Oocyte development in Hydra involves selection from competent precursor cells

We have investigated oocyte development in Hydra vulgaris, a member of one of the oldest metazoan phyla. We show that oocyte determination involves a mechanism that establishes a subset of precursor interstitial cells competent to differentiate into oocytes. The oocyte is singled out from this subset and the competence of the remaining cells to become oocytes dramatically decreases as they adopt the alternative nurse cell fate. Progression through the nurse cell differentiation program requires the presence of the oocyte. When the oocyte is removed from the egg field, nurse cells abort their differentiation program, undergo apoptosis, and are phagocytosed and degraded by somatic epithelial cells. However, in the presence of the oocyte, nurse cells differentiate and enter an unusual apoptosis program where they are phagocytosed by the oocyte, but are not degraded. We show that the oocyte is able to induce this unusual apoptosis program in immature nurse cells that have not completed differentiation. A new model for oocyte development in Hydra is discussed.