First Sex, Then Cheating

A love of big questions led Paul Turner to investigate why simple species have sex, with surprising results. Hint: viruses, too, are faced with a prisoner's dilemma.

Karen Hopkin
Dec 1, 2008
<figcaption> Credit: © Jason Varney | Varneyphoto.com</figcaption>
Credit: © Jason Varney | Varneyphoto.com

As a graduate student at the University of California, Irvine, Paul Turner declared he wanted to study vultures. "I had this strange appreciation for carrion birds," he says. "And I thought there must be something special about their digestive system that allowed them to tolerate and maybe even exploit the potentially pathogenic microbes that grow on rotting meat."

Rich Lenski, Turner's thesis advisor, was not enthusiastic. "I pointed out to Paul some of the scientific challenges of studying that sort of system," Lenski says. "Keeping a lab full of vultures would have been a logistical nightmare," laughs Turner. Not to mention the need for decaying carcasses.

Instead, Turner turned his attention to more accessible microbial systems. "Paul was immediately excited when I told him about the viruses and plasmids that infect bacteria, and...

Turner's research "says that evolutionary theory and thinking is not simply for big organisms, it impacts every level of biology." - Michael Travisano

"When it comes to microbial experimental evolution, Paul is among the top people in the field," says Santiago Elena of the Institute of Molecular and Cellular Biology of Plants in Valencia, Spain. "His work is high quality and many of his projects are absolutely innovative. Nobody is doing anything like it. So if you call five people in the field and ask them for names, Paul's will definitely show up."

Bringing back sexy

As a student in the Lenski lab in the early 1990s, Turner used a bacteria-plasmid system to investigate host-parasite interactions. In particular, he wondered whether the presence of a large population of uninfected hosts might "tempt" the parasite, in this case a plasmid, to become more infectious. For plasmids, that means that they'd become more likely to propagate themselves from one bug to the next via conjugation, the bacterial equivalent of sex. The alternative would be for the plasmid to sit tight and simply wait for the cell to divide, so it will be transmitted to the progeny.

"What we assume is that there's an inability to maximize those things simultaneously," says Turner. "So either the plasmid becomes more virulent, and infectiously transfers itself more quickly to new hosts. Or it becomes less virulent and doesn't transfer to new hosts as often, instead being transmitted to daughter cells."

That assumption was upheld: "Plasmids evolve to be better conjugators to the detriment of being able to move across generations and vice versa," says Turner. Although, in the end, whether a plasmid would opt to be more infectious had little to do with how many uninfected hosts were around. Despite the complications, "the experiment was a really elegant demonstration of the choices faced by a parasite," says Pennsylvania State University's Siobain Duffy, one of Turner's first students.

Of course, it may be a bit of a stretch to consider a plasmid a parasite, and conjugation akin to virulence. Which is why, when he started his postdoc in Lin Chao's lab at the University of Maryland in 1996, Turner switched to studying viruses. Chao was working on a bacteriophage called phi6, an RNA virus whose genome is divided into three segments that are sort of like chromosomes. As a result of this segregation, phi6 can readily undergo a kind of genetic recombination. "When multiple viruses enter a single cell, they can swap segments like you would shuffle a deck of cards," says Turner. Such shuffling was of interest to Turner for two reasons: First, because it's the same way that influenza and other viruses of medical concern generate genetic variation. And second, because it provided a system for studying the potential advantages of sex.

"Sex does provide an opportunity to bring together good combinations of genes," says Turner. "That's one of the longstanding arguments for why it evolved. But it can just as easily tear apart good combinations of genes. So the question is: Why on this planet are there so many sexual species?" Turner was hoping phi6 might lead him to an answer.

Addicted to it

The experiment itself was fairly straightforward. Turner and his team challenged phi6 to adapt to growing on an unfamiliar host bacterium they had in the lab. And they tweaked the conditions so that they could compare two populations: One in which the viruses infected cells singly; and one in which multiple viruses were allowed to co-infect a single cell and, presumably, to swap "chromosomes" while doing it. "The theory states that when you have a sexual process occurring, those populations will be at an advantage," says Turner. "They'll be able to create greater genetic variability, so you would expect them to evolve faster." But after hundreds of generations, he says, "we actually got the opposite result." The loner phage adapted perfectly well to their new host. But in the experiments in which co-infectors were allowed to swap genes, the researchers ended up selecting for viruses that were addicted to sex: They were no longer able to infect cells efficiently all on their own. So these phage turned out less fit than the loner phage, not more so.

"It took some time to figure out what was going on," says Turner. But what he realized was that he'd inadvertently bred a flask full of cheaters. When viruses co-infect cells, they can make liberal use of proteins that have been produced by the other viruses, such as stealing another virus' capsids when assembling new infectious particles. "So in a sense, what we did was select for viruses that were very good at stealing proteins from other viruses," says Turner. That would explain why the light-fingered phi6 were kings of co-infection, but did poorly when faced with having to infect a cell without other phage to borrow from.

Then, Turner went a step further and related the data to another situation where cheating is "selected for," even though it leads to a large penalty: the prisoner's dilemma. In this popular game theory problem, Duffy explains, "you and your friend rob a bank and are brought in for questioning by the police. If you both keep you mouths shut, you'll both get a small prison term. But if either one flips, the cops will cut you a deal: You'll go free and the other guy will get the full sentence. The dilemma is that the number of prison years served overall is far lower if you both keep your mouth shut," she says. "But the temptation is to cheat, because then you, personally, will be fine."

In people, this is an easy problem to picture. "But it's been difficult to prove that it can ever happen in a biological population," says Turner. "We essentially stumbled on this result because of the environmental treatment we imposed." Faced with the challenge of infecting a bacterium, viruses that were offered an opportunity to cheat readily accepted, and then rapidly spread through the population. Once that happens, Turner says, "suddenly everyone's a cheater. And the cost of cheating when everybody else is a cheater is generally a decrease in fitness, a result that's consistent with the prisoner's dilemma."

That observation, published in Nature in 1999, put Turner on the map. "It's classic. It's clean. It's one of those 'why didn't I think of that' experiments," says Susanna Remold of the University of Louisville, Turner's former postdoc. "It's something that's intuitively obvious once somebody has actually done it."

Duffy agrees. "It's a clear, unambiguous, strong demonstration of a fundamental concept in biology and it's going to continue to be cited for years to come." What's more, the study bridges the gap between microbial ecology and experimental evolution, two fields that don't always intersect. "Most evolutionary biologists don't really think about these sorts of systems. I mean, most people don't even think a virus is an organism," says Michael Travisano of the University of Minnesota. "This study says that evolutionary theory and thinking is not simply for big organisms, it impacts every level of biology."

Serenity and spice

After his stint in Chao's lab, Turner went on to do two more postdocs: One in Elena's lab in Valencia studying the evolution of vesicular stomatitis virus, an RNA virus that infects eukaryotic cells; and a second at the NIH working on the virus that causes chicken pox. "He could certainly have gotten a faculty position after being in Lin's lab," says Travisano. "But that's not what Paul wanted to do. He's interested in the science, not just his career."

Turner carried that enthusiasm with him when he set up his own lab at Yale University in 2001. "Paul is just jazzed about the big ideas," says Remold. "His first concern is always: Is a problem interesting? Is it cool, is it fun? If the idea is good, Paul will say, 'let's make it happen'." That follow-the-idea attitude makes for some interesting lab meetings. "We would often have these hilarious conversations where Paul would be totally pie-in-the-sky and his graduate students would have to shoot him down, saying, 'no, Paul, it can't be done.' In other labs, the PI is the voice of realism. But Paul has fun with the science. So everybody else does, too."

Turner is also pretty unflappable, which Travisano says allows him to focus on the problem at hand. "If everybody's bouncing off the walls and there's one guy who doesn't get ruffled, it gives him an edge, maybe lets him see connections that other people might miss. Paul is just very calm." "He's like that even in crisis situations," adds Remold. "The freezer's defrosting, there are thousands of strains possibly about to go down the tubes. Everyone is freaking out and Paul is saying, 'these things happen.' He's able to roll with it. It's a good approach to model."

As is his ability to tell a story. "I'll always go to hear his talks, even though I pretty much know what he's going to say," says Vaughn Cooper of the University of New Hampshire. "Because Paul tells a good story even if you've heard it before. He recognizes the importance of narrative in science. It isn't a report, it's a history. There are people, there are conflicts. Even at the cellular level. That's what you want to listen to."

Remold agrees. "Paul is definitely into the soap opera of biology: Who's doing what to whom and why. All the things he doesn't do in his real life, he studies how organisms do to each other—quirky, sneaky things that capture the imagination. Paul is good at telling a story, but also good at seeing a story," she says. "If Paul gets excited about something, it's liable to be something big that, if it works, will change the way we think about things."