BACULOVIRUS GENOMICS: A RESOURCE FOR BIOCONTROL

Baculoviruses are the most studied group of invertebrate viruses and are unique among the invertebrate viruses in how important they have become to many other fields of research. This has largely come about through their development as one of the premiere eukaryotic protein expression systems available today (Kost and Condreay, 1999). Despite this wide use ofbaculoviruses, much of their biology remains to be deciphered. By combining the knowledge being derived from baculovirus genome analysis with studies into the biology of these viruses and their hosts, many advances are being made in our understanding of how these viruses work. Baculovirus genome analysis is also providing the basis for elucidating the roles of many baculovirus genes. This is giving new leads into how baculoviruses can be better utilised or improved, and supplying genes that have the potential to be used effectively in other pest control strategies. Baculoviruses are characterised by circular double-stranded DNA genomes that range in size from 80-180kbp (van Regenmortel et a!., 2000). This genome may carry up to 200 genes, the majority of which have no proven function. The genome is packaged into rod-shaped nucleocapsids that are enveloped. A defining criterion of the baculoviridae is their biphasic life cycle. Infection of an insect commences with the ingestion of polyhedral inclusions bodies (PIBS) that dissolve in the alkaline midgut of a susceptible host. The released virions, occlusion derived virus (ODV), must penetrate the peritrophic matrix lining the insect midgut, attach to midgut cells, and infect those cells. The virus must then replicate in the nucleus of the infected cell, avoiding any host defences such as apoptosis and/or sloughing. For most baculoviruses the infection then spreads to many tissues in the infected larva. This dissemination phase requires the virus to bud from the cell rather than be occluded in polyhedra. This alternate budded virus (BV) form of the virus has a modified host membrane envelope and disseminates via the tracheal system to get beyond the gut barrier. The virus is then free to infect, replicate and spread throughout much of the host insect. Late in the infection cycle in each cell, a switch to very late gene expression leads to de novo envelope synthesis and occlusion of progeny virions to generate more ODY. One of the final acts of the infectious process is the dissolution of the infected larva by proteolytic and chitinolytic enzymes to release the PIBS into the environment where they are free to re-infect another susceptible host.