E. coli populations from the long-term evolution experiment (LTEE). The center population evolved the capacity to use citrate.WIKIMEDIA; BRIAN BAER, NEERJA HAJELASince 1988, Richard Lenski has been propagating the same 12 cultures of E. coli at Michigan State University, observing how they change over time. These cultures are grown on a low-glucose medium that includes citrate. Every day, members of the Lenski lab transfer the cultures to new media at a dilution of 1:100.
The team’s 2008 finding that some of the bacteria could use citrate as a carbon source under aerobic conditions was considered a game-changer—a potential example of how a new species could emerge (E. coli’s inability to metabolize citrate aerobically is one of its defining phenotypic features). Lenski and colleagues attributed the 15-year delay in the appearance of citrate-eating E. coli to the slow accumulation of “potentiating mutations,” genetic changes that provide no discernible advantage at the time but set the stage for future adaptation. Whether a particular culture has a citrate-friendly genetic background depends on its history, the researchers proposed, an idea called historical contingency.
Now, the authors of a study published this month (February 1) in the Journal of Bacteriology suggest that the delay ...
