Symbiosis – harmony or harm?

We have all experienced relationships which are beneficial and others that are not. The same can be seen throughout nature. Originally defined by German scientist Heinrich Anton de Bary, symbiosis describes a close association between two species, principally a host and a symbiont, which lives in or on the host. While some partnerships may be advantageous or neutral to one or both parties, others may have a more detrimental effect.

Mutualistic symbiosis:

screen-shot-2016-10-02-at-22-27-45The first of the symbioses involves relationships between two different species which benefit both organisms. Mutualistic symbiosis can involve organisms of all shapes and sizes from stinging ants and bullhorn acacia trees, a relationship where the tree provides the ants with food and shelter in return for protection from herbivores, to the alliance between oxpeckers and zebras, in which the bird enjoys a readily available food source while the zebra has any parasites living on it removed.

One of the most well studied forms of mutualistic symbioses is that of the ruminant (i.e. cattle and sheep etc.), as these organisms play an important role in our agriculture and nutrition. Ruminants host an extensive microbial population in the largest of their four stomachs, the rumen. A mutually beneficial relationship exists between these two organisms because the rumen microbes are able to digest the plant matter consumed by the ruminant. In doing so, they produce fatty acids, which can be used by both parties for energy. Carbon dioxide is also released in this process, providing the rumen microbes with the oxygen-free environment they need to survive (these microbes are predominantly anaerobic so are poisoned by oxygen).

Parasitic symbiosis:

screen-shot-2016-10-02-at-22-27-52In contrast to mutualistic symbiosis, the interaction between two organisms may be less savoury in nature. Parasitic symbiosis describes a relationship between organisms where the symbiont benefits at the expense of its host. Unfortunately for the host, this generally causes it harm, whether this be in the form of disease, reduced reproductive success or even death. The symbiosis between birds, such as the cuckoo and the reed warbler, known as brood parasitism, is a characteristic example of a parasite-host relationship. Rather than building her own nest, the parasitic cuckoo will lay her eggs in a reed warbler’s nest, leaving the warbler to raise this egg along with her own offspring. Once hatched, the cuckoo chick then ejects the warbler’s young from the nest, allowing it to receive all the food that its “adopted” mother provides.

Unsurprisingly, this antagonistic relationship has led scientists to question why warblers raise these parasitic chicks if the practice is so harmful. It has been suggested that cuckoos engage in a kind of “evolutionary arms race” with its chosen host, based on the host’s ability to recognise a parasitic egg. In this ongoing contest, the evolution of a host species to become more adept at spotting and rejecting a parasitic egg may result in a subsequent evolution in the cuckoo to counter this change. This may be to lay eggs with greater similarity to the host’s or to move towards a new host species. Such a process could continue indefinitely.

screen-shot-2016-10-02-at-22-28-01An even more detrimental relationship exists between the parasitoid wasp and its hosts, which include a range of insects from ants to bees. Similarly to cuckoos, these wasps rely on their host to facilitate the development of their young, but do so by either laying their eggs inside the host or gluing them to its body. Once hatched, the wasp larva will feed on the host, usually until it dies.

Commensal symbiosis:

Symbiosis does not necessarily have to be beneficial or detrimental to the host organism. Commensal symbiosis describes a relationship in which one organism benefits while the host is unaffected. This may be in the form of shelter, transportation or nutrition. For example, throughout their lifecycles small liparid fish will “hitch a ride” on stone crabs, providing them with transportation and protection from predators while conserving energy. The crabs, meanwhile, appear to be neither benefitted nor harmed.

One case of commensalism which may come as a surprise involves Candida Albicans, a species of yeast known to cause the fungal infection Candidiasis in humans. Contrary to popular belief, C. Albicans can be pathogenic or commensal depending on which phenotype it has. Under normal circumstances, C. Albicans reside in our gastrointestinal tract undergoing a commensal symbiotic relationship with us (i.e. causing us no harm). This interaction is actually the default existence for C. Albicans. When changes occur in the body’s environment, however, a “switch” in phenotypes to the pathogenic form can occur, placing a temporary hiatus on the usual commensal relationship.

A plethora of symbiotic relationships exist throughout the natural world, from the tiny microbes inhabiting the ruminant gut to the large acacia trees housing ants. They can offer both organisms the harmony of a mutually beneficial association, as is the case with the oxpecker and the zebra, or be parasitic and work in the favour of one player while harming the other, as seen with the parasitoid wasp. In some instances, one organism can gain benefit without impacting the other either positively or negatively. As illustrated by C. Albicans and cuckoos, a symbiotic interaction may change or evolve according to the environment or evolution of the host, respectively. Symbiosis is clearly a highly important aspect of nature which many organisms rely on for survival, and one that will continue to fascinate scientists and non-scientists alike both now and in the future.

Post by Megan Barrett.

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