Going back to move forward
Evolutionary innovations may require some previous adaptations to be reversed.
Bacteria and other organisms are constantly under pressure to survive in the face of ever-changing environmental challenges. They generally adapt to these challenges through genetic mutations that modify features they already have. However, occasionally a species may acquire an entirely new characteristic — known as an evolutionary innovation — that allows it to colonize a new environment or adopt a new mode of life.
The Lenski Experiment, which began in 1988, is an ongoing study of the evolution of bacteria grown in the laboratory. The experiment started with twelve identical populations of bacteria and has so far tracked the genetic mutations that have been acquired by the populations over tens of thousands of generations. Fifteen years into the experiment, bacteria in one of the populations evolved the ability to exploit a new food source, a molecule called citrate. The bacteria in this population have multiple mutations in a gene called gltA,which encodes an enzyme called citrate synthase. However, it was not clear how these mutations contributed to the ability of the bacteria in this population to use citrate.
Erik Quandt and colleagues have now used a variety of genetic and biochemical techniques to examine the mutations in gltA. They found that one mutation occurred before the bacteria evolved the ability to use citrate, and others occurred afterward. The first mutation in gltA increased the activity of the citrate synthase enzyme, which paved the way for a key mutation affecting citrate transport into cells that allowed the bacteria to consume the new food source. However, once the bacteria evolved the ability to use citrate, and more mutations in other genes refined this process, the increased citrate synthase activity became detrimental. At this point, the bacteria acquired a second gltA mutation that lowered citrate synthase activity to a level below what it had been in the original bacteria before the first gltA mutation.
The Lenski Experiment presents a rare opportunity to examine the complete history of an evolutionary innovation. Quandt and colleagues’ findings show that evolutionary ‘reversals’ may be necessary to adjust cell processes in different ways as an innovation first evolves and is further refined. A challenge for future work is to identify the other mutations that, together with the first gltA mutation, were necessary for the bacteria to evolve the ability to use citrate.
To find out more
Read the eLife research paper on which this eLife digest is based: “Fine-tuning citrate synthase flux potentiates and refines metabolic innovation in the Lenski evolution experiment” (October 14, 2015).
Read a commentary on this research paper: “Metabolism: Evolution retraces its steps to advance”.
