Evolved Relationships: Symbiosis in the natural world

Like the eusociality theory, the role of symbiosis in the evolution of life itself is still debateable and although we may never know the full extent of its role 1.5 billion years ago, scientists can make an educated guess (Aanen & Eggleton 2017). So far we’ve seen that symbiosis can drive the evolution of behaviours, structures, responses, new genes and new species as well as playing an important role in critical ecosystems and services such as coral reefs, seagrass meadows, pollination, primary production thanks to mycorrhiza and immunity.   Some argue that symbiosis is so heavily integrated in individuals that we should study the evolutionary history and development of organisms with consideration of   the effects of their symbiotic microbiota, this new concept is called Ecological Evolutionary Development or Eco-Evo-Devo (Gilbert et al.2015). Another consideration is the possibility that symbiosis paved the way for major evolutionary changes, this idea is known a...

So we have seen how mutualistic interactions can benefit ecosystems and even forge the evolution of a new branch of life such as the eusocial termites but what happens when an organism loses it’s host? For some such as the Bobtail squid and many coral species it can mean certain death, while generalists have the chance to have a new lease of life on a similar host…but not without consequences. Studies by Appelgren et al. (2016) & Appelgren et al. (2018) found that while the hen flea Ceratophyllus gallinae can swap mingling great tit Parus major and collared flycatcher Ficedula albicollis birds during the breeding season, the fitness and reproductive success of the flea reduces depending on how specialized they were to their original host. They also found that the fleas were affected differently in different geographical locations. Appelgren at al. (2018) suggested that because hen fleas cannot disperse independently, they become genetically isolated while living with their h...

Unlike the bee, parasite and virus, some symbiotic relationships can form positive feedback loops that can fortify the basis of an ecosystem instead of destroying it. A good example of this is the positive feedback loop between seagrass, bivalves and their sulphur oxidizing bacteria. In 2012, de Fouw et al. demonstrated that Lucinidae bivalves play an important role in the health and abundance of seagrass meadows. Seagrasses are an important costal marine ecosystem that provide habitat, food and shelter for a vast array of marine and coastal animals and are natural carbon sinks (Ugarelli et al. 2017). A turtle eating seagrass (Lindgren 2013) Seagrasses evolved from terrestrial angiosperms approx. 100 million years ago and mostly rely on the nitrogen fixing bacteria in their rhizosphere to neutralize toxic sulfur buildup from degrading biomass (Ugarelli et al. 2017). However studies have shown that the sulfur oxidizing activity of Lucinidae bivalve symbionts further re...

Although many symbionts help their hosts, others work together to harm their hosts. In some cases pathogens such as viruses and bacteria work together to double the damage to their host. One example of this the parasitic mite Varroa destructor and the deformed wing virus of Apis mellifera honey bees. Varroa destructor feeds of a single hole on a honey bee which its young will also feed from, it is still unclear whether the mites also stimulate immune suppression (Di Prisco et al. 2016). The deformed wing virus suppresses the immune system of their host by enhancing the transcription of genes which downregulate the bee’s immune response. DWV also causes deformation of wings, disrupts bee behaviour and reduces lifespan (Stamets et al. 2018).  A Varroa destructor mite feeding off a honeybee pupa (Anderson 2017). Scientists have found that these two pathogens have a mutualistic relationship whereby the mite acts as a vector for the virus and both actively weaken the...

Aside from being able to initiate the development of new organs and changes in behaviour, holobiont relationships can have a really interesting effect on the immune system of the host. An area of particular interest recently is how nitrogen fixing bacteria affect the immune system of their plant hosts.   Almost all terrestrial plants share symbiotic relationships with rhizobial bacteria or mycorrhizal fungi to gain access to otherwise unavailable nutrients (Heijden et al. 2017; Mcnear 2013). The bacteria or fungi will extract nitrogen or other nutrients from the environment and pass it onto the plant roots in exchange for carbon produced through photosynthesis (Mcnear 2013; Heijden et al.2017). While most plants exchange with fungi, legumes have a unique relationship with symbiotic with bacteria that will invade the root system and grow in chambers known as nodules within them (Mcnear 2013). Symbiotic bacteria colonize nodules in the legume roots and exchange Nitrogen fo...