stage, while examples of the latter include ectoparasitoids. The feeding
activity of ectoparasitoid larvae probably stimulates the host’s immune
system, given that simple abrasion of the insect cuticle activates both
haemocytes and humoral defence molecules, such as antimicrobial
peptides (Breyet al., 1993). However, ectoparasitoid larvae obviously
cannot be encapsulated and also tend to rapidly kill their host by secreting
proteases, which preorally digest internal tissues. Another strategy for
passively avoiding encapsulation is for parasitoids to develop in specific
host organs, so that larvae are protected from immune recognition. As
previously noted, some Diptera in the family Tachinidae develop from
eggs that are ingested by the host (Salt, 1968). After hatching, first-instar
larvae move through the midgut and enter ganglia, salivary glands or
flight muscle. Platygasterids (Hymenoptera) likewise spend most of their
immature life in the gut or ganglia of their dipteran hosts (Leiby and Hill,
1923). A final means by which parasitoids passively avoid elimination is
by having surface features that are either not recognized as foreign or that
haemocytes do not adhere to (Strand and Pech, 1995). For example, host
haemocytes do not attach to proteins that coat the eggs and larvae of
parasitoids in the genusCotesia(Hayakawa and Yazaki, 1997; Asgari
et al., 1998). At the other extreme, the eggs of parasitoids in the genus
Asobarastrongly adhere to the fat body ofDrosophilalarvae, with the
strength of adhesion being directly proportional to the egg’s ability to
avoid encapsulation (Kraaijeveld and van Alphen, 1994). While some
tachinids develop in host organs, others develop directly in the host’s
haemocoel. Larvae of these species have hooks on their posterior
spiracles, which they use to puncture the host’s body or trachea to form a
respiratory funnel as a conduit for oxygen. The host then encapsulates the
fly larva but, possibly because of the respiratory funnel, the capsule is
unable to kill the fly larva, which continues to develop normally (Clausen,
1940; Salt, 1968).
Active suppression of the host immune system is usually mediated by
maternal factors injected into the host at oviposition, rather than by the
parasitoid larva. The most studied of these factors are polydnaviruses
(PDVs) which are divided into the ichno- and bracoviruses, on the
basis of their association with endoparasitoid wasps in the families
Ichneumonidae and Braconidae, respectively (Webb, 1998). PDVs are the
only viruses with segmented DNA genomes, and all appear to share a
similar life cycle (Fig. 7.2). PDVs persist as stably integrated proviruses in
the genome of the wasp and replicate asymptotically in the ovaries of
females. When the wasp lays an egg into its host, it injects a quantity
of virus, which then infects host immune cells and other tissues. PDVs
do not replicate in the wasp’s host, but specific viral gene products,
whose primary function is to suppress the host’s immune system, allow
the wasp’s progeny to successfully develop. Thus, a true mutualistic
relationship exists, as viral transmission depends on wasp survival
and wasp survival depends on PDV-mediated immunosuppression of
the host. PDVs disrupt encapsulation by inducing specific alterations in136 M.R. Strand