The bobtail squid, whose mutualism with luminescent bacteria is an example for the new model. Photo by megpi.As a concrete example for their model, the authors refer to the mutualism between bobtail squid and a species of bioluminescent bacteria, which colonize the squid's light organ and makes it glow. Short of some kind of complicated squid-bacterium signaling system, how does a squid ensure that its light organ is only colonized by bacterial strains that will pay it back and generate light?
They charge a cover.
More specifically, while the squid's light organ supplies food for colonizing bacteria, it is also full of toxic reactive oxygen compounds. In order to take advantage of the food supply, a bacterium has to clear out these toxins—and conveniently, the bacterial enzyme that generates light consumes oxygen and removes the toxins in the course of the light reaction. So the only bacterial strains that colonize the squid's light organ are those that can pay the cost of eating up the oxygen-based toxins and still make a "profit" on the food supply provided by the squid, generating light in the process.
The trick in setting up this screening is to find the right balance of cover charge and reward for prospective mutualists. The cover charge paid by high-quality partners has to be high enough that low-quality partners won't accept it, and the reward offered for paying that high cost must be sufficiently good to make it worthwhile.
The authors suggest that this model should also apply not just to other mutualisms in which a host takes on microbial partners, such as plants' partnerships with nitrogen-fixing bacteria, or animals' interactions with the bacteria living in their guts—but also to interactions like obligate pollination mutualism or ants' protection interactions with some plants.
Yuccas and ant-protected plants have to screen mutualists, too—and may impose their own cover charges. Photos by jby and Alistair Rae.In the case of obligate pollination mutualism, like the one between yuccas and yucca moths, the cover charge is the effort involved in pollination—to guarantee a supply of yucca seeds for their larvae to eat, yucca moths must deliver plenty of pollen and do relatively little damage to the flower as they lay their eggs in it. There do exist yucca moth species who don't pollinate, but lay their eggs on yucca flowers after they've been pollinated and are starting to develop into fruit. The new model would predict that the lower effort of this strategy is reflected in a lower payoff, maybe a lower rate of survival for the eggs of these "bogus" yucca moths.
In the case of ant-protected plants, the cover charge is the effort involved in defending a host plant from other ant colonies that would like to occupy it. As it happens, parts of an ant-plant that are better protected grow to provide better food and shelter for the ants occupying them, which gives a competitive advantage to a colony of effective defenders trying to fight off a colony of less-effective defenders.
Both of these scenarios, and similar ones in other interactions, suggest ways to test for self-screening mechanisms like the one described in this new model. The model suggests that active screening using signaling between interacting species should be rare in nature, and that a simple cost/benefit structure usually underlies the process of establishing associations between partners. I'll be very interested to see whether new experimental or observational data further supports the self-screening hypothesis.
Archetti, M., Úbeda, F., Fudenberg, D., Green, J., Pierce, N., & Yu, D. (2011). Let the right one In: A microeconomic approach to partner choice in mutualisms. The American Naturalist, 177 (1), 75-85 DOI: 10.1086/657622
Weyl, E., Frederickson, M., Yu, D., & Pierce, N. (2010). Economic contract theory tests models of mutualism. Proc. Nat. Acad. Sci. USA, 107 (36), 15712-6 DOI: 10.1073/pnas.1005294107