Unlike classical pathogenic viruses, PDV particles do not replicate in the infected host tissues (Wyder et al, 2003) where they act as potent regulators of lepidopteran host immunity. Experimental evidence for this role came from the initial observations by Vinson (1972) that fluids from the calyx, a tissue located between the ovarioles and the lateral oviducts of some parasitoid females, protected the parasitoid eggs from being encapsulated by host lepidopteran circulating immune cells (haemocy tes). PDVs were later found in these fluids, and their life cycle investigated from their production by calyx cells to their uptake by host cells (Rotheram, 1973; Krell and Stoltz, 1980) (Figure 9.1 and Figure 9.2). In 1981, Edson et al. demonstrated that virus-free eggs of the ichneumonid parasitoid Campoletis sonoren-sis artificially injected into the haemocoel of their permissive host, Heliothis virescens, were always encapsulated (Edson et al., 1981). In contrast, addition of calyx-fluid extracts or purified C. sonorensis ichnovirus (CslV) to the injected eggs had a protective effect, reducing encapsulation rate by approximately 75%. The protection afforded to the injected eggs was lost if calyx fluid or purified viruses were previously irradiated with ultraviolet light, a treatment which alters DNA, indicating that PDVs were responsible for the impairment of the encapsulation process via the expression of encapsidated genes, that would thus act as virulence genes.
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