Corals rely on a symbiotic relationship with Symbiodiniaceae dinoflagellates, receiving photosynthates in exchange for inorganic nutrients. The breakdown of this mutualistic relationship causes coral bleaching, which can result in mortality and coral reef declines. Despite the importance of this symbiosis, the cellular and molecular processes involved are not well understood. Like corals, the upside-down jellyfish Cassiopea exhibits a mutualistic endosymbiosis with Symbiodiniaceae, yet Cassiopea features many advantages over corals in laboratory settings: its short life cycle can be closed in the laboratory, and if fed regularly, Cassiopea polyps can remain aposymbiotic and reproduce. This study focused on establishing Cassiopea as a model organism to further elucidate the molecular mechanisms underlying this important cnidarian-Symbiodiniaceae symbiosis. To this end, an optimized method using menthol and DCMU was developed to generate aposymbiotic Cassiopea polyps, which was validated through confocal microscopy. A high-quality genome assembly of Cassiopea from the Red Sea was generated and gene expression profiles were compared between aposymbiotic and symbiotic polyps, revealing potential genes and pathways involved in symbiosis. Symbiotic polyps exhibited 1,395 upregulated and 1,576 downregulated genes, for a total of 2,971 differently expressed genes. While reef-building corals depend on this symbiosis for survival, Cassiopea does not. Gene Ontology enrichment and Kyoto Encyclopedia of Genes and Genomes pathway analyses revealed that Cassiopea’s symbiotic relationship is associated with changes in terms and pathways related to growth and increased metabolism, such as ribosome biogenesis, DNA replication, spliceosome, and cell cycle, which are also upregulated in symbiotic coral larvae. Furthermore, genes encoding for proteins involved in fatty acid metabolism were upregulated during symbiosis, in line with prior findings in Aiptasia. However, symbiosis in Cassiopea downregulated glutathione to L-glutamate conversion but upregulated glutathione S-transferase, in contrast to its downregulation in symbiotic anemones observed in previous studies, suggesting different mechanisms for ROS scavenging in cnidarian systems. Despite the different responses observed between Cassiopea and Aiptasia, this study indicates that both share similar cellular responses and metabolic pathways during symbiosis. These findings suggest that Cassiopea is a valuable model organism for studying symbiotic relationships and could provide new insights about the mechanisms underlying cnidarian-Symbiodiniaceae symbiosis.
| Date of Award | Jun 2023 |
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| Original language | English (US) |
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| Awarding Institution | - Biological, Environmental Sciences and Engineering
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| Supervisor | Manuel Aranda (Supervisor) |
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