Long - term experimental evolution in Escherichia coli . XI . Rejection of non - transitive interactions as cause of declining rate of adaptation J Arjan

Background: Experimental populations of Escherichia coli have evolved for 20,000 generations in a uniform environment. Their rate of improvement, as measured in competitions with the ancestor in that environment, has declined substantially over this period. This deceleration has been interpreted as the bacteria approaching a peak or plateau in a fitness landscape. Alternatively, this deceleration might be caused by non-transitive competitive interactions, in particular such that the measured advantage of later genotypes relative to earlier ones would be greater if they competed directly. Results: To distinguish these two hypotheses, we performed a large set of competitions using one of the evolved lines. Twenty-one samples obtained at 1,000-generation intervals each competed against five genetically marked clones isolated at 5,000-generation intervals, with three-fold replication. The pattern of relative fitness values for these 315 pairwise competitions was compared with expectations under transitive and non-transitive models, the latter structured to produce the observed deceleration in fitness relative to the ancestor. In general, the relative fitness of later and earlier generations measured by direct competition agrees well with the fitness inferred from separately competing each against the ancestor. These data thus support the transitive model. Conclusion: Non-transitive competitive interactions were not a major feature of evolution in this population. Instead, the pronounced deceleration in its rate of fitness improvement indicates that the population early on incorporated most of those mutations that provided the greatest gains, and subsequently relied on beneficial mutations that were fewer in number, smaller in effect, or both. Background Twelve populations of Escherichia coli B were founded from a common ancestor, and they have evolved for 20,000 generations in a uniform environment with glucose as the density-limiting resource [1–3]. During that period, the populations adapted to this environment by the substitution of spontaneous beneficial mutations. Figure 1 shows the overall fitness trajectory of the 12 populations, measured in competition with the common ancestor. As shown by the excellent fit of the hyperbolic curve, the rate of fitness gain relative to the ancestor has strongly decelerated over time. Published: 30 October 2002 BMC Evolutionary Biology 2002, 2:19 Received: 6 September 2002 Accepted: 30 October 2002 This article is available from: http://www.biomedcentral.com/1471-2148/2/19 © 2002 de Visser and Lenski; licensee BioMed Central Ltd. This article is published in Open Access: verbatim copying and redistribution of this article are permitted in all media for any purpose, provided this notice is preserved along with the article's original URL.