Why did peacocks develop such elaborate tails? A study that tracks how a gene spreads through a population of yeast could finally help answer such knotty evolutionary questions.

How could yeast help to uncover the mysteries of the peacock's tail? Credit: Punchstock

Evolutionary biologists have come up with several models to explain how competition to reproduce affects traits such as the peacock's tail. Most agree that a trait giving individuals an advantage in competing for mates will spread through the population. But they disagree about how preferences for that trait arise in the first place. Some argue that bright tails, for example, could indicate that the male is strong and healthy; others say that preferences for such a trait arose arbitrarily. Reaching a firm conclusion is difficult because the traits have already evolved, and depend on multiple genes.

David Rogers, a molecular biologist from Imperial College London, UK, and his colleague Duncan Greig, an evolutionary biologist at University College London, have now developed a system that allows them to watch how the preference for a trait affects the spread of a gene through the population. Their model organism: the humble yeast.

It is an important advance on what we have been able to do before. It allows us to look at the genetic details of how selection works. Malte Andersson , University of Gothenburg, Sweden

"People have been coming up with theoretical models of sexual selection for years as a way of explaining how traits evolved," says Rogers. "But unless you can measure fitness — that is, how genes spread through the population — you can't really test these models."

"In the system we developed, we followed a single allele of a gene and saw how it spread through a population," he adds. The research is reported in Proceedings of the Royal Society B1.

Sexual competitions

The scientists chose yeast because it reproduces quickly, and has well-understood genetics — ideal for studying sexual selection.

Yeast is a microbe that can reproduce asexually, by dividing into two daughter organisms that are genetically identical to the parent. But it can also have sex, with two yeast cells merging to mix up their DNA, creating a new, genetically unique individual.

Some yeast cells give off a pheromone to attract others to mate with them. Stronger concentrations of that pheromone make a cell more attractive — so in theory, the gene that expresses the pheromone will spread through the population.

The researchers modified a group of 'signaller' yeast to give off extra pheromones and mixed them up with yeast that signalled at normal levels.

After a further genetic tweak to ensure they could not reproduce asexually, they pitted the microbes against each other in two sexual competitions.

Are you receiving me?

In the first scenario, the scientists put an excess of signaller yeast cells among a smaller number of 'receiver' cells that respond to the pheromone, so that there was strong competition to find a mate. The scientists found that the gene to produce a strong pheromone signal spread quickly through the population.

Conversely, an excess of receiver cells made it easier to find a mate, and the gene did not spread as quickly.

"The results of our experiment are quite simple and obvious. What is important is that we have shown that yeast can be used to study sexual selection. It should allow rapid progress in testing models," says Rogers.

"It is an important advance on what we have been able to do before," agrees Malte Andersson, an evolutionary ecologist at the University of Gothenburg in Sweden. "It allows us to look at the genetic details of how selection works."

Only the strength of the signaller trait was altered in this research, but future experiments could create receivers with greater or lesser preferences for the pheromone trait. The preference gene could also be linked to a genetic marker — a piece of known DNA that can be easily observed — that also weakens the yeast's vitality.

This would allow scientists to test competing theories about why preferences evolve. For example, one argument suggests that a preference for a particular male trait will evolve and spread through the population only if the trait offers a direct benefit to the female. Rogers hopes that other researchers will now use the yeast system to tackle such questions.