EVOLUTION OF COMPETITIVE FITNESS IN EXPERIMENTAL POPULATIONS OF ESCHERICHIA-COLI - WHAT MAKES ONE GENOTYPE A BETTER COMPETITOR THAN ANOTHER

An important problem in microbial ecology is to identify those phenoty pic attributes that are responsible for competitive fitness in a parti cular environment. Thousands of papers have been published on the phys iology, biochemistry, and molecular genetics of Escherichia coil and o ther bacterial models. Nonetheless, little is known about what makes o ne genotype a better competitor than another even in such well studied systems. Here, we review experiments to identify the phenotypic bases of improved competitive fitness in twelve E. coli populations that ev olved for thousands of generations in a defined environment, in which glucose was the limiting substrate. After 10000 generations, the avera ge fitness of the derived genotypes had increased by similar to 50% re lative to the ancestor, based on competition experiments using marked strains in the same environment. The growth kinetics of the ancestral and derived genotypes showed that the latter have a shorter lag phase upon transfer into fresh medium and a higher maximum growth rate. Comp etition experiments were also performed in environments where other su bstrates were substituted for glucose. The derived genotypes are gener ally more fit in competition for those substrates that use the same me chanism of transport as glucose, which suggests that enhanced transpor t was an important target of natural selection in the evolutionary env ironment. All of the derived genotypes produce much larger cells than does the ancestor, even when both types are forced to grow at the same rate. Some, but not all, of the derived genotypes also have greatly e levated mutation rates. Efforts are now underway to identify the genet ic changes that underlie those phenotypic changes, especially substrat e specificity and elevated mutation rate, for which there are good can didate loci. Identification and subsequent manipulation of these genes may provide new insights into the reproducibility of adaptive evoluti on, the importance of co-adapted gene complexes, and the extent to whi ch distinct phenotypes (e.g., substrate specificity and cell size) are affected by the same mutations.