Back in December PNAS published a paper from Oliveira et al. called “Evolutionary limits to cooperation in microbial communities” . My research interests lie right at the intersection of evolution and cooperation so I was fascinated by the idea that evolution imposes limits on cooperation. In this paper, Oliveira et al. examine the evolutionary dynamics of a community of microbes that can exchange a number of valuable secretions between different strains. This experimental setup enables the evolution of cooperation if one genotype focuses on producing one secretion and shares that secretion with a different strain while also gaining access to that other strain’s secretions. Ultimately, however, they found that cooperation only evolved under specific and limited conditions because of the fitness decrease that occurs when an individual isn’t close enough to receive secretions from another strain.
Because I enjoy reading papers that are not in my immediate specialty, I frequently track the interesting information I learn in both the background literature review section as well as the main results of the study. In this feature, I discuss those bits of information that I found relevant to my interests and just generally cool.
- Oliveira et al. describe a number of studies that have been able to easily select for microbes that cooperate in the laboratory even between genotypes. It has even been suggested that species that are obligate mutualists with other genotypes could therefore be unable to be cultured in monoculture laboratory settings.
- On the other hand, empirical evidence of microbes in natural settings seems to suggest that competition prevails over cooperation in nature. This and the above point combine to make it pretty clear that we don’t have a great idea of how much cooperation is really going on in the microbial world all around us (and in us!).
- There is a possible Black Queen dynamic in this system if the microbes both started with the ability to produce two valuable but costly secretions and then each lost the ability to produce one because they could use the secretions from the other. The Black Queen hypothesis refers to the dynamic of trait loss when the trait is a public good that can be scavenged from other organisms’ efforts. This is closely related to the idea of cheaters, where an organism loses the ability to perform an altruistic act but still benefits from the altruism of other organisms.
- To determine some of the evolutionary effects on microbes in a two-secretion system, Oliveira et al. grew four genotypes in low, intermediate, and highly-mixed environments. The genotypes were (1) a strain that could produce both secretions, (2) two strains that could only produce one of the secretions, and (3) a strain that couldn’t produce either of the secretions. They found that at low mixing the strain that produced both secretions was fittest, likely because the secretions did not diffuse far enough for “cheating” strains to use them. At high mixing they found that the genotype that doesn’t produce either secretion was most successful, likely because the public goods were distributed throughout the medium and therefore were easily accessible to cheaters. Finally, at intermediate mixing a cross-feeding situation was able to exist where the two genotypes that could only produce one secretion were successful, likely because they could share their resources mostly with each other.
- Oliveira et al. also found that increasing the cost to produce the secretion or increasing how many secretions are necessary favors single-secretion genotypes over full secretors. The higher cost seems to make it all the more beneficial to drop one costly trait as long as the strain can get that secretion from other organisms. An increase in the number of secretions required, however, makes the cooperative cross-feeding fragile due to how many other genotypes an organism must find in order to succeed.
- While cooperative cross-feeding intuitively seems like a good thing, Oliveira et al. actually found that the more cooperation there was, the lower the group productivity compared to a monoculture of the full secretor. This is likely due to the very loss of traits that led to cooperation in the first place: fewer organisms producing a secretion means less secretion overall.
- They conclude with the idea that cross-feeding cooperation seems like it will be fragile and rare in natural systems unless mechanisms are in place that this experimental design did not include. Possible mechanisms include detection of and movement towards partner genotypes to avoid cheaters and any other mechanism that allows partners to help and police each other.
Since I use Avida to explore the evolutionary dynamics of cooperation, I now of course can’t stop thinking about ways to implement these experiments and extend them to determine if a mechanism like movement could lead to more stable cooperation. In the meantime though, I highly recommend you check out the full paper if you found the above seven bits of trivia as interesting as I did.
 Oliveira, Nuno M., Rene Niehus, and Kevin R. Foster. “Evolutionary limits to cooperation in microbial communities.” Proceedings of the National Academy of Sciences 111.50 (2014): 17941-17946.