Mainstreaming Caenorhabditis elegans in experimental evolution

Abstract

Experimental evolution provides a powerful manipulative tool for probing evolutionary process and mechanism. As this approach to hypothesis testing has taken purchase in biology, so too has the number of experimental systems that use it, each with its own unique strengths and weaknesses. The depth of biological knowledge about Caenorhabditis nematodes, combined with their laboratory tractability, positions them well for exploiting experimental evolution in animal systems to understand deep questions in evolution and ecology, as well as in molecular genetics and systems biology. To date, Caenorhabditis elegans and related species have proved themselves in experimental evolution studies of the process of mutation, host-pathogen coevolution, mating system evolution and life-history theory. Yet these organisms are not broadly recognized for their utility for evolution experiments and remain underexploited. Here, we outline this experimental evolution work undertaken so far in Caenorhabditis, detail simple methodological tricks that can be exploited and identify research areas that are ripe for future discovery.

Keywords: Caenorhabditis; evolution; experimental evolution.

Figure 1.

(a) Life cycle of C. elegans at 25°C, annotated with key life-history features pertinent to experimental study. (b) A schematic diagram indicating some of the many genetic and environmental manipulations possible in rearing worms. See main text for more details on methods and examples.

Figure 2.

Example fitness and phenotyping assays in Caenorhabditis. (a) Competitive fitness on Petri dishes (or in liquid). Two strains of worm are inoculated on each plate and serially transferred. Change in frequency of strains is determined over time by the use of a fluorescent transgenic marker [18,32]. (b) Non-competitive liquid fitness assays. Worms are inoculated into wells seeded with bacterial food in high-throughput microtitre plates. Absorbance change is measured over time and growth parameters determined for each well [35,36]. Alternatively, worms in liquid culture can be put through a biosorter to count, measure and sort the animals. (c) Microfluidic devices have been designed for worm sorting and phenotyping [37]. For example, worms are added to a microfluidic chip and stopped at a junction. Worms can be automatically scored for length, or sorted by fluorescence or another trait ranging from chemotaxis to fecundity [37]. (d) Animal behaviour also can be assessed in relatively high-throughput assays. For example, videos of worm movement when added to a plate with a chemoattractant or environmental gradient permit worm-tracking equipment or software [26], coupled with image processing, to determine and track the skeleton of the worm. Worm trajectories can then be quantified in terms of speed, curvature and other features. (Online version in colour.)