Antonie van Leeuwenhoek 73: 35–47, 1998. 35 c 1998 Kluwer Academic Publishers. Printed in the Netherlands. Evolution of competitive fitness in experimental populations of E. coli: What makes one genotype a better competitor than another? Richard E. Lenski , Judith A. Mongold, Paul D. Sniegowski 1 , Michael Travisano 2 , Farida Vasi, Philip J. Gerrish & Thomas M. Schmidt Center for Microbial Ecology, Michigan State University, East Lansing, Michigan 48824, USA; 1 Present address: Department of Biology, University of Pennsylvania, Philadelphia, PA 19104, USA. 2 Present address: Department of Plant Sciences, University of Oxford, Oxford OX1 3RB, UK; ( author for correspondence) Received 24 January 1997; accepted 1 October 1997 Key words: cell size, competition, evolution, fitness, nutrient specificity, selection Abstract An important problem in microbial ecology is to identify those phenotypic attributes that are responsible for competitive fitness in a particular environment. Thousands of papers have been published on the physiology, biochemistry, and molecular genetics of Escherichia coli and other bacterial models. Nonetheless, little is known about what makes one 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 evolved for thousands of generations in a defined environment, in which glucose was the limiting substrate. After 10000 generations, the average fitness of the derived genotypes had increased by 50% relative 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. Competition experiments were also performed in environments where other substrates were substituted for glucose. The derived genotypes are generally more fit in competition for those substrates that use the same mechanism of transport as glucose, which suggests that enhanced transport was an important target of natural selection in the evolutionary environment. 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 elevated mutation rates. Efforts are now underway to identify the genetic changes that underlie those phenotypic changes, especially substrate specificity and elevated mutation rate, for which there are good candidate loci. Identification and subsequent manipulation of these genes may provide new insights into the reproducibility of adaptive evolution, the importance of co-adapted gene complexes, and the extent to which distinct phenotypes (e.g., substrate specificity and cell size) are affected by the same mutations. Introduction What phenotypic properties make one bacterial geno- type a superior competitor to another in a certain envi- ronment? Unfortunately, we cannot provide any gener- al answers to this question at the present time. Nonethe- less, we believe that this question is sufficiently impor- tant to the field of microbial ecology that it is worth- while to consider how one might go about getting an answer, even if only in some special circumstances. To that end, we will summarize our progress to date in understanding the phenotypic – and ultimately genet- ic – bases of enhanced competitive fitness in a mod- el experimental system. This review focuses on one aspect of a long-term evolution experiment that is on- going in our laboratory. Other reviews on experimental studies of microbial evolution can be found in papers by Dykhuizen & Hartl (1983), Hall (1983), Levin & Lenski (1983), Mortlock (1984), Dykhuizen & Dean (1990), and Lenski (1992, 1995). Before we describe our experimental system, let us consider the general nature and scope of the problem.