haemocytes, which include cytoskeletal rearrangements, inhibition of
adhesion, alterations in the number of circulating haemocytes and induc-
tion of apoptosis (reviewed by Webb, 1998; Schmidtetal., 2001). In some
PDV–parasitoid systems, disruption of haemocyte function requires that
cells be directly infected by PDV, whereas, in others, virus infection of
possibly other tissues results in production of secreted proteins, whichInteractions between Larval Parasitoids and Their Hosts 137
Fig. 7.2. Life cycle of polydnavirus-carrying parasitoids. Proviral DNA of the polydnavirus
is integrated into the genome of all cells in the adult wasp including eggs. Polydnaviruses
replicate in a specialized type of cell in the ovaries of female wasps called calyx cells.
Circularized polydnavirus DNAs excise out of the genome of calyx cells, circularize and
are packaged into infectious particles (virions). Virus particles are then stored in the
lateral oviducts. The female wasp injects a quantity of virus particles along with an egg,
venom and other proteins when it oviposits into a host. Hosts are usually larval-stage
Lepidoptera. Virus particles enter and migrate to the nuclei of host cells, such as
haemocytes. Once in the nucleus, the viral DNA molecules usually remain episomal
(i.e. do not integrate into the host’s genome). However, certain viral genes are transcribed
in host cells, and the resulting viral proteins disrupt encapsulation and other physiological
processes in the host (see text). The wasp egg is already infected with polydnavirus, as
its nucleus contains integrated provirus. Thus, when the egg hatches all cells of the
parasitoid larva also contain polydnavirus in a proviral form. The parasitoid larva
completes its development by feeding on the host. At maturity, the larva emerges from
the host and pupates and, during the pupal phase, the virus then begins to replicate again
in calyx cells. The parasitoid later ecloses as an adult wasp and mates, and female wasps
then seek hosts to repeat the life cycle.