More than meets the eye - Endosymbiosis

New symbiotic relationships can be hard to identify and understand especially when they occur inside another organism. Holobiont relationships can have a broad range of effects on the host and it's microbial communities. 

One organism well known for its diversity and complexity in symbiotic partners is coral.

Just this year five new types of endosymbiotic algae have been discovered in coral in the red sea (Osman et al. 2020). While coral algae symbiosis were originally thought to be mutualistic, studies have found that coral – algae symbiosis runs on a spectrum between mutualism and parasitism depending on the species involved and the level of stress on the coral (Baker et al. 2018). Other Cnidarians such as some species of jellyfish and anemones also hold mutualistic cyanobacteria within their surface mucus layer for energy exchange. It is believed that they also play a role in host immune responses however, their role in this is still varied and unclear (Ortega & Thome 2018). Cnidarians harness the power of photosynthesis through their algae.  

The upside down jellyfish Cassiopia xanachama derives energy from symbiotic zooxanthellae (Nash 2013)

Other holobionts can be highly species specific for example, the Hawaiian Bobtail squid Euprymna scolopes. This squid has evolved two symbiotic organs: The light organ and the accessory nidamental organ. The Bobtail squid shares a highly specific mutualism with the bioluminescent bacteria Vibrio fischeri. Early inoculation of the light organ with V. fischeri causes the organ to mature and allows the squid use the bioluminescence as camouflage (Bosch 2019 & Montgomery & McFall-Ngai 1994). The ANG organ houses a variety of mutualistic bacteria that are believed to protect the squid’s eggs from harmful bacteria (Kerwin & Nyholm 2017).

The Bobtail Squid Eupryma scolopes uses bio luminescent bacteria to mimic moonlight for camouflage (Stiefel 2012)

A recent genome analysis of the squid has found that the two organs may have developed by completely different evolutionary mechanisms. Genomic analysis by Belcaid et al. (2019)  found that while the light organ may have developed from a mutated eye genes, the ANG has a high number of orphan genes (genes found only in one genus or species with no phylogenetic history) suggesting the rapid evolution of the organ in response to selection pressures yet to be discovered.

For more information on the Bobtail squid go here: https://www.quantamagazine.org/new-squid-genome-shines-light-on-symbiotic-evolution-20190219/

Thankyou for reading 😊

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