How bacteria can spontaneously adapt to environmental changes
An international team of scientists has for the first time observed an evolutionary strategy called 'bet hedging' under laboratory conditions. The term bet hedging describes the way in which organisms ensure the survival of their species in rapidly changing environments by generating offspring suited to different living conditions. It is possibly one of the earliest evolutionary adaptation techniques. The researchers from Germany, the Netherlands and New Zealand have published their observations of bet hedging in the bacterial species Pseudomonas fluorescens in the journal Nature. In their experiments, the scientists introduced Pseudomonas strains into two different culture media. The variant that out-competed other variants in one medium was then placed in the other medium and vice versa, where the previously advantageous mutation would no longer be beneficial. As a consequence, new mutations and hence new variants evolved to compensate for this disadvantage. The researchers found that the bacteria ultimately developed strains with the same genetic makeup, which always produced two different variants, in both media. Those bet-hedging pseudomonades prevailed over other genotypes that adapted by mutation only. While evolutionary changes commonly take place over generations in which carriers of advantageous mutations prevail over those with less advantageous characteristics, bet hedging produces a generation of offspring that are genetically identical. However, some of the offspring are ideally adapted to the current environment. Others are better suited to completely different conditions, so that they would be able to survive in case of drastic environmental changes. The phenomenon has been observed in organisms ranging from bacteria to plants and animals. Bacterial pathogens, for instance, show varying cell surfaces (e.g. smooth vs rough), thus spreading the risk of detection by the host's immune system. 'Our experiments provide evidence that risk-spreading is a very successful strategy to rapidly adapt to changing environments,' said one of the authors of the study, Dr Christian Kost of the Max Planck Institute for Chemical Ecology in Germany. 'If the same genotype generates several variants at the same time, it may respond faster to major changes in the environment.' The study concludes: 'Insights into the underlying molecular details reveal how evolution tinkered with central metabolism to generate a strategy that could reasonably - one might think - have taken tens of thousands of generations to evolve. The rapid and repeatable evolution of bet hedging during our experiment suggests it may have been among the earliest evolutionary solutions to life in variable environments, perhaps even preceding the evolution of environmentally responsive mechanisms of gene regulation.' Pseudomonas fluorescens, which has an extremely flexible metabolism, is a bacterium found in soil and water. Some Pseudomonas fluorescens strains are useful in the protection of plants against parasites and other biocontrol scenarios. In culture, the bacteria produce an antibiotic. Pseudomonas fluorescens is also used in milk to make yoghurt. The cells divide every 52 minutes making them particularly suitable for the study of evolution.
Countries
Germany, Netherlands, New Zealand