News Release

CHAPEL HILL — Cockroaches, birds, rabbits and humans – just to name a few species – all reproduce sexually. But for two centuries, biologists have wondered why sexual reproduction is so common, since it actually poses serious risks.

Now scientists at the University of North Carolina at Chapel Hill and the University of Houston have found evidence for one possible reason. Sexual reproduction may perpetuate itself by making each successive generation more resistant to genetic mutations and by helping natural selection remove the most harmful mutations from the genome.

In that way, sex may ensure its own survival, said Dr. Christina L. Burch, assistant professor of biology in UNC’s College of Arts and Sciences.

The study, published as a letter in the March 2 issue of the journal Nature, is the first to provide evidence that sexual reproduction results in organisms that are more robust (less susceptible to harm from genetic mutations). It’s also the first to show that sexual reproduction results in negative epistasis, which is a pattern of interactions between genes that helps remove the most harmful mutations from the gene pool.

"There are a lot of organisms – and this includes humans – that acquire more than one mutation in their genome every time they reproduce," said Burch. "Since most mutations are harmful, that’s kind of shocking. How is it that populations don’t go extinct when they’re acquiring all these mutations all the time?"

The ability of these organisms to reproduce sexually may provide an answer, Burch said.

Sexual reproduction also results in recombination: the introduction of new combinations of genes into the population. Recombination is risky, Burch said, because certain combinations of genes don’t work very well together.

Burch and colleagues used a model artificial gene network to observe what happens as recombination continues over time. At first, recombination did indeed produce many gene combinations that didn’t work well together. But as the model ran, and gene networks evolved, both recombination and mutations began to have smaller effects.

And, over time, recombination resulted in the elimination of many mutations by bringing them together into individual genomes. These combinations of mutations were then more likely to be eliminated from the population through natural selection.

"Imagine two parents who each have a different mutation," Burch said. "After evolution in our simulations, those mutations have little to no effect when they appear alone. When the parents reproduce, half of their offspring will inherit just one of those mutations. One-fourth of the offspring will inherit zero mutations. But one-fourth of them will inherit both of the mutations.

"And if those two mutations, when they appear together, are quite harmful, then natural selection can quickly remove those individuals from the population. And now on average, your offspring have fewer mutations than your parents did."

This pattern, called negative epistasis, in which mutations have small effects alone but large effects in combination, has been difficult to prove in real organisms. By using artificial gene networks to model the evolutionary process, Burch and colleagues were able to demonstrate that negative epistasis is at least theoretically plausible.

The model used in the study is a mathematical equation that models a hypothetical gene network in which various genes switch each other on or off. The model can be applied to many different gene networks and isn’t based on any one organism, Burch said.

"We tried hard to make sure that the numbers of genes and numbers of interactions between them were biologically realistic," Burch said. "But we used a more general model to show that, regardless of the structure of your network, this phenomenon occurs."

Along with Burch, other authors of the study are lead author Dr. Ricardo B.R. Azevedo, doctoral candidate Rolf Lohaus, and master’s student Suraj Srinivasan, all of the University of Houston; and Kristen K. Dang, a doctoral candidate in the department of biomedical engineering, jointly housed at UNC and N.C. State University.

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Note: Burch can be reached at (919) 843-2691 or by email at CBurch@bio.unc.edu.

College of Arts and Sciences contact: Dee Reid, (919) 843-6339 or deereid@unc.edu
News Services contact: Deb Saine, (919) 962-8415 or deborah_saine@unc.edu