Picking the right mate is a formidable challenge for most humans, but baker’s yeast manages it without a brain or the ability to move around. A recent study found that when food is abundant, yeast spores mate with big partners that can take advantage of the feast, while in lean times, they prefer to mate with small spores that need less food.
“This sort of parallels what you see in higher organisms. Males that are stronger, whether for genetic reasons or environmental reasons, might be more attractive because they can invest more in their signal to attract females,” says Duncan Greig, research group leader at the Max Planck Institute for Evolutionary Biology and one of the study’s authors. “We wondered if the same sort of thing would happen with yeast.”
Saccharomyces cerevisiae, also known as brewer’s or baker’s yeast, has a lot of options when it comes to reproduction. Most of the time a yeast cell “buds,” growing a tiny clone that eventually pinches off to become its own cell. But when there’s not enough food, it turns to sex.
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Yeast cells don’t have sexes, but in lean times the cell will divide into spores, which come in two complementary mating types, to wait out the bad times in a dormant state. Spores are equivalent to our sperm and eggs, with half the genetic material of the parent. No one knows how long they can survive without food, but researchers claim to have resuscitated yeast spores from 45 million-year-old amber. They used them to make beer.
Yeast romance begins when spores find themselves on food. Each type starts producing chemical signals, or pheromones, to attract spores of the opposite mating type. When a spore senses a pheromone, it develops a bulge that stretches out toward the source—a process known as “shmooing” because it makes the spores look like a 1940s cartoon creature of the same name. Two shmoos stretch until they touch, then fuse into one cell.
If a spore can’t find a mate, it will bud by itself. To see how well yeast play the dating game, the researchers first looked at what makes spores successful when they are forced to reproduce alone. They found that large spores bud faster in food-rich environments, whereas small spores bud faster in poor ones.
The researchers then placed spores together in groups of three, allowing one spore to choose between a large or small spore of the opposite mating type. Spores were more likely to choose large mates when food was abundant and small mates when times were tough.
So how does a spore know which suitor is the right size? The researchers aren’t sure, but they think it comes down to pheromones. Spores that produce lots of pheromones are better at attracting mates. In a previous study, the team found that pheromones are costly, and they’re produced in larger quantities by healthy spores that bud efficiently. Although the new study didn’t look at pheromones directly, it is likely that spores also produce more when the spores are the right size for their environment. The researchers think pheromones are the yeast’s version of a peacock’s tail, proving to potential mates that a spore is strong.
Most people associate courtship more with bird songs and Valentine’s cards than with single-celled fungus, but Greig’s team has shown that we may need to expand our thinking. “Things that can’t think and can’t have actual strategy can nevertheless have very complex behavior,” says Greig. “We call it mate choice, but obviously it’s not choice as we would think of it. It’s a program that’s evolved because it increases fitness.” Yeast are perfect for studying the evolution of sex and courtship because they can reproduce both sexually and asexually, and they are easy to manipulate in a lab.
“Yeast are a lot more like us than we realize,” says Catherine Jackson, research director at the Institut Jacques Monod in Paris, who was not involved in the study but who conducted some of the research that inspired it. “They can provide lots of information to us, at all levels—even how processes operate in humans.”