Microcosm_ E. Coli and the New Science of Life - Carl Zimmer [43]
Adams thought he might have contaminated his original colony with another strain, so he shut down the experiment and started over again. After the new colony had adapted to the low-glucose diet, Adams spread the microbes on plates again. And again he discovered the same big-and small-splotch makers. Adams ran the experiment a few more times, and he found that it took about 200 generations for the two types of microbes to emerge. He realized that a single clone was evolving time and again into two distinct types of E. coli.
Those two types turned out to be ecological partners. The large colonies are inhabited by microbes that do a better job than their ancestors at feeding on glucose. One of the waste products they give off is acetate. E. coli can survive on acetate, although it grows more slowly on it than on glucose. Adams discovered that some of his E. coli were becoming more efficient at feeding on acetate than their ancestors were. The acetate feeders grow slowly, but they aren’t driven to extinction because they are taking advantage of a food that the faster-growing bacteria aren’t eating. A food chain had emerged spontaneously in Adams’s lab as organisms began to depend on one another for survival.
Other scientists have confirmed Adams’s findings with experiments of their own. And they’ve created new kinds of ecological diversity from a single E. coli ancestor. Instead of a glucose-only diet, Michael Doebeli and his colleagues at the University of British Columbia supplied E. coli with both glucose and acetate. After a thousand generations, Doebeli found that the bacteria had evolved into big and small colonies. But they were different from the big and small colonies that Adams had produced. Both colonies in Doebeli’s experiment fed on glucose and acetate. The difference between them was a matter of timing. The big colonies fed on glucose until it ran out, and then they turned to acetate. The small colonies switched over sooner, so that they had a head start.
Doebeli and his colleagues then looked closely at how the genes in each colony had evolved. Typically, when E. coli is feeding on glucose, it keeps the genes for digesting acetate tightly repressed. If it made both sets of enzymes at the same time, they would get snared in a metabolic traffic jam. When the time comes to switch sugars, the bacteria must first destroy the enzymes for glucose and then build enzymes that can break down acetate. Doebeli found that in the small colonies, natural selection had favored mutants that stopped repressing their acetate genes. When glucose and acetate were available, these mutants fed on both kinds of sugar but did a lousy job of it compared with the glucose specialists in the large colonies. They got a reward for this sacrifice, however: they could leap quickly to take advantage of acetate while the big colony slowly retooled itself.
These experiments on E. coli may shed light on how new species form. Nature has formed its own petri dishes in Nicaragua, where dead volcanoes have filled with rainwater. These crater lakes are completely isolated from neighboring lakes and rivers, but on rare occasion a hurricane can sweep fish into them. In Lake Apoyo, which formed about 23,000 years ago, two species of cichlids live together. One of the fish, known as the Midas cichlid, is a big creature that roots around in the muck and crushes snails. The other fish, the arrow cichlid, is a thin, quick-darting creature that hunts for insect larvae in the open water. Their DNA indicates that the Midas cichlid was swept into the lake after it formed and that the arrow cichlid evolved from it. The split may have taken only a few thousand years.
Whether scientists study cichlids or E. coli or any other organism, they face the same question: Why specialize? Why don’t organisms evolve to become jacks-of-all-trades instead? There may simply be limits to how well one organism can do many things. Sooner or later they encounter a trade-off. A