Abstract
Baculoviruses (Baculoviridae) are occluded DNA viruses that are lethal pathogens of the larval stages of some lepidopterans, mosquitoes, and sawflies (phytophagous Hymenoptera). These viruses have been developed as biological insecticides for control of insect pests and as expression vectors in biotechnological applications. Natural and laboratory populations frequently harbor covert infections by baculoviruses, often at a prevalence exceeding 50%. Covert infection can comprise either non-productive latency or sublethal infection involving low level production of virus progeny. Latency in cell culture systems involves the expression of a small subset of viral genes. In contrast, covert infection in lepidopterans is associated with differential infection of cell types, modulation of virus gene expression and avoidance of immune system clearance. The molecular basis for covert infection may reside in the regulation of host–virus interactions through the action of microRNAs (miRNA). Initial findings suggest that insect nudiviruses and vertebrate herpesviruses may provide useful analogous models for exploring the mechanisms of covert infection by baculoviruses. These pathogens adopt mixed-mode transmission strategies that depend on the relative fitness gains that accrue through vertical and horizontal transmission. This facilitates virus persistence when opportunities for horizontal transmission are limited and ensures virus dispersal in migratory host species. However, when host survival is threatened by environmental or physiological stressors, latent or persistent infections can be activated to produce lethal disease, followed by horizontal transmission. Covert infection has also been implicated in population level effects on host–pathogen dynamics due to the reduced reproductive capacity of infected females. We conclude that covert infections provide many opportunities to examine the complexity of insect–virus pathosystems at the organismal level and to explore the evolutionary and ecological relationships of these pathogens with major crop and forest pests.
Highlights
Baculoviruses are large occluded dsDNA viruses that infect insects
The occlusion body (OB) protects the virions, known as occlusion derived virions (ODVs), from environmental factors. These viruses can be readily distinguished into two groups: nucleopolyhedrovirus and granuloviruses that clearly differ in the structure of their OBs (Adams and Bonami, 1991)
Through the examples provided it will become evident that mixed-mode transmission involving long-lived viral OBs in the environment and vertical transmission from infected adult insects to their offspring has key implications for the persistence, spatial dispersal and genetic diversity of insect baculoviruses (Cory, 2015)
Summary
Baculoviruses are large occluded dsDNA viruses that infect insects. The collapse of insect outbreaks in some species is associated with epizootics of baculovirus disease suggesting that these pathogens can regulate insect populations (Anderson and May, 1980; Cory and Myers, 2003; Elderd et al, 2013). Through the examples provided it will become evident that mixed-mode transmission involving long-lived viral OBs in the environment and vertical transmission from infected adult insects to their offspring has key implications for the persistence, spatial dispersal and genetic diversity of insect baculoviruses (Cory, 2015).
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