Live Evolution Witnessed In Controlled Environment Of Microbial Predator And Prey
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.
or 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.
morevia sciencenews
