Recently, several studies on coral gametogenesis from the molecular and cellular perspectives have been performed and molecules related to coral gametogenesis have been identified 8, 9, 10, 11, 12. Spermatogonial proliferation, meiotic differentiation and spermiogenesis take place within each spermary 3, 6, 7. After spermatogonia form clusters, they infiltrate into the gonadal mesoglea and form spermaries that consist of dozens of spermatogonia surrounded by a thin layer of mesoglea. On the other hand, spermatogenesis begins with spermatogonial proliferation alongside the gonadal mesoglea 6, 7. Consequently, the oocytes are enveloped by a thin mesogleal layer in which they increase in size by accumulating yolk until they reach the maturation period 3. After the oogonia differentiate into early oocytes by entering meiosis, the oocytes enlarge in size and infiltrate into the gonadal mesoglea. Oogenesis begins with the proliferation of oogonia alongside the gonadal mesoglea, which is a gelatinous extracellular matrix. corals, sea anemones, hydras and jellyfish) are generally thought to possess no true organs, the specific site of gamete production is often referred to as a gonad (testis/ovary) 4. Corals develop their gametes in a specific site of each endodermal mesentery 3, 4. Generally, the annual spawning pattern of corals exhibits a single annual gametogenic cycle that is completed in less than 12 months 3, 4. The common characteristics of gametogenesis (e.g., processes, specific site of gametogenesis within the polyp and the cycle of gametogenesis) among species have been revealed 3, 4, 5. To date, coral gametogenesis has been investigated in many species and locations worldwide mainly from ecological and histological perspectives 3, 4, 5. Gametogenesis is the initiation of sexual reproduction and creates mature gametes that contain all the material to drive fertilization and embryonic development.

Sexual reproduction also allows corals to foster genetic diversity, an advantage in adapting to environmental changes. In sexual reproduction, the gametes or larvae released into the ocean allow corals to colonize different habitats and create genetic links between one reef region and another 2. Sexual reproduction of corals is among the most important means of establishing new coral colonies, populations and hence coral reefs. Stony corals are indispensable in building the framework of coral reefs, which possess the highest levels of marine biodiversity on earth 1. The present study provides not only the first evidence for the potential involvement of FPs in coral oogenesis but also an insight into a cellular strategy underlying coral sexual reproduction. These results raise the possibility that EaRFP plays a role in protecting the oocytes from oxidative stress from the early to late stages of oogenesis.

Furthermore, recombinant EaRFP was shown to possess an H 2O 2 degradation activity. Immunohistochemical analysis revealed that the expression of EaRFP starts in the early stage of the oocyte and continues until the maturation period. The combination of the ovarian-cell separation method and the RT-PCR analysis revealed that the oocytes, but not the ovarian somatic cells, are the cells expressing EaRFP. Microscopic observation and quantitative RT-PCR revealed that EaRFP is almost exclusively expressed in the ovary of the adult coral. We identified a clone encoding a novel red fluorescent protein cDNA of E. We then performed a differential screen to identify genes related to oogenesis in the stony coral Euphyllia ancora. To date,the molecular and cellular mechanisms underlying coral sexual reproduction remain largely unknown.