Yesterday A Close friend and Now Turns out to be a Backstabbing Bandit: A parasitic algae symbiont’s story and its upside-down Castle

When we hear the word “symbiont” what usually crosses our mind is a helpful and healthy relationship between two different organisms. But in reality, there is a less complex relationship between a symbiont and its host. A symbiont’s task is to provide what the host cannot provide for itself such as nitrogen fixation, sugar synthesis and many other functions, but the same processes may also produce a negative relationship. Symbionts adapt by gradually evolving (virulent evolution), and this involves infecting the host to survive. Usually symbionts infect their hosts through vertical transmission but horizontal transmission also occurs in the wild, where the host discards excess number of symbiont.

There are two ways by which a symbiont can affect its host, and that is by vertical and horizontal transmission -  vertical is when parents transmit/infect symbiont which is passed onto its offspring and horizontal is when symbiont affects neutral/affected host from the environment. A parasitic relationship is detrimental for the host (which is caused mainly by the free-living symbiont), affecting the host’s fitness and growth rate. This occurs by outgrowing the host and making the symbiont’s colony costlier which leads to the uptake of nutrients. Stalling the host’s growth takes place and in some cases, shrinking the host’s body mass.

The experiment involves the infection of the upside-down jellyfish polyp with free living symbiont and also observation of the vertically infected polyp. Comparison of growth and shape with statistical results from both host was performed. Sachs and Wilcox (2005) states that horizontal transmission of Symbiodium microadriaticum causes a striking reduction in Cassiopea xamachana polyp compared to vertical transmission, which retains helpful and discards harmful and excessive symbionts.

From the virulence theory of evolution and the experiment, we know that free living symbionts that infect the host via horizontal transfer may gradually evolve into a parasitic symbiont. In the case of the upside-down jellyfish, they are a perfect pump factory for symbiont spawning as they rapidly multiply. The free-living organism will infect new host and may change its strategy to take advantage of the host, which will gradually evolve into a more forceful symbiont that harmful to its host. From the experiment itself, Sachs and Wilcox (2005) mentions that ‘algal expulsion rate per unit of host biomass was significantly higher in the horizontal treatment’, but in the end, they also mention that ‘total expulsion did not differ between treatments’. From this, we know that symbionts that come from the environment are more aggressive in siphoning benefits from the host. Scientists (Fine 1975; Ewald 1983; Bull 1994) foretold that, ‘virulence theory predicts that horizontal transmission of symbionts promotes the evolution of parasitism’.

This paper shows us a good understanding of how a symbiont (microalgae) can turn from a handful to a harmful residence for its host and gives us a deeper understanding of virulence evolution. The paper also has a very brief and structured method section, which gives detailed data and step to step on how the algae symbiont infects its host. However, if the author was able to provide more visual charts and diagrams as part of the results section using primarily statistical data, it would visualize the discussion better in my opinion. Overall it is a very interesting paper and definitely gives the reader a base understanding of how inconsistence the relationship between the host and a group of symbionts is, and potentially opens new fields for further research within symbiosis relationship.

Bibliography

Joel L. Sachs* and Thomas P. Wilcox (2005). A shift to parasitism in the jellyfish symbiont Symbiodinium microadriaticum, 425- 429. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC1560209/