How do mutations affect growth and fermentation rates in yeast?

In the yeast Saccharomyces cerevisiae, glucose is converted to energy in oxygenated environments via fermentation rather than respiration.  Scientists are curious to discover why there is this preference for a relatively inefficient method of utilising glucose.

Aysha Sezmis and colleagues from the McDonald and Marshall labs within the Centre for Geometric Biology are looking at this question through the lens of evolutionary biology.  They recognised that although fermentation might be less efficient than respiration, it converts glucose into energy more quickly than respiring competitors.  In populations that have evolved with abundant resources, individuals that most rapidly convert resource into biomass and energy are favoured, even at the cost of efficiency.

Aysha and her colleagues were interested to find out whether mutations that have appeared repeatedly in experimental populations of yeast allowed the mutant populations to achieve greater fitness through their ability to ferment glucose at a wider range of glucose concentrations.

They re-created 6 mutations that repeatedly evolve in yeast evolution experiments by using a gene deletion technique and grew the different yeast populations in varying concentrations of glucose and ammonium sulphate.  Across these concentration gradients, they were able to detect if fermentation was the preferential method for acquiring energy by measuring ethanol; a metabolite of fermentation.  They then compared these rates with those of the ancestral population.  They were able to measure growth rates, maximum yields and fitness for each population at each combination of glucose and nitrogen concentration.

A ‘heat map’ showing the amount of ethanol produced for each of the six strains of the yeast S. cerevisiae at different nitrogen and glucose concentrations.

The team found that simple selection for high growth rates can drive the evolution of a preference for fermentation at glucose concentrations where respiration is preferred by the ancestral strain.  But most interestingly they found that nitrogen concentrations also played a part and the fermentation phenotype may only be ‘engaged’ at very low nitrogen concentrations.  They concluded that the mounting evidence for the importance of nitrogen abundance in the switch from respiration to fermentation as a preferred mechanism should be considered in future studies.