mutualism Archives - EcoEvo@TCD

Any designer will tell you that choosing the right colour combinations are essential to strike the right tone and balance in a room, particularly if your goal is to attract clients. Well, what if your room is a web and your client is a moth?

This is just the situation the rather drab and dreary coloured Cyrtophora unicolor finds itself in. These spiders live almost exclusively on large moth prey, which are attracted by pale colours and twinkling lights, a problem if your evolution has led you to optimize your “I’m a brown leaf” appearance. Enter the small but enchanting Argyrodes fissifrons (a member of the dewdrop spiders). These spiders by contrast are endowed with a glistening silver and black pattern. These little guys face a rather different dilemma: they are kleptoparasites (food thieves!), meaning that they use the web of another larger spider species for feeding and reproductive territory.

Somewhere along the lines these two species found each other and struck up what is the first recorded example of an arthropod predator mutualism based on colour. Predator mutualisms, where both species benefit from the relationship, are rare due to conflicts of interest. Most dewdrop spiders live off the webs of others and suffer high levels of aggression from the hosts (somewhat understandably!). It was noticed however that A.fissifrons and C.unicolor seemed to coexist quite happily and that webs of C.unicolor even seemed to intercept more prey when A.fissifrons was present. The reason: The twinkling silver body of A.fissifrons. Against a leafy background the silver A. fissifrons is quite conspicuous and even attractive to the primarily visually oriented Hawkmoth prey. It is thought that the moths read the silver as some kind of a cue, potentially reflecting the moonlight at a similar intensity or wavelength as the stars, used by the moths for celestial navigation. These large moths however are much to big for little A.fissifrons to tackle; they prefer the smaller “accidental” prey. The result: These two species live quite contentedly side-by-side, one providing the house and the other the decoration.

Most animals that use colour signals use them as warnings, as mating indicators, service providers (as is the case in cleaner fish), camouflage and mimicry. If your ecological and evolutionary pressures demand a more low-key and less vibrant costume, adding a splash of colour from a friend with similar interests it seems may lead to joint benefits. Co-evolutionary related colour patterns, such as those associated with mimicry, usually arise from antagonistic interactions among species so it is interesting to see examples like these and wonder whether some might also arise from, or might eventually evolve as a result of, synergistic interactions among species.

Author

Deirdre McClean: mccleadm[at]tcd.ie

Photo credit

wikimedia commons

No man is an island; the same could be said for the millions of life forms that populate our planet. Think of all the ways in which organisms interact with each other through predation, parasitism and the countless symbioses. Sometimes a pair of interacting partners can become inextricably linked such is their mutual dependence. Each one may provide the other with a resource it’s unable to obtain on its own.

A recent collaboration explored instances when these interactions lead to the loss of a trait and showed the fragility of this situation. One of the examples the authors use is an ant species that farms fungus. The fungus provides the ants with all the arginine (an amino acid) they need so they have lost the ability to synthesise it themselves. Thus anytime an ecological interaction involves some provision of a resource by one partner to another the evolutionary pressure is removed and the trait can be lost in the species receiving the goods. In other words we end up getting ‘compensated trait loss’ due to the ecological interaction. This can tighten a symbiosis from a facultative to an obligatory one.

But the fragility of compensated trait loss should be obvious now. In the ant example, were the fungus to go extinct the ant would disappear along with it. It’s like the ecological interaction is undermining all the good work done by natural selection in providing the ancestral ants with all the traits they need. The authors reckon that trait loss is “grossly underestimated” which puts many species in a precarious position in this age of mass extinction. Although there have been some instances where the trait was recovered, in flagrant disregard for Dollo’s law. Some of these law breakers include parasitic insects who regained their ability to synthesise lipids once the provision was lost.

A difficulty in studying these systems is how to detect when trait loss is taking place. A decreased expression of some gene in some members of a population would probably be reported as natural variation. But with ever improving molecular techniques we will be able to get a better estimate of the number of compensated trait loss interactions.

Author

Adam Kane: kanead[at]tcd.ie

Photo credit

wikimedia commons

Tag: mutualism

Complementary colours

No animal is an island