Parallel evolution on a Petri dish

How predictable can evolution be? This question is surely hard to answer. Even the smallest bacterial genomes have hundred of thousands of nucleotides that can be mutated and the genotypic space to explore can be huge. Moreover, even in a well controlled environment like a laboratory evolutionary experiment the selective pressures applied can be quite complex. Can we expect evolution to follow always the same path if we repeat experiments?

Recent work from our lab shows that the answer can be yes. We carried out experimental evolution using swarming motility in the bacterium Pseudomonas aeruginosa. Our experiments started with the wild-type strain PA14, which is mono-flagellated – it has a single polar flagellum. When we grow a colony of PA14 for 24 h on a Petri dish prepared with a minimal medium soft agar recipe, the colony forms a neat shape with a characteristic branched pattern. We passaged the bacteria to a new plate every day, and after only a few days we saw the bacteria evolve a different phenotype where they cover the entire plate. We call this phenotype “hyperswarming”.


The hyperswarmer evolution experiment

P. aeruginosa becomes a hyperswarmer by getting a point mutation in certain residues of a gene called fleN. The mutation makes the bacterium, which is normally mono-flagellated, become multi-flagellated.

Perhaps even more interesting, when we repeated the experiment several times we always found hyperswarmers with point mutations in the same gene. This is surprising because there are many genes and pathways known to affect swarming motility and therefore there were potentially many paths open for evolutionary adaptation.

Our experiments show that evolution can be, to some extent, predictable. The swarming experiments are based on a simple, well-established, microbiology protocol. In a recent article in Trends in Microbiology we encourage microbiologists to think about the evolutionary implications of their experiments.

Read also the piece by Carl Zimmer in his column “Matter” at the New York Times, which includes videos of swarming colonies.

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