Brewing beer is a tricky business. A lot of the ingredients, like the yeast and the hops, don’t go into the finished product and need to be filtered out. In the case of yeast, this is made much easier by the process of flocculation: once all the sugars in the brew have been consumed during fermentation, the yeast falls out of solution and collects at the bottom of the tank. The amount of yeast left in the brew affects both the taste and the cost of production: flocculation has to happen at the correct rate to produce a good beer at a good price. Because of this, artificial selection pressure on flocculation has been very heavy during the domestication of yeast (Saccharomyces cerevisiae) for brewing (Jin and Speers, 1998).
Flocculation is controlled by a family of genes called the flocculins (Jin and Speers, 1998). During artificial selection, individual flocculin genes could have evolved quickly, as has been documented for different genes in numerous other domestication episodes (for example, the teosinte branched1 gene in maize (Wang et al, 1999)). Additionally, the family as a whole could have been selected to increase in size through duplication, with each extra duplicate in the family providing more raw material on which selection could act – examples of which are extremely rare.
To search for increases in gene family size during yeast evolution, Hahn et al (2005) measured the sizes of all gene families shared by five species of yeast. They then measured the change in size of each gene family along the phylogeny of these five species, and compared this to the change expected by chance, to identify those families that got significantly bigger or smaller during yeast evolution.
They found that the flocculin gene family increased dramatically in size along the branches leading to S. cerevisiae (brewer’s yeast). While there are 6-11 flocculins in the other species of yeast, S. cerevisiae has 14 flocculins. This increase in gene family size may be adaptive: selection on flocculation could have caused retention of flocculin duplicates, each of which can specialize in a different aspect of flocculation.
The next step would be to measure the evolutionary rate and function of each member of the flocculin family in S. cerevisiae, to confirm that they have been retained by selection. However, this study is still a remarkable demonstration of the dramatic changes in gene copy number that can occur in response to strong selection, be it natural or artificial. As the authors say:
“This is the first example to our knowledge, however, of selection on gene family size being implicated in domestication.”
So the next time you raise a glass, remember the key ingredient!
Jin, Y-L. and Speers, A. (1998) Flocculation of Saccharomyces cerevisiae. Food Research International 31:421-440.
Hahn, M.W., de Bie, T., Stajich, J., Nguyen, C. and Cristianini, N. (2005) Estimating the tempo and mode of gene family evolution from comparative genomic data. Genome Research 15:1153-1160
Wang, R-L, Stec, A., Hey, J., Lukens, L. and Doebley, J. (1999) The limits of selection during maize domestication. Nature 398:236-239