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Witnessing what Darwin could only dream of: live evolution
05 March 2009
CNRS (Délégation Paris Michel-Ange)
Observing the mechanisms of evolution in order to understand how a species adapts to another under different ecological conditions was the goal of researchers at the Laboratoire Écologie et Évolution at CNRS, Université Pierre et Marie Curie and the École Normale Supérieure. They studied two bacteria - a predator and a prey - over 300 generations in a controlled environment. For the first time, these scientists were able to demonstrate that the coevolutionary process is dependent on ecological conditions. Indeed, under certain conditions, the prey becomes resistant to the predator, which itself evolves so that it can attack this new prey. In addition, the scientists issued a warning against the previously envisaged use of this predator (Bdellovibrio bacteriovorus) as a "living antibiotic" because, like other antibiotics, this could lead to the selection of resistant pathogenic bacteria.
Since publication of the Origin of Species by Charles Darwin 150 years ago, it has been known that one of the dynamics of evolution is natural selection. Its results depend on environmental conditions and interactions between the species present (competition, predation, parasitism, cooperation). Some twenty years ago, a new field of research - experimental evolution - started to develop, and it has enabled scientists to better understand the mechanisms underlying evolution. For example, one idea was to cultivate populations of bacteria under well-controlled conditions over a large number of generations. These populations are made up of numerous individuals that were initially identical from the genetic point of view. And because the turnover of generations was very rapid, just a few months were sufficient to observe the emergence of new mutants, constituting a source of genetically-different lines. Instead of reconstituting the past, the scientists thus became eye-witnesses to the appearance of new species.
Scientists in the Laboratoire Écologie et Évolution have used this type of experiment to understand how the environment influences the evolution of a pair of bacteria. One was the predator, Bdellovibrio bacteriovorus, and the other the prey, Pseudomonas fluorescens. The predator penetrated the prey and killed it by consuming it from the inside. This predator is a relatively common bacterium, one which British researchers have suggested could be used as a "living antibiotic". As for the prey, it benefits from a considerable capacity for adaptation: when cultured in a bottle of liquid medium (not agitated), it gave rise to two new forms (or species), each occupying an ecological niche: the WS (Wrinkly Spreader morph) formed a biofilm on the surface of the nutrient medium while the FS (Fuzzy Spreader morph) lived at the bottom of the bottle where low oxygen levels were present. The SM (ancestral smooth morph), the initial form, occupied the liquid phase of the oxygen-rich nutrient medium. Each species formed colonies with a different appearance, so that population diversification could be monitored.
During their experiment, the researchers worked on 36 populations of P. fluorescens, cultured in a liquid medium and sealed within 36 constantly-agitated bottles. They introduced the predator into half of these bottles. At regular intervals (2, 3 or 4 days), a fraction of each population (1% or 0.1%) was collected and inoculated into a new bottle filled with fresh culture medium. These transfers simulated an environmental disturbance and could be compared with a hurricane sweeping through a forest, flattening large trees but allowing other undergrowth species to develop. By varying the frequency and intensity of disturbances, some species were privileged over others, thus ensuring either the maintenance of biodiversity or, on the contrary, the proliferation of certain species. During the experiments, the frequency and intensity of transfers was varied, so that the researchers were able to simulate six different environmental conditions. Thus 20 successive transfers (corresponding to 300 generations) were performed, during which the predators and prey were preserved by freezing, making it possible to test the efficiency of predators on ancestral prey after evolution, or vice versa. By "manipulating" time in this way, the scientists were able to follow evolutions in predator efficiency and prey resistance.
At the end of the experiments, only the SM prey were present in predator-free bottles; no new species had appeared. In the other bottles, predator presence caused the natural selection of predator-resistant prey: the prey had thus evolved. The appearance of several types of resistant preys was observed, taking the SM, FS and WS forms. WS morphs adhered to the walls, were difficult to transfer and ultimately disappeared from the experiment. SM morphs grew more rapidly than FS, but were less predator-resistant. Depending on the conditions of transfer, one or the other was selected. The predator also evolved. It was able to adapt to FS resistant prey, but not to SM resistant prey. What makes prey resistant or predators capable of attacking them again remains poorly understood. Sequencing the genome of these bacteria will probably throw light on the underlying mechanisms. Whatever the case, these scientists showed for the first time that the coevolution of prey and predator is not systematic, but depends on the type of resistance displayed by the prey, which itself is selected as a function of ecological conditions. In the end, it is the environment that determines whether coevolution will occur, or not.
In addition, rather than using an antibiotic compound, two British researchers proposed a few years ago that B. bacteriovorus could be used as a predator to kill bacteria responsible for human diseases. They thought that no bacillus (a group of bacteria to which the prey, P. fluorescens, belongs) could develop resistance to this predator. These recent experiments have demonstrated the contrary. In the light of these findings, use of a bacterial predator as a living antibiotic could cause the selection of bacterial strains resistant to this predator, so that there would not necessarily be any further advantages to be gained over standard, chemical antibiotic therapy.
The predator bacterium, Bdellovibrio bacteriovorus, is equipped with a flagellum that enables it to seek actively for its prey (left). The right-hand image shows the predator inside its prey, in the process of feeding off the cytoplasm. © Edouard Jurkevitch
Pseudomonas fluorescens colonies in a Petri dish. From left to right: SM, FS and WS morphs. © Romain Gallet.