Wild sex is a staple of nature films, but there is one sex scene David Attenborough has never narrated: the mating ritual of yeast. That's right: yeast. Sex isn't just limited to lions, birds of paradise, and aphids; single-celled fungi do it too. Although most people don't like to hear the words 'fungus' and 'sex' used in the same sentence, yeast mating is a remarkable phenomenon and worth a closer look.
You can see this amazing scene, played out billions of times every day in wine vats and under oak trees, captured on film. As is the case with most wild mating rituals, filming yeast sex requires great patience - yeast are slow to commit and even when they do, they don't rush things. The beginning of foreplay itself takes several hours, as you can see below:
It may look like not much is happening in this scene, but there is some remarkable activity going on. The whole process begins with pheromones: the two sexes, evocatively called a and α, send amorous signals to each other in the form of small peptides, called mating factors. Although the mating factors are too small to see under a microscope, you can see their effects as the yeast become aroused. After several hours, you can see the formation of what biologists discreetly call the "mating projection" as a yeast cell starts to get ready for action. (As you watch the video again, pay careful attention to the behavior of the yeast in the top-center of the picture). As foreplay progresses, the cells continue to elongate, a behavior termed "shmooing". No, that's not derived from the word 'smooching', it's actually a highly technical term coined by yeast biologists in reference to the 1940's cartoon character, whose shape bears a striking resemblance to an aroused yeast:
As yeast shmoo and find a mate, the final process of yeast sex begins, and the two partners fuse their cells. This climactic scene of fungal ecstasy has unfortunately been censored from the video footage, but one intrepid researcher, who offers tantalizing glimpses of hot petri dish action on his blog, caught an intimate shot with his voyeur cam:
Other than the well-known scientific obsession with the sexual practices of other species, why are scientists interested in yeast mating? While yeast won't help you learn how to be a better lover, genetically, we can learn a lot about sexual reproduction from yeast. The fact that even some single-celled eukaryotes reproduce sexually is a powerful testament to the evolutionary advantage of sexual reproduction.
Just why sex is evolutionarily a good thing is still debated among biologists. (Note that I said evolutionarily - biologists are perfectly aware of non-evolutionary reasons why sex is a good thing.) One suggestion is that the genome shuffling that goes on in sexually reproducing organisms (recall that when you pass on one copy of your chromosomes to your kids, its a mixture of genetic material from both your father and your mother) enables a species to more efficiently 'try out' different combinations of mutations, some harmful, others beneficial, combinations which are then tested by natural selection. Yeast undergo meiosis, with its attendant chromosome recombination, just like human germ cells do. Thus, many of the evolutionary consequences of sex can be studied much more easily in yeast than in species like wildebeasts and humans. (For a whole series of Nature articles on the evolutionary benefits of sex, check here.)
There is an even more compelling reason to study yeast mating that has nothing to do with sex itself. The biological events that occur inside a yeast cell in response to mating pheromone is a classic example of a widely shared signaling pathway, a reusable information-processing module of the kind I wrote about here. In this case, the information-processing module is called a MAP kinase pathway; this is a pathway that plays a tremendously important role in human biology, including normal biology and diseases like cancer. The yeast MAP kinase pathway is so similar to the human version, that you can replace the yeast pheromone receptor with a human hormone receptor, add human hormone, and thereby prompt the yeast mating response.
The MAP kinase pathway is such an important pathway, and yeast are so easily studied, that many research groups focus on yeast mating, including the Alpha Project, a group trying to produce a detailed computer model of this system. Boeing engineers can model their engines much, much better than biologists can model cellular systems, so scientists currently focus much of their modeling efforts on simple, genetically tractable organisms like yeast.
What can we really learn by making computer models of yeast mating? Like Boeing engineers, scientists are interested in knowing 'what happens if we tweak this, or rewire that, or remove this component?' At Boeing, they can do their tweaks on a computer and predict, with very high confidence, what's going to happen in the real engine. In biology, we can't yet - we actually have to go make all of the tweaks and see what happens. We consider ourselves lucky if we can do all of the experiments, feed everything that happened back into a computer model, and get the computer model to 'retrodict' what should happen. We don't yet know what variables are important and which are not, or what level of abstraction is best for building a computer model, and we don't really know why, over the course of evolution, the MAP kinase pathway was put together the way it is. My hope is that once we understand that, we'll know how to build models of human cells that can lead to true predictions of what will happen when you tweak the system with a drug.
That's heady stuff, based on studies of fungal sex. Biologists will take any experimental system they can get, no matter how strange (or depraved?), if it will promise them a chance of learning something new about ourselves.