The following is from a short post on The Loom, by Carl Zimmer.
You want a little Science with your bread musings? How about an assertion that the little beasties that make your bread rise are making a choice? Really. This is some amazing stuff, kind of fun to wrap your head around!
"[Yeast] This single-celled fungus–for which we should give thanks for bread, beer, and wine–can reproduce in several ways. Most of the time, it produces buds that eventually split off as free-living cells of their own. Its daughters are identical to itself, carrying the same two sets of chromosomes. Sometimes, however, life get rough for yeast, and they respond by making spores, each with only one set of chromosomes. Later, when times get better, the spores can germinate. In some cases the yeast cells that emerge just grow and divide. But they can also have sex. One yeast cell merges with another one, combining their DNA to produce a new yeast cell with two sets of chromosomes.
What makes yeast sex especially interesting is that the cells communicate with each other first. A yeast cell produces a pheromone that can cause another cell to stop dividing and start crawling towards the source of the signal. These pheromones divide yeasts into two groups. Yeast cells carry one of two genes for making pheromones and will only mate with yeast cells that produce the opposite type.
But if you surround a yeast cell with a ring of pheromone producers, the yeast will not just pick a partner at random. It will exercise a choice. The cell will measure the pheromones coming from each suitor, and it will creep its way to the strongest source....
... the scientists compared how much pheromones each kind of yeast produced. They found that yeast of higher quality churned out more pheromones than yeast of lower quality. So a yeast that chooses to mate with a strong pheromone producer will be endowing its offspring with good genes.
Smith and Grieg’s experiment makes me think about the yeast in a glass of wine in a different way: I now imagine an ocean of love songs. But it also makes me appreciate just how far-reaching Darwin’s ideas about the evolution of sex have turned out to be. The same rules apply–to bird, frog, and fungus alike."