New & Noteworthy

You Can Thank Fruit Flies for Those Yummy Beers

October 14, 2014

A wonderful side effect of the close relationship between yeast and fruit flies is great tasting beer. Image from Wikimedia Commons

It is as simple as this, beer tastes good.  And if a new study in Cell Reports by Christiaens and coworkers pans out, you can thank fruit flies for some of those delicious flavors.

No, fruit flies aren’t in your beer. Instead, they have forced the evolution of our favorite beast, Saccharomyces cerevisiae, down a path towards making the aromatic compounds that make beer so darned tasty.

See, yeast can’t get around on their own and so they often rely on insects to move to new pastures. In order to have this happen, they need to attract insects. Plants have worked this out by evolving colorful flowers and sweet nectar. And one way that yeast may do this is by generating aromas that fruit flies find irresistible.

The researchers in this study first stumbled onto this possibility around fifteen years ago. Back then the P.I. was a graduate student who left his yeast flasks out on the bench over the weekend.  Over that same weekend fruit flies escaped from a neighboring Drosophila lab and invaded the yeast lab.

In a “you got peanut butter on my chocolate” moment, the yeast researchers found the fruit flies swarming around one set of flasks and ignoring some of the others. A quick look at the flasks showed that fruit flies were ignoring the yeast strains in which the ATF1 gene was knocked out.

The ATF1 gene encodes the alcohol acetyltransferase responsible for making most of a yeast’s fruity acetate esters.  So it makes perfect sense that fruit flies ignored strains deleted for ATF1 because they didn’t smell as good anymore. To confirm this hypothesis, the authors did a fun, controlled experiment.

In this experiment, the authors set up a chamber where they could use cameras to track fruit fly movement.  One corner of the chamber had the smells from a wild type yeast strain and another corner had smells from that same strain deleted for ATF1.  As you can see in the video here, the fruit flies cluster in the corner with the wild type strains.  Fruit flies definitely prefer yeast that can make flowery sorts of acetate esters.

Christiaens and coworkers took this one step further by actually looking at the effect these chemicals had on Drosophila neurons.  They used a strain of fruit fly containing a marker for neuronal response, so that the researchers could “see” how the flies were reacting to wild type and atf1 mutant yeast smells.  As expected from the previous experiments, the olfactory sensory neurons responded differently to each smell.

To confirm that the esters were responsible for this difference, the authors observed the effect of adding esters back to media in which the atf1 mutant yeast were growing. They found that as more esters were added, the activity pattern of the Drosophila neurons shifted towards that seen with the wild type yeast.

OK, so fruit flies like good smelling yeast.  The next question the researchers asked was whether this had any effect on the dispersal of the yeast – and it definitely did.

To test this, they labeled wild type and atf1 mutant yeast with two different fluorescent markers, so the strains could be distinguished from each other. They then spotted each strain opposite from one another on a specially designed yeast plate and let a fruit fly roam the plate. They then removed the fly and the original spots of the yeast cells.

After letting the plate incubate for 48 hours, so that any yeast cells that had been moved around on the plate could grow up into colonies, they washed the plate to remove the cells that had been dispersed by the fly and used flow cytometry to determine the amount of each strain. They found that wild type yeast were transported about four times more often than the aft1 mutant yeast.

These results show that fruit flies are more likely to disperse yeast if the yeast are producing fruity smells.  Given the close relationship between fruit flies and yeast, and the fact that insect vectors are very important for yeast out in the wild, it is reasonable to think that yeast may smell good in order to attract fruit flies to carry them to new places.

This research also again points to the importance of expanding studies to include more than one organism (see our last blog here). By increasing the diversity of organisms in an experiment, we can learn much more about how things work in the real world. And maybe even learn why yeast evolved to give us such delicious beer.

by D. Barry Starr, Ph.D., Director of Outreach Activities, Stanford Genetics

Categories: Research Spotlight

Tags: beer , Saccharomyces cerevisiae , Drosophila

Time Flies Like an Arrow, Fruit Flies Like a Grande Yeast

July 31, 2014

Yeast need working mitochondria to make these bananas extra attractive to fruit flies. Image from Wikimedia Commons

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

Categories: Research Spotlight

Tags: mitochondria , Saccharomyces cerevisiae , Drosophila

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