Nurse eggs form through an active process of apoptosis in the spionid Polydora cornuta (Annelida)

The production of nurse eggs is fundamental to poecilogony in some species of spionid annelids. In species such as Polydora cornuta, nurse-egg production varies among females and ingestion of nurse eggs varies among young, resulting in a form of poecilogony with divergent phenotypes for females (e.g., fecundity and per-offspring investment) as well as for larvae (e.g., trophic mode, size, and stage at hatching). We tested the hypothesis that nurse eggs of P. cornuta form through an active developmental process and specifically, through apoptosis. Results of a TUNEL assay indicate nuclear fragmentation occurs in a process that is characteristic of apoptosis. Cellular indicators of apoptosis in nurse eggs include activation of caspase-3, a positive Annexin V reaction indicating exposure of phosphatidylserine on the outer cell membrane, and invagination of the membrane to form yolk vesicles. These results indicate that formation of nurse eggs in this population of P. cornuta occurs through an active, adaptive process. Furthermore, while apoptosis also occurs in some cells of P. cornuta embryos, it was not detected until later in development. This suggests that nurse eggs originate through heterochrony in a developmental process (apoptosis) that is common to all young of P. cornuta.