In the microbe-eat-microbe world, the weak often prevail against the strong, provided the weak are present in larger numbers, on the order of 99:1. And yet even when the weak qualify as the 99%, their occupy movements do not always spread. If they did—if the weak were ever able to declare “winner take all”—they would reduce genetic diversity. In fact, genetic diversity often endures competitive struggles between weak and strong. But how?
One idea is that less-competitive microbial strains may be confined to niches that exclude more-competitive strains. This idea, a variation of positive frequency-dependent selection model, holds that genetic diversity can be sustained by patchily structured populations and communities. To put this idea to the test, researchers at ETH Zurich studied isolates from a local natural population of the highly social bacterium Myxococcus xanthus.
M. xanthus lives in soils almost all over the world and is capable of social interactions; that is, individuals join forces to go hunting together for other bacteria and fungi. In times of need, several bacteria from this species can jointly form fruiting bodies with spores that can survive without water or nutrients for a long period of time. This works particularly well with bacteria that are closely genetically related. If the individuals are too genetically different, they might mutually impede and destroy one other.
At ETH Zurich, Olay Rendueles, Ph.D., working in the laboratory of Gregory J. Velicer, Ph.D., organized a sort of M. xanthus tournament across a number of petri dishes. First, she checked which strains prevailed in one-to-one duels. Here, she found that the more competitive strains always prevailed and destroyed the weaker ones. Diversity was therefore lost. Next, she confirmed that if a less-competitive strain outnumbered the stronger strain by a ratio of 99:1, the weaker strain prevailed.
After these preliminaries, the tournament got more interesting. Dr. Rendueles arranged the duels in a checkerboard fashion over four fields, with the weaker strain predominating on a white field and the stronger strain predominating on the black field, then the numerically superior strain always prevailed in the respective field.
If the less competitive strain prevailed in its field thanks to its numerical superiority, then it successfully held on to this field. The more competitive strain could not take over this niche. A social barrier between the genetically different strains hindered the assault.
Dr. Rendueles discovered that although two different strains can fight one another when single individuals are in direct contact, they cannot fight remotely; for example, by using antibiotics to kill the opponent. Overall, diversity was therefore retained across the total population, which encompassed all four fields.
These results were presented June 4 in Current Biology, in an article entitled, “Positively Frequency-Dependent Interference Competition Maintains Diversity and Pervades a Natural Population of Cooperative Microbes.”
“All strains that compete poorly at intermediate frequency are shown to be competitively dominant at high frequency in most genotype pairings during both growth and development, and strongly so,” wrote the authors. “Interference competition is often lethal and appears to be contact dependent rather than mediated by diffusible compounds. Finally, we experimentally demonstrate that positively frequency-dependent selection maintains diversity when genotype frequencies vary patchily in structured populations.”
These findings suggest that positive frequency dependent selection need not be diversity-reducing. Accordingly, this mechanism may vie with another mechanism, negative frequency-dependent selection, to explain how genetic diversity is maintained. In this negative form of selection, rare gene variants enjoy an advantage over frequent, dominant variants because they fall victim to predators less often. For example, different color variants of a butterfly population may escape a predator because they are better camouflaged than the majority of the population. This selective advantage persists only for as long as the camouflage gene variant remains rare.
The positive form of selection, the ETH Zurich researchers found, can contribute to high levels of local diversity. Among M. xanthus social groups in natural soil populations, this kind of selection “reinforces social barriers to cross-territory invasion and thereby also promotes high within-group relatedness.”
“This is the first experimental evidence of this theoretically predicted mechanism,” said Dr. Rendueles. She adds that it would have been a different story were the population evenly distributed; for example, in aqueous solution as found in the sea. In such a situation, where all the strains present are able to mix, only the most competitive or numerically superior strains of bacteria survive. This inevitably leads to a reduction in genetic diversity.
The ETH researchers concluded that positive-selection mechanisms that maintain patchily distributed diversity should be investigated more thoroughly in other species.