| Description |
The genetic sequencing of the yeast Saccharomyces cerivisiae is now complete. However, nearly two-thirds of its genes known to code for proteins have unknown functions. These genes would be inactivated by stop codons if they provided no useful function to the yeast. When mutations occur within genes that perform necessary functions for the organism, they are eliminated by selection, but unnecessary genes can accumulate these mutations without effecting the fitness of the organism. For many of these genes, even deliberate disruption fails to produce an obvious phenotype. Three hypotheses were considered to account for genes which can be disrupted without being fatal to the yeast: that they could be selfish genes, that they provide a selective advantage in some but not all environments, or that they provide a slight but significant advantage in all environments. Disrupted strains of FY10 Saccharomyces cerivisiae were tested in head-to-head competitions against their parent strain. Close to seventy percent of these strains demonstrated a selective disadvantage in a standard environment. These results support the theory that those strains with disruptions in non-essential genes had a small but significant decrease in fitness. To discover whether these results would be consistent in many environments, similar competition experiments were carried out with four representative strains with selection coefficients from across the range of values. In each environment, yeast strains were grown in batch culture of a total volume of 2 ml and back-diluted daily into fresh medium, (with the exception of some 0 dilution competitions). Samples of the inoculated medium were taken periodically throughout each competition. Relative concentrations of each strain were determined by replica plating. Data from these experiments were graphed, and used to calculate correlation coefficients to demonstrate whether the fitness effects were significantly different from zero. The results of the experiments showed distinct differences between the fitness of each strain in varied conditions. Moreover, the fitness of each strain was not consistently greater or less than that of the parent strain for every environment. Thus, the fitness of each strain is conditional upon the environment in which the competition is carried out. However, there were no systematic patterns that might clarify the nature of the defects in the various strains. |
