Bacteria in Lab Tests Seem to Direct Own Evolution
In studies that appear to sharply contradict conventional ideas about the nature of genetic mutations and their role in evolution, two research groups have independently found that some bacteria seem to be able to recognize changes in their environment and choose--or even stimulate--the mutations that will be most beneficial to them.
These mutations, which enable the bacteria to prosper in the altered conditions, then are passed along to the bacteria’s progeny. In a sense, the bacteria thus seem to be directing their own evolution--a startling concept that has not been seriously considered by biologists for more than 50 years.
The new results seem to support the largely discredited views of the 18th-Century biologist Jean-Baptiste Lamarck, who argued that acquired traits could be passed on to progeny. His theory would postulate, for example, that a blacksmith who developed strong muscles in his work would be destined to have children with stronger-than-average muscles.
Lamarck’s views were fervently adopted by Soviet geneticist Trofim Denisovitch Lysenko, a Stalin confidant who held great sway in the Soviet Union from the 1930s to the 1950s. His erroneous theories stifled Russian biology and are widely viewed as the cause of the inadequate agricultural production in the Soviet Union that persists today.
But the new results suggest that Lamarck’s theories have a germ of truth.
“The evolutionary implications are very important” because they will change our views of how mutations arise and how microorganisms evolved into more complex creatures, said molecular biologist Barry G. Hall of the University of Connecticut. “But it is equally important to recognize that we have seen just a very few cases, all from one organism.”
“It would be a great leap in belief to say that something like this could operate in more complex organisms,” added biologist John Cairns of the Harvard School of Public Health.
The conventional wisdom of biology holds that mutations in deoxyribonucleic acid (DNA), the chemical blueprint of all organisms, occur continuously and randomly when chemicals or radiation alter the identity of individual chemicals within DNA or, more often, when a cell’s DNA is not copied perfectly during replication.
Most Are Harmful
Most such mutations are harmful and the mutated organism dies. But in some cases, biologists believe, the mutation is preserved and, at some later date, allows the altered organism to move into a different ecological niche under the pressures of environmental stress.
In all cases, according to the conventional wisdom, the mutations occur before the environmental stress.
But Hall’s and Cairns’ experiments indicate that mutations also can occur in response to environmental stress, a possibility that most scientists had thought unlikely. Cairns’ results are published in today’s issue of the British journal Nature. Hall’s have been accepted for publication in the journal Genetics.
In one experiment, Cairns and his colleagues studied an intestinal bacterium called Eschericia coli that had been genetically altered so it could not use the sugar lactose as an energy source.
If the bacteria are allowed to proliferate in a growth medium containing other sugars, a very small number will mutate during replication and reacquire the ability to use lactose. But if the bacteria are placed on a growth medium containing only lactose as an energy source, they remain alive without proliferating--and thus, according to the conventional wisdom, with little chance to mutate.
But within a few days after the bacteria were placed on a medium containing only lactose, Cairns observed a sharp increase in growth indicating that a significant number of bacteria had mutated to be able to use lactose. But he observed no increase in the number of other mutations, indicating that the overall mutation rate had not increased. Two other experiments involving different types of mutations showed the same results.
These experiments suggest “that bacteria have mechanisms for making just those mutations that adapt the cell to an available energy source,” said molecular biologist Franklin W. Stahl of the University of Oregon in a commentary in the same issue of Nature.
Hall’s results are, in Cairns’ words, “much more flamboyant.” He studied E. coli that are unable to use a different sugar, called salicin. In this case, however, two separate mutations are required before the bacteria can use the sugar.
Frequency of Mutations
One of these two mutations occurs infrequently in proliferating E. coli, Hall said, while the second mutation occurs so infrequently that it is never observed in the laboratory. But when the altered bacteria are placed in a growth medium containing only salicin, he said, within two weeks both mutations occur and the bacteria start proliferating.
“The mutations occur, or occur more frequently, only when they are of benefit to the cell,” Hall said.
In both Hall’s and Cairns’ studies, there is no evidence to indicate how the mutations are occurring, and Hall predicts that many investigators will begin seeking such evidence. “It will be an exciting problem to see what are the rules of this game and how general it is.”