The enduring utility of continuous culturing in experimental evolution

Abstract

Studying evolution in the laboratory provides a means of understanding the processes, dynamics and outcomes of adaptive evolution in precisely controlled and readily replicated conditions. The advantages of experimental evolution are maximized when the selection is well defined, which enables linking genotype, phenotype and fitness. One means of maintaining a defined selection is continuous culturing: chemostats enable the study of adaptive evolution in constant nutrient-limited environments, whereas cells in turbidostats evolve in constant nutrient abundance. Although the experimental effort required for continuous culturing is considerable relative to the experimental simplicity of serial batch culture, the opposite is true of the environments they produce: continuous culturing results in simplified and invariant conditions whereas serially diluted batch cultures are complex and dynamic. The comparative simplicity of the selective environment that is unique to continuous culturing provides an ideal experimental system for addressing key questions in adaptive evolution.

Keywords: Chemostat; Continuous culture; Experimental evolution; Microbial evolution; Turbidostat.

Figure 1

Continuous versus discontinuous modes of selection used in microbial experimental evolution. A. Serially diluted cultures experience variations in nutrient level and cell density over each growth cycle, including turbidostat-like and chemostat-like phases. B. Chemostat cultures grow at a set dilution rate and experience constant nutrient limitation akin to that seen in batch cultures just before nutrient exhaustion. C. Turbidostats can grow cultures at their maximal growth rate by tuning dilution rate based on culture density, generating constant population size and selection pressure.