The team used samples from Michigan State University evolution biologist Richard Lenski’s long-term E. coli experiment that has been ongoing for more than 24 years since February 1988. Credit: G.L. Kohuth, Michigan State University
years ago researchers at Michigan State University (MSU) reported
discovering a novel, evolutionary trait in a long-studied population of
Escherichia coli, a rod-shaped bacterium commonly found in the lower
intestine of mammals. The E. coli added a helping of citrate to its
traditional diet of glucose, even though other E. coli can’t consume
citrate in the presence of oxygen.
These same biologists have now analyzed this new trait’s genetic origins and found that in multiple cases, the evolving E. coli
population needed more than one mutational step before the key
innovation took hold. Complex traits, like using a new food source, are
thought to be difficult and arise rarely, making the research of broad
interest to both evolutionary biologists and public health scientists.
The findings, reported in this week’s journal Nature,
document the step-by-step process by which organisms evolve new
functions. The study also highlights the importance of evolutionary
changes that alter the physical arrangement of genes, leading to new
patterns of gene regulation.
normally can’t digest citrate when oxygen is present because they don’t
express the right protein to absorb citrate molecules. Citrate is a
salt or class of citric acid commonly found in fruit such as lemons. So
how did this mutation occur?
find the answer, postdoctoral researcher Zachary Blount and MSU Hannah
Distinguished Professor of Microbiology and Molecular Genetics Richard
Lenski analyzed dozens of complete genome sequences from bacteria that
had evolved this new trait and had been sampled and stored at different
time points in the history of the lineage.
National Science Foundation’s Division of Environmental Biology partly
funded the research, as did the NSF-supported BEACON Center for the
Study of Evolution in Action.
The team used samples from Lenski’s long-term E. coli
experiment that started in February 1988 and has been ongoing for more
than 24 years. The experiment allows Lenski and his students and
colleagues to study more than 56,000 generations of bacterial evolution.
In terms of generations, it is the longest running evolution experiment
Twelve populations of E. coli
live in an incubator in Lenski’s laboratory producing about seven new
generations every 24 hours. Each day, scientists take one percent of
each population and transfer it into a new food source, a flask
containing fresh glucose, which the bacteria readily eat, and citrate,
which one population discovered how to eat after more than 30,000
generations. The researchers also take samples every 500 generations,
and freeze them for later study.
they freeze the samples, when something new emerges the scientists can
go back to earlier generations to look for the steps that happened along
the way, which is what occurred in this case.
researchers found that at least three mutations were required for the
bacteria to effectively use citrate when oxygen is present. One or more
mutations were necessary to set the physiological stage for the two
later events. Then a critical gene duplication occurred that effectively
re-wired the expression of a previously silent gene.
“These bacteria have evolved to consume a food resource—citrate—that no wild E. coli
uses. Three mutations are required for this to happen, and they must
occur in a specific order,” said George Gilchrist, NSF program manager
for the BEACON Science and Technology Center. “This study shows that the
first mutation is required to set the stage for the next two, but
surprisingly, this turns out to occur repeatedly and independently in
different populations. What this suggests is that complex traits, at
least in the microbial world, can evolve quickly and repeatedly.”
Additional co-authors include Jeff Barrick, University of Texas and Carla Davidson, University of Calgary.
Source: National Science Foundation