the start of his paper, it appears that males engage in this after-the-fact courtship so that females will lay eggs immediately after their courtship. If scientists prevent the males from courting during copulation, the female flies fly away without laying any eggs.

Scientists are only starting to test hypotheses about postcopulatory sexual selection, but its effects could turn out to be huge. In fact, it may have bearing on our food supply. Some studies indicate that plants can choose between the pollen of other plants that land on them in order to fertilize their seeds. Plants may even abort fruits that don't come from suitable mates. Such are the unexpected directions in which Darwin's initial ideas have traveled.

Photo: Stefano Baldacci, Wikimedia

Sex gives nature much of its spice. Fireflies flash through the night to find mates; a flower’s perfume lures insects to carry pollen to distant partners; male bullfrogs croak to impress females. Currently, biologists understand the molecular nuts and bolts of sex fairly well. But the why of sex is still fairly mysterious. Bacteria don’t have to search for mates; they just grow and divide in two. An aspen tree can simply send out shoots that grow into new trees. No muss, no fuss with finding a partner, fertilizing an egg, and joining two genomes.

Darwin felt the whole subject of sex was "hidden in darkness.” In this month's Origins essay, Carl Zimmer sheds light on sex with his examination of progress since Darwin's day in understanding the "why." Today, scientists use genomics and other 21st century tools to search for answers. They are finding hidden signs of sex in the DNA of supposedly asexual organisms and are tracking the evolutionary impact of sex among living populations of animals and plants. Some use sophisticated mathematical models to assess the conditions under which sex can arise. These efforts are providing new hints about how sex first emerged some 2 billion years ago and about the forces that have made it so widespread. The new studies bolster a handful of hypotheses: Sex may speed up evolution, for example, or it may provide a better defense against parasites. In the past, scientists have focused on just one of these hypotheses at a time, but today many argue that several forces may be at work at once.

—Elizabeth Pennisi

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