Testicular somatic cells in the stony coral Euphyllia ancora express an endogenous green fluorescent protein

In a variety of organisms, adult gonads contain several specialized somatic cells that regulate and support the development of germline cells. In stony corals, the characteristics and functions of gonadal somatic cells remain largely unknown. No molecular markers are currently available that allow for the identification and enrichment of gonadal somatic cells in corals. Here, we showed that the testicular somatic cells of a stony coral, Euphyllia ancora, express an endogenous green fluorescent protein (GFP). Fluorescence microscopy showed that, in contrast to the endogenous expression of the red fluorescent protein of E. ancora ovaries that we have previously reported, the testes displayed a distinct green fluorescence. Molecular identification and spectrum characterization demonstrated that E. ancora testes expressed a GFP (named EaGFP) that is a homolog of the GFP from the jellyfish Aequorea victoria and that possesses an excitation maximum of 506 nm and an emission maximum of 514 nm. Immunohistochemical analyses revealed that the testicular somatic cells, but not the germ cells, expressed EaGFP. EaGFP was enclosed within one or a few granules in the cytoplasm of testicular somatic cells, and the granule number decreased as spermatogenesis proceeded. We also showed that testicular somatic cells could be enriched by using endogenous GFP as an indicator. The present study not only revealed one of the unique cellular characteristics of coral testicular cells but also established a technical basis for more in-depth investigations of the function of testicular somatic cells in spermatogenesis in future studies.