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Multiple symbiont systems and the evolution of mutualism

Posted by TFayle on 29 May 2012 at 08:37 GMT

The evolution of mutualism is usually studied using host-symbiont systems in which only a single resident species inhabits any one host individual. But, although less tractable as study systems, symbioses in which multiple resident species co-exist on a single host are probably widespread. The evolutionary dynamics likely to occur in these systems will differ in interesting and informative ways from those observed in one host-one symbiont systems. Furthermore, they provide an opportunity to study the evolution of more highly specific host-symbiont relationships, since these may have started out as low-specificity, multiple-symbiont systems.
This paper documents one interesting case in which multiple decapod symbiont species co-habit on individual coral hosts. The decapods all clean the coral of sediment, thus increasing coral fitness. However, all species contributed to cleaning the coral to the same degree. Corals hosting more decapod species experienced higher rates of cleaning, independent of decapod species identity.
We published a study of a similar system recently (Fayle et al. 2012, http://onlinelibrary.wile...). Here, multiple ant species inhabit individual epiphytic ferns in the canopy of tropical rainforest. The ants protect the ferns from herbivores, with relatively little variation between ant species in the degree to which they contribute towards fern protection. The higher the biomass of ants, the greater the degree of protection afforded. In addition, the larger the fern is, the larger the area of available habitat for ants. It might be supposed that this would lead to resident ants increasing levels of patrolling in order to encourage ferns growth (and so gain more nesting space). However, larger ferns support more ant species, not larger colonies of particular species, and so this behaviour is unlikely to be selected for, and ferns must make do with the protection provided by background levels of ant foraging. The coral-decapod system described by Stier et al. is also potentially at this two-way by-product mutualism stage, with no incentive for residents to invest in coral growth by increasing rates of sediment removal for the public good.
The study of systems like this has the potential to shed light on the evolution of mutualistic relationships in which partners invest reciprocally in each other (i.e. in which there is altruism, rather than just by-product mutualism). We need to accumulate further datasets (with multiple symbiont species) on systems with variation in the degree to partners invest in each other, thus providing snapshots of the development of altruistic relationships through evolutionary time. Perhaps colleagues with similar datasets might like to get in touch?
Fayle T.M., Edwards D.P., Turner, E.C., Dumbrell A.J., Eggleton P. & Foster W.A. (Online early). Public goods, public services, and by-product mutualism in an ant-fern symbiosis. Oikos DOI: 10.1111/j.1600-0706.2011.20062.x

No competing interests declared.