Meeting Abstract

P3.29  Jan. 6  Evolutionary changes in the pH tolerance range of experimental Escherichia coli lineages. CULLUM, A.J.*; CURRIN, T.J.; BENNETT, A.F.; Creighton Univ.; Creighton Univ.; Univ. of California, Irvine acullum@creighton.edu

When a population experiences a change in an environmental variable, evolutionary changes in the relationship between fitness and the variable are expected. The most well-examined and dependable such change is an increase in fitness at the new value of the variable; i.e. the direct response. Correlated responses, however, are less well examined. For example, as a population adapts to a change in an environmental variable, does its tolerance range also shift? In this study, we examine the evolutionary changes in the acid/base tolerance range of lines of the model organism Escherichia coli allowed to adapt to media with different pH values. A common ancestral genotype was used to found six replicate lineages that evolved for 2000 generations in each of the following environments: pH 5.4, pH 7.8 and the ancestral environment of pH 7.1. The pH tolerance ranges of the ancestral and evolved lineages are determined to the nearest 0.1 unit by propagating populations in media of differing pH values for up to 20 generations. Populations that die out entirely or show continued decline in density at a particular pH are considered to be outside their tolerance range. Our results to date are surprising in that, relative to the ancestral genotype, none of the pH 5.4 or pH 7.8 lineages show an increase in their tolerance range in the direction of their environmental shift; in fact, some lineages even show a contraction in their tolerance range in that direction. We are currently measuring the other end of the tolerance range for these two groups of lineages, as well as the full tolerance range of the pH 7.1 lines that continued to evolve in the ancestral condition.