From Single to a Symbiotic Relationship: How “Relationship Status” Changes Gene Expression in Corals & Symbiotic Algae

The relationship between adult corals (the host) and algae of the Symbiodiniaceae family (the symbiotic partners, previously known as zooxanthellae) has long been studied by researchers; however, relatively little is known about the beginnings of this mutualistic relationship.

The majority of coral larvae are born “single” without their symbiotic partners. Instead, they acquire their algal symbionts from the local environment in their early life history stages via a process called horizontal transmission. While larval stages are not the only coral life history stage at which horizontal transmission can occur, it provides a unique opportunity to study the beginning of this mutualistic relationship.

For the first time, we were able to simultaneously track changes in gene expression in both the coral host and its algal symbiont during colonisation. This was done at the coral’s larval stage using dual RNA-seq, an approach that has been recently been used to understand host and parasite interactions in many human diseases such as leprosy, malaria and tuberculosis. Gene expression is the processes by which the information stored in DNA is converted into instructions for making molecules, like proteins. However, not all genes are expressed all the time! Think of them like a light switch: they can be turned on or off based on changes in their environment. In this case, the beginning of a mutualistic relationship.

While there is little field evidence that algal symbionts infect coral in their larvae stage, our most recent paper shows that they are not only capable of doing so, but are also able to establish a mutualistic relationship with their (larval) host that is normally associated with adult corals. This early relationship between larvae and algal symbionts may give the coral a “head start” in life after it settles! This is because the symbionts will already be at a nutritionally-useful density at a time when most coral settlers are undergoing initial symbiont infection. In other words, their symbiotic partners are already making them meals every day while those corals and symbionts starting their relationships a bit later in life are only beginning to go out for the occasional coffee and dinner! Although this idea has not yet been verified, pre-infection of coral larvae with appropriate symbionts shows potential in improving coral survival rates post-settlement and thus may be of benefit in conservation efforts.

Like us, corals have to put their guard down a bit when entering in to a relationship. In this case, we provide further evidence that, upon entering its coral host, the algal symbiont subverts the normal cellular protection mechanisms of its host, in a manner that resembles the evasion of the human immune system by pathogens, such as the tuberculosis bacterium.

All relationships have their bumpy roads, and corals and their symbionts are no different! We also provide evidence for the involvement of a novel coral gene family called “SCRiPs” in establishing the symbiotic relationship the host side of the interaction, and identify candidate molecular markers for the “healthy” and “unhealthy” states of coral. Too bad those relationship markers are limited to corals and their symbionts!

PAPER

Mohamed, A.R., Andrade, N., Moya, A., Xin Chan, C., Negri, A.P., Bourne, D.G., Ying, H., Ball, E.E. and Miller, D.J. (2020). ‘Dual RNA‐seq analyses of a coral and its native symbiont during the establishment of symbiosis’. Mol Ecol. Accepted Author Manuscript. DOI:10.1111/mec.15612

FOR FURTHER INFORMATION

Prof. David Miller (Australia, AEST)
E: david.miller@jcu.edu.au

Dr. Amin Mohamed (Germany, CEST)
E: am_rd85@yahoo.com