Gene expression alterations from reversible to irreversible stages during coral metamorphosis.

For corals, metamorphosis from planktonic larvae to sedentary polyps is an important life event, as it determines the environment in which they live for a lifetime. Although previous studies on the reef-building coral Acropora have clarified a critical time point during metamorphosis when cells are committed to their fates, as defined by an inability to revert back to their previous states as swimming larvae (here referred to as the "point of no return"), the molecular mechanisms of this commitment to a fate remain unclear. To address this issue, we analyzed the transcriptomic changes before and after the point of no return by inducing metamorphosis of Acropora tenuis with Hym-248, a metamorphosis-inducing neuropeptide. Gene Ontology and pathway enrichment analysis of the 5893 differentially expressed genes revealed that G protein-coupled receptors (GPCRs) were enriched, including GABA receptor and Frizzled gene subfamilies, which showed characteristic temporal expression patterns. The GPCRs were then classified by comparison with those of Homo sapiens, Nematostella vectensis and Platynereis dumerilii. Classification of the differentially expressed genes into modules based on expression patterns showed that some modules with large fluctuations after the point of no return were biased toward functions such as protein metabolism and transport. This result suggests that in precommitted larvae, different types of GPCR genes function to ensure a proper environment, whereas in committed larvae, intracellular protein transport and proteolysis may cause a loss of the reversibility of metamorphosis as a result of cell differentiation.