Ultrastructure of Cotesia plutellae Bracovirus in Its Replication at Wasp Ovarian Calyx

Cotesia plutellae bracovirus (CpBV) is a polydnavirus symbiotic to an endoparasitoid, C. plutellae. Despite rich information on CpBV genome, there has been little known on its viral replication mode from proviral to episomal form. This study illustrates fine structures of the epithelial cells producing CpBV with a reference to non-producing ovarian epithelial cells. The ovarian epithelial cells of teneral females (within 12 h after emergence) were characterized by large nucleus and rich rough endoplasmic reticulum. CpBV particles were present only at the calyx region, in which follicle epithelial cells exhibited virogenic structures. Though a matured CpBV particle found in the calyx lumen was encapsidated in a single envelop containing multiple nucleocapsids, numerous free nucleocapsids were observed in the calyx epithelial cells and appeared to undergo assembly step to a final multiple capsid form. The multiple capsid forms appeared to be released into the oviduct lumen. The epithelial cells bordering the oviduct lumen showed phagocytosis presumably due to clearing cellular debris. At the calyx area close to the common oviduct, the epithelial cells appeared to maintain protein synthetic activity due to highly developed rough endoplasmic reticulum, but showed a marked decrease in the viral production.