An ultrastructural study of polyembryonic parasitoid embryo and host embryo cell interactions.

The morula-stage embryo of the polyembryonic egg-larval parasitoid Copidosoma floridanum forms outside the host embryo and secondarily invades the host body. Electron microscopic analyses of cellular interactions between the extraembryonic syncytium of the parasitic morula and the host embryonic epithelial cells showed that morula penetration into the host embryo did not cause obvious damage to the host cells, except for the abrasion of the embryonic cuticle. Epithelial cells of the host embryo extended microvilli toward the invading C. floridanum morula and also adjacent host cells in the same way. Shortly after settlement of the morula within the host body cavity, gap junctions and adherens junctions with host cells were formed. The morula was then surrounded by a cyst comprised of host cells into which host tracheoles were invaginated.