The synchronized spawning is postulated to be controlled by the circadian clock mechanism, which is based on transcription-translation feedback loops and maintains a free-running period of approximately 24 h 12. In this process, which has been observed worldwide, individuals from the same species synchronously release their eggs and sperm to the water column, where fertilization occurs 9, 10, 11. The most well-known sexual reproduction phenomenon in cnidarians is the mass spawning of corals. Although they can reproduce asexually, their long-term ecological success relies on sexual reproduction 7, 8. Within cnidarians, the anthozoans, a group that includes corals and sea anemones, are among the morphologically simplest extant eumetazoans. An informative group of animals for studying evolution are cnidarians, as they are basal metazoans and considered to be a sister group of the Bilateria 6. Sex is thought to have arisen once and to have been present in the last eukaryotic common ancestor 3, 4, 5 therefore, it is an important trait in evolutionary biology. It generates the conditions for sexual selection, which is a powerful evolutionary force driving morphological, physiological, and behavioral changes in many species 1, 2. Sexual reproduction is the predominant mode of procreation in almost all eukaryotes, from fungi and plants to fish and mammals. Our findings reveal the potential mechanisms that may enable the mesenteries to serve as a gonad-like tissue for the developing oocytes and expand our understanding of sexual reproduction in cnidarians. Real-time qPCR revealed that light exposure has a minor effect on expression levels of most examined transcripts, implying that temperature change is a stronger inducer for spawning in Nematostella.
Concurrently, processes related to the cell cycle, fatty acid metabolism, and other housekeeping functions are downregulated. We identified an array of processes involving numerous receptors, circadian clock components, cytoskeleton, and extracellular transcripts that are upregulated upon spawning induction. To uncover molecular mechanisms and pathways underlying spawning, we characterized the transcriptome of Nematostella females before and during spawning induction. The sea anemone Nematostella vectensis, which has emerged as an important model organism for developmental studies, can be induced for spawning by temperature elevation and light exposure. In cnidarians, long-term ecological success relies on sexual reproduction.
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