Originally published as Genetics Published Articles Ahead of Print on January 19, 2009.

Genetics, Vol. 181, 1057-1063, March 2009, Copyright © 2009
doi:10.1534/genetics.108.100297

The Strength of Selection Against the Yeast Prion [PSI+]

Joanna Masel1 and Cortland K. Griswold2

Department of Ecology and Evolutionary Biology, University of Arizona, Tucson, Arizona 85721

1 Corresponding author: Department of Ecology and Evolutionary Biology, 1041 E. Lowell St., University of Arizona, Tucson, AZ 85721.
E-mail: masel{at}u.arizona.edu

The [PSI+] prion causes widespread readthrough translation and is rare in natural populations of Saccharomyces, despite the fact that sex is expected to cause it to spread. Using the recently estimated rate of Saccharomyces outcrossing, we calculate the strength of selection necessary to maintain [PSI+] at levels low enough to be compatible with data. Using the best available parameter estimates, we find selection against [PSI+] to be significant. Inference regarding selection on modifiers of [PSI+] appearance depends on obtaining more precise and accurate estimates of the product of yeast effective population size Ne and the spontaneous rate of [PSI+] appearance m. The ability to form [PSI+] has persisted in yeast over a long period of evolutionary time, despite a diversity of modifiers that could abolish it. If mNe < 1, this may be explained by insufficiently strong selection. If mNe > 1, then selection should favor the spread of [PSI+] resistance modifiers. In this case, rare conditions where [PSI+] is adaptive may permit its persistence in the face of negative selection.




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A. K. Lancaster, J. P. Bardill, H. L. True, and J. Masel
The Spontaneous Appearance Rate of the Yeast Prion [PSI+] and Its Implications for the Evolution of the Evolvability Properties of the [PSI+] System
Genetics, February 1, 2010; 184(2): 393 - 400.
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