What Selective Benefits Counterbalance Formation of Deleterious Mutations?
A propensity for expansion and contraction of chromosomal DNA may help to account for the frequency with which selection for loss of function produces strains containing large deletions. About 20% of mutants selected on the basis of an inability to express pcaHG, the genes encoding the enzyme that acts upon protocatechuate (Fig. 1), contained deletions extending at least 7 kb into the dca region required for growth with dicarboxylic acids (Fig. 2). At first glance, a selective benefit that might be associated with such large deletions is not evident. In a broader sense, a system that gives rise to the deletion mutations as occasional accidents in some cells may be tolerated if it allows other members of the population to contain multiple tandem copies of genes that can be called upon by intermittent selective forces. This concept of evolutionary tradeoffs also provides an interpretation for shorter deletions that are frequently recovered after selection for spontaneous mutations causin
