New & Noteworthy
July 31, 2014
Here at SGD we tend to have a totally positive opinion of yeast. As we have said before, they give us bread, booze, a great model organism, and even our livelihoods. But in truth, Saccharomyces cerevisiae has a few minor faults.
For example, you can thank yeast for all those irritating fruit flies buzzing around your brown bananas. Fruit flies aren’t attracted to the rotting fruit itself. They are instead attracted to chemicals the yeast cells are pumping out as they nosh on that old banana.
In a new study, Schiabor and coworkers set out to identify the genetic differences that make some yeast strains more attractive to fruit flies as compared to other strains. They found that the flies can actually tell the difference between “petite” yeast, with defective mitochondria, and “grande” yeast whose mitochondria are normal. The mitochondria play a huge role in determining which volatile chemicals a yeast will release, and so determine which yeast are the most attractive to a fruit fly. But the mitochondria are probably not involved in the way that you might be thinking…
In the first experiment, the authors tested a bunch of different yeast strains to find the ones that fruit flies prefer. As expected, they found a wide range of yeast attractiveness. They decided to focus on BY4741 as the more appealing strain and BY4742 as the less appealing one.
Schiabor and coworkers chose these two strains both because they are isogenic and because they are the strains from which the systematic yeast deletion collection was made. These two attributes mean that it should be relatively easy to track down the genetic difference in each strain’s attractiveness to fruit flies.
The first obvious candidate was the different auxotrophies in each strain. Although the strains are isogenic overall, they have a few small differences: BY4741 is a met17 mutant and is mating type a, while BY4742 is a leu2 mutant and is mating type α. Since amino acids are very important in creating various volatile chemicals, the mutations in the amino acid biosynthetic genes seemed a likely cause of the difference in the way the two strains smelled to fruit flies. However, the authors found that none of the auxotrophic mutations mattered. When they mated the two strains and did tetrad analysis to obtain every possible genetic combination, they found that each of the eight new strains was preferred over BY4742.
Given the non-autosomal inheritance of attractiveness, an obvious candidate was the mitochondria. This hunch was confirmed in a couple of ways. First, Schiabor and coworkers showed that every strain except BY4742 grew well on glycerol, and second, they found that an isolate of BY4742 with functional mitochondria, BY4742g, was as attractive to fruit flies as BY4741. Apparently their stock of BY4742 had lost mitochondrial function (which can happen fairly easily for some strains), and clearly the mitochondria matter here!
Through a series of experiments we don’t have the space to describe here, the authors found that the lack of attractiveness was not due to an inability to respire. Instead, by growing each strain on different nitrogen sources, they were able to provide evidence that mitochondrial functions like proline catabolism and/or branched amino acid anabolism were more likely to be involved. It can sometimes be hard to remember that the mitochondrion is more than the powerhouse of the cell we all learned about in high school: a lot of very important metabolic reactions other than respiration happen within the mitochondrial compartment.
The authors think that yeast with good working mitochondria are the most useful to fruit flies, which is why fruit flies have evolved to be attracted to those yeast. This all makes sense, as yeast and fruit flies have a mutually beneficial relationship. Yeast serve as food for fruit fly larvae, and the ethanol they produce also protects those same fruit fly larvae from predators. Fruit flies can open up parts of the fruit the yeast can’t get to and help move the yeast to different places.
The bottom line is that you can blame yeast mitochondria for that swarm of fruit flies hovering over your fruit bowl. One day maybe we can come up with a way that our fruit will only allow petite yeast to grow. Then we’ll have a bit of time to enjoy fruit that isn’t attractive to fruit flies. Until, of course, the flies evolve to prefer petite yeast…
by D. Barry Starr, Ph.D., Director of Outreach Activities, Stanford Genetics