HUGHES, Bradley/S. *; CULLUM, Alistair/J.; BENNETT, Albert/F.; University of California, Irvine; Creighton University; University of California, Irvine: Experimental Evolutionary Adaptation to Environmental pH in the bacterium

Abstract�The evolutionary responses of organisms to changes in abiotic environmental factors�such as temperature and pH�have been of longstanding interest, because they are naturally variable and can sometimes reach stressful or even lethal levels. We discuss our current research using the bacterium Escherichia coli to study evolutionary responses over 2000 generations across an environmental acidic-alkaline pH range. Populations of E. coli (in six-fold replication) were exposed to one of four different constant conditions: benign ancestral pH of 7.2, stressful novel acidic pH of 5.4, stressful novel alkaline pH of 8.0, and benign novel pH of 6.4. All four groups showed improved relative fitness in their own specialized pH environment (direct fitness response). One might expect that a fitness gain in a specialized environment would result in a fitness loss in other environments as a cost of specialization, or tradeoff. Our results seem to provide support for tradeoffs when examined at a large group level. However, closer investigation of the individual 8.0 pH lines revealed that although some exhibited tradeoffs, other lines demonstrated no tradeoffs and actually increased relative fitness in all tested pH environments. Supported by NSF Grant IBN9905980 and NASA Grant 632731