17.2: Mutualisms

Introduction to Mutualisms

Mutualism

A mutualism occurs when two species benefit from their interaction. For example, termites have a mutualistic relationship with protists that live in the insect’s gut (Figure \(\PageIndex{2}\)a). The termite benefits from the ability of the protists to digest cellulose. However, the protists are able to digest cellulose only because of the presence of symbiotic bacteria within their cells that produce the cellulase enzyme. The termite itself cannot do this: without the protozoa, it would not be able to obtain energy from its food (cellulose from the wood it chews and eats). The protozoa benefit by having a protective environment and a constant supply of food from the wood-chewing actions of the termite. In turn, the protists benefit from the enzymes provided by their bacterial endosymbionts, while the bacteria benefit from a doubly protective environment and a constant source of nutrients from two hosts.

Lichen are a mutualistic relationship between a fungus and photosynthetic algae or cyanobacteria (Figure \(\PageIndex{2}\)b). The glucose produced by the algae provides nourishment for both organisms, whereas the physical structure of the lichen protects the algae from the elements and makes certain nutrients in the atmosphere more available to the algae. The algae of lichens can live independently given the right environment, but many of the fungal partners are unable to live on their own.

Figure \(\PageIndex{1}\): (a) Termites form a mutualistic relationship with symbiotic protozoa in their guts, which allow both organisms to obtain energy from the cellulose the termite consumes. (b) Lichen is a fungus that has symbiotic photosynthetic algae living in close association. (credit a: modification of work by Scott Bauer, USDA; credit b: modification of work by Cory Zanker)

Types of Mutualisms

Resource-resource relationships

Mutualistic relationships can be thought of as a form of "biological barter" in mycorrhizal associations between plant roots and fungi, with the plant providing carbohydrates to the fungus in return for primarily phosphate but also nitrogenous compounds. Other examples include rhizobia bacteria that fix nitrogen for leguminous plants (family Fabaceae) in return for energy-containing carbohydrates.

Service-resource relationships

Figure \(\PageIndex{2}\): The red-billed oxpecker eats ticks on the impala's coat, in a cleaning symbiosis.  Photograph by Muhammad Mahdi Karim.

Service-resource relationships are common. Three important types are pollination, cleaning symbiosis, and zoochory. In pollination, a plant trades food resources in the form of nectar or pollen for the service of pollen dispersal. Phagophiles feed (resource) on ectoparasites, thereby providing anti-pest service, as in cleaning symbiosis ()Figure \(\PageIndex{2}\). Elacatinus and Gobiosoma, genera of gobies, feed on ectoparasites of their clients while cleaning them and zoochory is the dispersal of the seeds of plants by animals. This is similar to pollination in that the plant produces food resources (for example, fleshy fruit, overabundance of seeds) for animals that disperse the seeds (service). Plants may advertise these resources using color and a variety of other fruit characteristics. Another type is ant protection of aphids, where the aphids trade sugar-rich honeydew (a by-product of their mode of feeding on plant sap) in return for defense against predators such as ladybugs.

Service-service relationships

Figure \(\PageIndex{3}\): Ocellaris clownfish and Ritter's sea anemones live in a mutual service-service symbiosis, the fish driving off butterflyfish and the anemone's tentacles protecting the fish from predators. Photograph by Jan Derk in the Public Domain.

Strict service-service interactions are very rare, for reasons that are far from clear. One example is the relationship between sea anemones and anemone fish in the family Pomacentridae: the anemones provide the fish with protection from predators (which cannot tolerate the stings of the anemone's tentacles) and the fish defend the anemones against butterflyfish (family Chaetodontidae), which eat anemones (Figure \(\PageIndex{3}\). However, in common with many mutualisms, there is more than one aspect to it: in the anemonefish-anemone mutualism, waste ammonia from the fish feeds the symbiotic algae that are found in the anemone's tentacles. Therefore, what appears to be a service-service mutualism in fact has a service-resource component.

A second example is that of the relationship between some ants in the genus Pseudomyrmex and trees in the genus Acacia, such as the whistling thorn and bullhorn acacia. The ants nest inside the plant's thorns. In exchange for shelter, the ants protect acacias from attack by herbivores (which they frequently eat when those are small enough, introducing a resource component to this service-service relationship) and competition from other plants by trimming back vegetation that would shade the acacia. In addition, another service-resource component is present, as the ants regularly feed on lipid-rich food-bodies called Beltian bodies that are on the Acacia plant.

Evolution of Mutualisms

Many mutualisms are thought to have evolved from antagonistic interactions. Several studies have determined ecological or evolutionary conditions driving transitions from mutualism to antagonism (mutualism breakdown), yet limited theory has been developed to mechanistically explain the evolutionary transitions from antagonism to mutualism. Models of virulence predict evolution of reduced antagonism, perhaps toward mutualism, in cases of parasitism where host fidelity is high and availability of alternate hosts is low. In these models, host fidelity is achieved when lineages of parasites and their host are tightly linked through vertical transmission from parent to offspring. 

Compelling evidence for the ‘virulence theory’ comes from interactions involving microbial parasites. For example, Wolbachia are endosymbiotic bacteria that live inside the cells of their invertebrate hosts. They are transmitted directly from mother to offspring, and spread through populations by manipulating the reproduction of their hosts. The most common reproductive manipulation responsible for the spread of these bacteria, called “cytoplasmic incompatibility,” arises when infected males mate with uninfected females, resulting in fewer offspring than normal. There are fitness costs for the hosts associated with Wolbachia infections, most commonly involving a reduction in egg production. The virulence theory predicts and evidence suggests that there is selection for the bacteria to evolve a more benign lifestyle, changing the bacterium from being parasitic to more mutualistic, where Wolbachia-infected hosts have higher rates of egg production than their uninfected counterparts. 

Reciprocal evolutionary responses are most likely important to the evolution of symbiotic mutualisms, and subsequent diversification may be the result of the mutualism improving niche width, population size, or transition into adaptive zones.  However, with the exception of plant-pollinator interactions, it is unclear how important mutualistic coevolution is as a driving force of diversification, especially as compared to the well-established mechanisms associated with competitive and antagonistic coevolution.

Plant-pollinator interactions are powerful tools of evolution because pollinators transport gametes (via pollen) between flowers.  Plant-pollinator relationships are thought to be critical to the historical diversification of angiosperm flowers. In fact, up to 25% of plant speciation events could be a result of interactions with pollinators. 

The obligate pollinating seed-predation mutualisms of the yucca and yucca moth and fig and fig wasps are attributed to coevolution diversification because the intimacy of the association may prevent species from making phylogenetically drastic switches to new partners. Interacting with a distant relative of your partner may not be an option if coevolution has created incompatibilities in specialized traits required for the interaction (Figure \(\PageIndex{4}\)).

For instance, if laying eggs into yucca flowers requires a particular complement of ovipositor traits and moth behaviors, we might predict finding host switches only between closely related yuccas. Initial analyses of yucca and yucca moth phylogenies suggest that the trees are somewhat congruent, although there have been at least two instances of moths switching hosts to quite distantly related yuccas. The interaction between yuccas and yucca moths is clearly one that is highly specialized and coevolved yet we do not find a phylogenetic pattern of perfect matching between plant and insect.

Figure \(\PageIndex{4}\): Comparison of host and parasite phylogenetic trees. The dotted lines indicate host associations of the parasites. a) Matching speciation events between host and parasite show a strict pattern of cospeciation. b) Host switches to distant relatives are shown by the crossing lines. Some parasites use closely related hosts, but most have jumped to a distant relative.  Figure from Segraves 2010.

However, there is also evidence opposing the primacy of mutualistic coevolution in these relationships. Figs have been shown to be successfully pollinated regardless of pollinator species and pollinator host-switch may be more frequent than co-speciation events (Figure \(\PageIndex{5}\)). There is also evidence that geographic isolation plays a significant role in yucca diversification. These together suggest that while coevolution plays a role in the emergence of these mutualisms, it is not the exclusive mechanism and may not even be the primary one.

Figure \(\PageIndex{5}\): Cophylogenetic comparison of the pollinators associated with the 15 fig species studied. Ficus and pollinator phylogenies used were indicated with symbol of figs and wasp separately. Tip-name colors indicate subgeneric classification. Extensive phylogenetic incongruence is shown for each pair of phylogenies.  Figure modified from Wang et al. 2021.

Attributions

This chapter was written by Aaron Howard.

References 

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Wang, G., Zhang, X., Herre, E.A. et al. Genomic evidence of prevalent hybridization throughout the evolutionary history of the fig-wasp pollination mutualism. Nat Commun 12, 718 (2021). https://doi.org/10.1038/s41467-021-20957-3

Weeks, A., Turelli, M., Harcombe, W., Reynolds, K. & Hoffmann, A. From parasite to mutualist: rapid evolution of Wolbachia in natural populations of Drosophila. PLoS Biol. 5, e114 (2007).

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