Timing of Glycoprotein Expression and the Evolutionary Success of Baculovirus Infection of Lepidopteran Hosts

University Of California-Berkeley, Berkeley CA

Investigators

Abstract

Autographa californica M Nucleopolyhedrovirus (AcMNPV) is the best-studied species of the Nucleopolyhedrovirus (family Baculoviridae), a genus comprised of viral pathogens that infect caterpillars (i.e., larvae of butterflies and moths in the order Lepidoptera). AcMNPV has been studied extensively as a potential microbial pesticide because it can induce fatal infection in a wide range of pest species that cause billions of dollars worth of damage each year (e.g., corn, tobacco, tomato, cotton). The unusually wide host range of AcMNPV suggests that its infection strategy is highly efficient at overcoming the defensive responses of many larval lepidopterans, one of which is sloughing infected midgut cells. AcMNPV requires two different forms to complete its natural infection cycle. One form, called ODV, is eaten by caterpillars and infects the midgut cells lining the gut lumen. The second form, budded virus or BV, transmits infection beyond the midgut to nearly all other host tissues causing death within a few days. AcMNPV has two traits that are highly unusual for a virus. First, most ODV particles contain multiple nucleocapsids which all enter a single midgut cell; because a midgut cell can be successfully infected by virions containing only one nucleocapsid, this infection strategy appears inefficient. Second, the synthesis of the major BV envelope protein, gp64, begins early during infection. Typically, structural proteins are only made late during infection, so this early synthesis of gp64 is highly unusual. gp64 is essential for BV infectivity, and because it is absent from ODV, it must be synthesized in midgut cells in order for infection to progress. The hypothesis to be tested is that after infection of a midgut cell by ODV containing multiple nucleocapsids, some of the entering nucleocapsids invade the nucleus where they express gp64 (and other viral genes) while others bypass the nucleus and bud from the basal plasma membrane containing newly synthesized gp64. Thus, early expression of gp64, coupled with packaging of multiple nucleocapsids in ODV, may accelerate the onset of systemic infection and thereby counter the caterpillar's defensive midgut cell-sloughing response. This hypothesis will be tested by comparing the pathogenesis of an AcMNPV recombinant (Ac-wt) that exhibits the normal, wild type (early and late) gp64 synthesis with that of a recombinant that that can only synthesize gp64 late during infection (Ac-late). Both of these viruses contain a reporter gene permitting temporal and spatial tracking of the viral infection as it moves through the tissues. Dose/mortality relationships for Ac-wt and Ac-late will be determined in larvae of the Tobacco Budworm and the Beet Armyworm, two major agricultural pests. If the hypothesis is correct, Ac-late should be slower in establishing systemic infection and also should require larger dosages to yield mortalities comparable to Ac-wt. Additionally, ODV nucleocapsids of both recombinants will be radiolabeled to determine, directly, whether and when they are transported into secondary target tissues. Finally, an AcMNPV recombinant lacking the ability to synthesize gp64 will be used both to confirm that gp64 is essential for systemic infection of the two insect species, and to quantify rates of midgut cell sloughing by these hosts. This research is designed to answer basic questions about the evolution and ecological significance of baculovirus infection strategies and the defense responses of caterpillars. The results will integrate the disease ecology of AcMNPV at the molecular and organismal levels, adding to our basic knowledge of baculoviruses and their utility in controlling agricultural pests.