SGM Journals

Research explained · Baculovirus infection

Why could a baculovirus infect caterpillars by injection but fail when eaten?

The caterpillars took in different forms of the baculovirus in the two tests. When they ate the mutant in its protected form, infection failed. Injecting the mutant’s budded form still caused infection. The researchers traced the feeding defect to a missing gene called pif. [1]

See original studies View the SGM research record

Feeding, injection and genetic rescue

Feeding the occluded form
Caterpillars ate the mutant virus in its occluded form, with the particles enclosed in protective bodies.
The mutant did not start an infection when the larvae ate it. [1]
Injecting the budded form
The team injected the budded form of the mutant and wild-type virus.
The researchers found similar infectivity after injecting the budded form of either virus. [1]
Restoring the missing factor
The team identified ORF7 and named it pif. Mutant virus produced in cells supplying the missing factor could infect larvae through feeding even though its genome lacked the gene.
The result points to a role for the protein in the incoming particle, early in infection. [1]

What are occluded and budded virus forms?

Some baculovirus particles are enclosed in protective occlusion bodies. Other particles leave the cell by budding. The researchers fed larvae the occluded form and injected the budded form. Their comparison changed the route of exposure and the virus form at the same time. [1]

How did the researchers isolate one gene from a larger deletion?

The mutant had lost about 4.5 kilobases of DNA, or about 4,500 DNA letters. The researchers supplied pieces of wild-type DNA and looked for the one that restored infectivity through feeding. The result led them to the gene they called per os infectivity factor, or pif. [1]

Where does the factor act?

Later experiments in another baculovirus found several oral infectivity factors forming a complex on the surface of particles from occlusion bodies. The original rescue narrowed the defect to an early stage of infection. Which receptor or step involved the individual factor remained open. [1][2]

Original research

  1. Kikhno et al. (2002), Journal of General Virology. Characterization of pif, a gene required for the per os infectivity of Spodoptera littoralis nucleopolyhedrovirus. Primary oral exposure, injection and genetic rescue experiments.
  2. Peng et al. (2010), Journal of Virology. Baculovirus per os infectivity factors form a complex on the surface of occlusion-derived virus. Later experimental study of a multi-protein complex on the virion.