Activation and detection of a latent baculovirus resembling Mamestra brassicae nuclear polyhedrosis virus in M. brassicae insects.

A laboratory culture of Mamestra brassicae insects (MbLC) has been found to harbor a latent baculovirus infection. The latent virus was activated by feeding the M. brassicae larvae with either the closely related Panolis flammea nuclear polyhedrosis virus (NPV), or the distantly related Autographa californica NPV. Restriction fragment profiles of the activated virus DNA showed that it is very closely related, if not identical, to M. brassicae NPV. Polymerase chain reaction amplification of polyhedrin gene sequences demonstrated that the latent virus was present throughout the life cycle of the insect; eggs, larvae, pupae, and adults. We failed to detect the presence of a latent virus in a second culture of M. brassicae insects, obtained from the environment and only recently adapted to growth in laboratory conditions, and thus these insects acted as an effective negative control in all the PCR and activation experiments. Using PCR analysis of DNA isolated from dissected tissues of fourth instar MbLC larvae, latent virus sequences were only detected in the fat body. Cell lines established from the isolated MbLC fat body tissue were also shown to harbor the latent virus sequences and should prove useful in further studies to elucidate the mechanisms of latency and virus activation.