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Michigan State University
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MEDIA CONTACT: Gregory Velicer
Center for Microbial Ecology
(517) 353-0809 or 353-3953 or
Sue Nichols
University Relations
(517) 353-8942, [email protected]
EMBARGOED FOR RELEASE 2 P.M. WEDS., APRIL 5, 2000.
CHEATING CAN PAY WHEN YOU'RE A BACTERIUM
EAST LANSING, Mich. - The adage that predicts cheaters will never prosper isn't necessarily true - at least among bacteria, according to an MSU study published this week in Nature.
Real-time evolution of bacterial social systems can be played out over thousands of generations on the stage of a culture dish. One group of bacteria called the myxobacteria exhibits fascinating social behaviors that can even involve altruism and cheating.
Researchers at Michigan State University have documented anti-social behaviors that suggest that cheaters may be common in natural populations of social bacteria.
The paper "Developmental Cheating in the Social Bacterium Myxococcus xanthus" appears in the April 6 edition of Nature, the British science journal. It continues the widely published work on experimental evolution from the lab of Hannah Professor Richard E. Lenski.
"Even in this relatively simple social system we see a cheating behavior," said Gregory Velicer, a research assistant professor at MSU's Center for Microbial Ecology and the paper's first author. "Now we know that social defection as an evolutionary strategy can happen at the bacterial level."
In evolutionary biology, social cheating is defined as behavior that does not contribute fairly to a cooperative process for the common good, yet provides an unfair reproductive advantage for the cheater.
Myxococcus xanthus is a soil-dwelling bacterium that exhibits several social behaviors. It moves in groups through the soil, swarming like a wolf pack toward prey, which it cooperatively digests and consumes. When times get tough due to low food supplies, the bacteria flock together into a multicellular structure called a fruiting body.
Within the fruiting body, Velicer explains, a minority of cells transform from actively growing rods into resting spherical spores. The hardy spores can then survive until they encounter a new food source and begin to grow again.
The process of fruiting body development and sporulation involves several stages of cell-to-cell communication mediated by distinct chemical signals. During this process many cells appear to sacrifice the chance to reproduce for the benefit of others, says Velicer. Many cells appear not to become hardy spores, but rather to die by literally spilling their guts. Compounds released by the dying cells may help other cells to become spores.
But the MSU research shows that not all Myxococcus strains are altruistic heroes for the common good. Some genetic variants are surprisingly proficient at freeloading.
Velicer, along with Lee Kroos, a professor in the Department of Biochemistry at Michigan State, and Lenski worked with some Myxococcus strains that had been evolved in liquid culture for 1,000 generations and with three other strains that had defined mutations in their developmental genes. Both the evolved and the defined mutants descended from the same common ancestor, known as the wild type, which displays all of the normal Myxococcus social behaviors. The evolved and defined mutants were all defective at forming fruiting bodies and spores when starved in isolation.
Logic would suggest that the defective mutants would suffer. But instead, they thrived.
When the mutants were mixed together with the wild type at the onset of food deprivation, some mutants had their sporulation level restored to various degrees. This restoration is due to the wild-type cells providing developmental signals that the mutants fail to make themselves.
Some of these mutants responded so well to the presence of the altruistic wild type that they actually sporulated better in such mixtures than did the wild type, even though these same mutants are very poor at making spores by themselves. This latter class of mutants is the cheaters, Velicer explains.
"The sporulation of cheaters is not merely rescued to normal levels by the presence of wild type, but the cheats actually form more spores than wild type in these mixtures," Velicer said.
Exactly how these cheaters win is not yet clear. But if they become too common in a population the overall social system suffers, Velicer said, just as too many insurance frauds would drive insurance companies into bankruptcy. In natural populations, therefore, cheaters may be common, but not so common that they eliminate the altruistic strains upon which their success depends.
This work gives insight into the population dynamics of the myxobacteria, which produce a wide variety of antibiotics (some of which may have medical applications) and play a key role in soil ecology around the world.
"We're seeing an impersonal demonstration of conflict and defection, which are important factors in the evolution of all cooperative social systems," he said. "While there is a significant level of cooperation in these bacteria, there may be enough conflict from cheaters to have prevented them from evolving into truly multicellular organisms. This may be what has held them back."
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