May 27, 2015
As anyone who watches a situation comedy knows, long range relationships are tricky. The longer the couple is separated, the more they drift apart. Eventually they are just too different, and they break up.
Of course if this were the end of the story, it would be the plot of the worst comedy ever. What usually happens in the sitcom is that one or both of them find someone more compatible and live happily ever after (with lots of silliness and high jinks).
Turns out that according to a new study by Hou and coworkers, our friend Saccharomyces cerevisiae could star in this sitcom. When different populations live in different environments, they drift apart. Eventually, because they accumulate chromosomal translocations and other serious mutations, they have trouble mating and having healthy offspring.
Now researchers already knew that big changes in yeast, like chromosomal translocations, affect hybrid offspring. But what was controversial before and what this study shows is that, as is known for plants and animals, smaller changes like point mutations can affect the ability of distinct populations of yeast to have healthy progeny. It is like Jerry Seinfeld being incompatible with a girl because she eats her peas one at a time (click here for other silly reasons Jerry breaks up with girlfriends).
The key to finding that yeast can be Seinfeldesque was to grow hybrid offspring in different environments. Hybrids that did great on rich media like YPD sometimes suffered under certain, specific growth conditions. Relying on the standard medium YPD masked mutations that could have heralded the beginnings of a new species of yeast.
See, genetic isolation is a powerful way for speciation to happen. One population generates a mutation in a gene and the second population has a mutation in a second gene. In combination, these two mutations cause a growth defect or even death. Now each population must evolve on its own, eventually separating into two species.
To show that this is a route that yeast can take to new species, Hou and coworkers mated 27 different Saccharomyces cerevisiae isolates with the reference laboratory strain S288C and grew their progeny under 20 different conditions. These strains were chosen because they were all able to produce spores with S288C that were viable on rich medium (YPD).
Once they eliminated the 59 pairings that involved parental strains that could not grow under certain conditions, they found that 117 out of 481 or 24.3% of crosses showed at least some negative effect on the growth of the progeny under at least some environmental conditions. And some of these were pretty bad. In 32 cases, at least 20% of the spores could not survive.
The authors decided to focus on crosses between S288C and a clinical isolate, YJM241, where around 25% of spores were inviable under growth conditions that required good respiration, such as the nonfermentable carbon source glycerol. They found that rather than each strain having a variant that affected respiration, the growth defect happened because of two complementary mutations in the clinical isolate.
The first mutation was a nonsense mutation in COX15, a protein involved in maturation of the mitochondrial cytochrome c oxidase complex, which is essential for respiration. The second was a nonsense suppressor mutation in a tyrosine tRNA, SUP7. So YJM241 was fine because it had both the mutation and the mechanism for suppressing the mutation. Its offspring with S288C were not so lucky.
Around 1 in 4 progeny got the mutated COX15 gene without SUP7 and so could not survive under conditions that required respiration. Which of course is why this was missed when the two strains were mated on YPD, where respiration isn’t required for growth.
So this is a case where the separated population, the clinical isolate YJM241, changed on its own such that it would have difficulty producing viable progeny with any other yeast strains. Like the narrator in that old Simon and Garfunkel song, it had become an island unto itself.
The researchers wondered whether this kind of change—a nonsense mutation combined with a suppressor—occurs frequently in natural yeast populations. They surveyed 100 different S. cerevisiae genome sequences and found that nonsense mutations are actually pretty common. Nonsense suppressor mutations were another story, though: they found exactly zero.
Apparently nonsense suppressor mutations are really rare in the yeast world, and Hou and colleagues wondered whether this was because they had a negative effect on growth. They added the SUP7 suppressor mutant gene to 23 natural isolates. It had negative effects on most of the isolates during growth on rich media, but it was more of a mixed bag under various stress conditions. Sometimes the mutation had negative effects and sometimes it had positive effects.
The fact that a suppressor mutation can provide a growth advantage under the right circumstances, combined with the fact that they are very rare, suggests that a new suppressor arising might help a yeast population out of a jam, but once the environment improves the yeast are free to jettison it. Suppressor mutations may be a transitory phenomenon, a momentary dalliance.
So, separate populations of yeast can change over time in subtle ways that prevent them from mating with one another. This can eventually lead to the formation of new species as the changes cause the two to drift too far apart genetically. It is satisfying to know that yeast drift apart like any other plant, animal, or sitcom character.
by D. Barry Starr, Ph.D., Director of Outreach Activities, Stanford Genetics
Categories: Research Spotlight