Do you know the phenomenon of coral reefs bleaching?
The first epigenetic study concerning the symbiosis between cnidarians and dinoflagellates.
Thinking about the coral reefs, the colors and the variety of fauna linked to them come to mind, but it does not immediately occur that they are also a source of livelihood on an economic level. In fact, in addition to tourism, they represent a source of income in the food sector as well as the wellbeing [1]. What is not known to all is that at the base of the coral reef ecosystem there is a symbiotic relationship between corals (cnidarians) and dinoflagellates. Specifically, dinoflagellates are photosynthetic unicellular algae which are mainly marine plankton. These two species establish a mutualistic symbiosis on a nutritional level. In fact, the dinoflagellate established within the cnidarian, thanks to photosynthesis, provides the fixed carbon to the cnidarian. On the other hand, the coral demolishes the macromolecules and returns to the dinoflagellate the waste products such as phosphates and nitrates [2].
This symbiosis is stable but sensitive to temperatures. This is demonstrated by the well-known events of climate change and increase in temperature due to anthropogenic stress, which in recent years together with pollution, always due to human action, has led to the so-called coral bleaching. This phenomenon occurs because the mutualistic relationship between the symbionts is lost and the dinoflagellate is expelled from the coral cells. All this leads to the death of the representative organisms of symbiosis as the nutrient exchange is missing. Corals bleaching does not only involve the death of the two species mentioned, but also affects the entire ecosystem connected to this symbiosis. This means that the other species supported by this relationship are also negatively affected, leading to a change on a larger scale. An important coral bleaching event occurred between 2014 and 2017 and it is estimated that coral reefs will only survive in a few areas of the ocean over the next eighty years. But what is the basis of this symbiosis? Molecular knowledge is still superficial so far, in fact, studies have always focused more on the obvious effects of symbiosis and their respective advantages. There was no epigenetic characterization. The first to tackle the research in this way are Manuel Aranda and his research group [3]. Epigenetics involves mechanisms of chemical modification on DNA that do not concern a change at the level of the sequence but regards subsequent modifications at the interactions and transcriptional regulation level.
The purpose of this study was to analyze the cnidarian genome in its entirety to evaluate possible modifications and implications in symbiotic mechanisms. Two models were used, respectively genus Aiptasia strain CC7 as cnidarian and the genus Symbiodinium strain SSB01 as dinoflagellate. The symbiosis between the two is species-specific and occurs through a phagocytosis of the dinoflagellate by the coral larva. Cultures of symbiont Aiptasia (6 replicates) and aposimbionte (6 replicates) were prepared from which DNA studies were made [4]. First of all, the study focused on comparing the whole genome methylation pattern between the aposymbiotic control and the symbiotic organism. A methylation of 6.37% of the CpG islands was thus observed on all the CpGs of the genome, these methylations were not random but were more localized in the body of the gene.
Fig 1. Representation of the intracellular symbiosis between cnidarians and dinoflagellates.
The components involved at the transcriptional level during the symbiotic phases are represented:
-red spots = membrane receptors;
-blue spots = apoptosis regulator;
-red arrows = transporters involved in nutrient exchanges.
This led to verify the appearance of spurious transcriptions. It has been previously demonstrated that histone H3K36me3 deposed along the genes actively transcriubed allows, through the recruitment of the methyltransferase (Dnmt3b), to methylate the gene body and preventing spurious transcriptions [5]. This relationship previously studied in murine models it is also preserved in Aiptasia as its Dnmt3b also binds the histone via its PWWP domain. it is possible to observe the decrease of spurious transcripts with the increase of methylation in the gene body. Finally, authors investigated how DNA methylation could be involved, at the transcriptional level, also in the establishment, maintenance and breakdown mechanisms of the symbiosis. Among the examples cited we report the involvement of some membrane receptors including TLR, SRB1, and C-type lectin necessary for the establishment of symbiosis, some genes involved in nutrient exchange such as the glucose transporter (from the dinoflagellate to the cnidarian) and the transporter of phosphate (from the cnidarian to the dinoflagellate). And again, genes involved in the breakdown of the symbiosis therefore implicated in the apoptosis and in the expulsion of the intracellular symbiont (Fig.1). These genes, thanks to the symbiosis, are differently methylated or differently expressed.
Surely this study lays the foundation for the knowledge of the role of DNA methylation in the different phases of symbiosis, and can also be exploited by acting directly on DNA to combat the coral reefs bleaching. Specifically, the symbiosis could be made, through methylation, more resistant to temperature and pollution. As mentioned however, this symbiosis is species-specific and the variety of species represented the coral reef is very high. It would therefore be advisable to carry out studies on other species and on a more representative number of samples, paying attention to the place of analysis of the specific coral reef.
References
- S. Sully, D. E. Burkepile, M.K. Donovan, G. Hodgson, R. van Woesik, A global analysis of coral bleaching over the past two decades. Nature communications (2019).
- S. K. Davy, D. Allemand, V. M. Weis, Cell biology of cnidarian-dinoflagellate symbiosis. Microbiol. Mol. Biol. Rev. 76, 229–261 (2012).
- Y. Li, Y. J. Liew, G. Cui, M. J. Cziesielski, N. Zahran, C. T. Michell, C. R. Voolstra and M. Aranda, DNA methylation regulates transcriptional homeostasis of algal endosymbiosis in the coral model Aiptasia. Science Advances (2018).
- I. Wolfowicz, S. Baumgarten, P. A. Voss, E. A. Hambleton, C. R. Voolstra, M. Hatta & A. Guse, Aiptasia sp. larvae as a model to reveal mechanisms of symbiont selection in cnidarians. Scientific Reports (2016).
- F. Neri, S. Rapelli, A. Krepelova, D. Incarnato, C. Parlato, G. Basile, M. Maldotti, F. Anselmi, S. Oliviero, Intragenic DNA methylation prevents spurious transcription initiation. Nature 543, 72–77 (2017).