Experimental evolution

Ever since Darwin, evolution – the result of the game of “chance and necessity”  as coined by J.Monod – has been held responsible for the great diversity of living organisms. The theoretical framework of evolution has been worked out to great details and is used to justify a posteriori the observation of biological structures and behaviours as arising from random mutations and selection of the fittest. However there are very few controlled evolution experiments that attempt to understand how an organism will a priori evolve in a new environment.
 
Does it evolve continuously or not? What are the genetic networks that contribute to that evolution? Do recombination mutants play a role or not? Is the organism evolving towards a unique form (solution)? If not, what characterizes the ensemble of observed solutions? Is there a limit to the evolvability (the power to evolve) of an organism? Moreover in a given population certain genetically identical individuals may adopt different phenotypes (behaviours). Can these different phenotypes co-exist? Is a phenotypic differentiation possible in a bacterial population?
 
Evidently the lack of controlled evolution experiments is due to the extremely long timescales of evolution, thousands of generations, which limit these experiments to simple fast breeding organisms such as bacteria and their phages. In fact, today on Escherichia coli there is really a single controlled evolution experiment: the one conducted for more than 15 years (>20,000 generations) by the Lenski lab at the University of Michigan. That experiment consisted in growing an E.coli population in a well defined medium by daily (100x) serial dilutions. Samples from the population were regularly taken and frozen for further study. This experiment is therefore time consuming and labour intensive.

The project we propose consists in launching an evolution experiment in a continuously fed automated chemostat in order to monitor with a low risk of pollution (due to minimal handling) and on a long timescale (months or years) the evolution of a bacterial population growing in a temperature gradient. The bacteria are continuously fed at a high temperature (~60°C) and removed at a low temperature (~37°C), thus creating an evolutionary pressure favouring invasion of the high temperature niches. Will they actually invade? By what type of mutations? Is there a limit to their evolvability, i.e. a maximal temperature beyond which they cannot adapt? Based on an original design by P.Marlière we constructed a chemostat consisting of an ensemble of Teflon and glass tubing and electro-switches controlled by a microprocessor responsible for the various cycles of the chemostat (bacteria feed, transfer, wash, etc.). We need to test that chemostat in real conditions over a few months and then duplicate it in 8-10 replicas: an evolution experiment can only be informative if it is reproduced identically a large number of times. In parallel with this high risk experiment we wish to conduct a number of more limited projects aimed at testing the evolution of cooperation in a bacterial population growing on a surface. 

 

Fig1: Schematic view of the setp. The top is heated at 60°C, the bottom is cooled at 30°C. Nutrients are provided from the top.

Fig2: Kymograph of the dynamics of E. coli in the temperature gradients. Red corresponds to high density and blue to low density. (here top is at the left and bottom at the right of the kymograph)