Apoptosis in gonadal somatic cells of scleractinian corals: implications of structural adjustments for gamete production and release.

Apoptosis is an evolutionarily conserved process of programmed cell death. Here, we show structural changes in the gonads caused by apoptosis during gametogenesis in the scleractinian coral, Euphyllia ancora. Anatomical and histological analyses revealed that from the non-spawning to the spawning season, testes and ovaries increased in size due to active proliferation, differentiation and development of germ cells. Additionally, the thickness and cell density of the gonadal somatic layer decreased significantly as the spawning season approached. Further analyses demonstrated that the changes in the gonadal somatic layer were caused by apoptosis in a subpopulation of gonadal somatic cells. The occurrence of apoptosis in the gonadal somatic layer was also confirmed in other scleractinian corals. Our findings suggest that decreases in thickness and cell density of the gonadal somatic layer are structural adjustments facilitating oocyte and spermary (male germ cell cluster) enlargement and subsequent gamete release from the gonads. In animal reproduction, apoptosis in germ cells is an important process that controls the number and quality of gametes. However, apoptosis in gonadal somatic cells has rarely been reported among metazoans. Thus, our data provide evidence for a unique use of apoptosis in animal reproduction.