Julia Shi

Usually, we think of evolution as a slow, gradual process — but Yale scientists have shown that vast changes in species can occur in just short periods of time.

A study spearheaded by Jason Shapiro GRD ’14 and biology professor Paul Turner focused on the evolution of viruses and bacteria. Their report, published in the journal Evolution on Jan. 30, provides some of the first evidence to support the theory that a mutually beneficial relationship can evolve from a parasitic one.

“We wanted to see if they would evolve back to being mutualist, and in almost every case, they did,” Shapiro said.

The viruses and bacteria originally had a parasitic relationship, in which the viruses thrived at the expense of the bacteria. Over time, though, they developed a mutualistic relationship, in which both parties benefited from one another’s presence.

“There’s very little research that examines how species can transition between [these relationships] … so that was our overarching motivation,” Turner said. His and Shapiro’s project is one of the few to date to show this transitory phenomenon — an approach he calls “experimental evolution.”

The recent paper expanded on a previous study that Shapiro and Turner conducted, which also focused on bacteria and viruses. In the original experiment, the viruses helped the bacteria grow in a mutualistic relationship.

In the first study, the infected bacteria grew better than they had without the virus, Shapiro said. The virus and bacteria then evolved together, exhibiting a mutualistic relationship. When they cultured the evolved virus with the original generation of bacteria, however, they noticed a parasitic relationship. Shapiro and Turner reversed this parasitism back to mutualism in their recent experiment.

“When we put the viruses back into the unevolved bacteria, who they had previously helped, it now hurt them,” Shapiro said. “What we wanted to know in the second paper was, could they evolve back?”

They did evolve back, taking just 20 generations to do so. M13, the virus used by the researchers, is categorized as a bacteriophage because it targets bacteria. Although mutualism and parasitism are both examples of symbiosis, or interactions between two different species, a transition from one end of the symbiotic spectrum to the other has not been observed in many research settings.

“The very satisfying outcome was that we saw movement along the continuum,” Turner said.

In both experiments, the symbiotic relationship between bacteria and viruses was characterized according to whether growth of the bacteria was inhibited or enhanced by the presence of the virus. The researchers inferred a mutualistic relationship in the event of enhanced growth and parasitic one in a case of inhibited growth.

Researchers have yet to find a scientific explanation for this kind of evolutionary behavior, Shapiro said. He speculated that a potential explanation for this could be the environment in which the bacteria and viruses grew. In the study, the researchers used ice-cube tray-like microplates that contain a grid of small wells. The scientists picked these microplates over flasks, another common method for growing cultures, which differ from microplates in the amount of oxygen and intermixing allowed in the environment, Shapiro said. These differences may have shaped the way that the bacteria and virus evolved.

According to Turner, the study supports a worldview of constant change and evolution.

“Evolution is something that’s taking place all of the time,” Turner said. “It’s a very dynamic biological world when it comes to strategies for reproduction.”

He added that many people have the false assumption that the behavior of a certain species is fixed. On the contrary, his research demonstrates the elasticity of organism behavior by showing fast shifts between mutualism and parasitism.

The results of the study also have implications for disease treatment. Several important bacterial pathogens, for both humans and plants, involve viruses similar to M13. In their paper, the researchers suggest that examining virus evolution in such bacterial infections may be helpful for treatment.

Approximately 265,000 people are infected with harmful E. coli strains — the bacteria used in the study — each year in the United States, according to the Centers for Disease Control and Prevention.

Jessica Pevner | jessica.pevner@yale.edu