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Few Genetic Paths From Here to There

September 12, 2012

Everyone knows that when the environment changes, those individuals with certain DNA differences useful in this new environment thrive while others wither.  But there hasn’t been a lot of work done to investigate how many DNA differences are available to a population for adapting to a particular environmental change.

How many paths lead to adaptation?

This may sound esoteric but the answer has real implications for speciation.  If there are few mutations possible and these mutations are very similar in terms of phenotype, then different populations will travel similar routes in their adaptations to the same environmental change.  This will definitely slow down speciation.  If on the other hand there are many genetic ways to adapt to the same change, then isolated populations will head down different paths leading to faster speciation.

In a new study out in GENETICS, Gerstein and coworkers found that at least for the environmental insult they used (low levels of the fungicide nystatin), there were very few paths to resistance. In fact, just four genes in the ergosterol biosynthesis pathway turned up in the 35 resistant lines they surveyed using whole genome sequencing.

Now that isn’t to say that there were just a few mutations.  There weren’t.  They found eleven unique mutations in the ERG3 gene, seven in ERG6, and one each in ERG5 and ERG7.  There were duplications, deletions, premature stop codons and missense mutations.  So there are lots of ways to mutate these few genes.

The small range of genes affected might suggest that adaptation favors populations evolving along similar paths since the same environmental effects result in the same adaptative mutations.  And yet, not all of these mutations in these few genes are created equally.  Different lines responded differently to other stressors.

For example, lines with mutations in the ERG3 gene responded poorly to ethanol while the other lines did very well.  And the lines with mutations in ERG5 and ERG7 responded less well to salt than the other lines.  So if one population was subjected to salt and nystatin and the other to ethanol and nystatin, the strains would almost certainly adapt with mutations in different genes.  Even within this narrow set of genes, there is room for adaptation by different routes.

While a useful first step, we don’t want to infer too much from this single study.  The researchers used a very specific environmental insult known to work through a specific pathway and found only mutations in that pathway.  The next study might want to focus on something like salt tolerance, a trait predicted to be achieved through multiple pathways.  Then we can get an even better feel for how many options a population has for adaptation.