Deletions in the Ac-iap1 gene of the baculovirus AcMNPV occur spontaneously during serial passage and confer a cell line-specific replication advantage

The PstI-I region of the Autographa californica multicapsid nucleopolyhedrovirus (AcMNPV) genome was previously shown to be a frequent target of spontaneous deletions during serial virus passage in TN-368 cells (Kumar and Miller, Virus Res. 7 (1987) 335). Analysis of two of these serial passage mutants showed that a portion of the Ac-iap1 gene was deleted. To directly test the effect of loss of Ac-iap1, three different deletions in Ac-iap1 were introduced into recombinant viruses and the ability of these viruses to replicate was examined in two cell lines, TN-368 and SF-21, as well as in two species of insect larvae, Trichoplusia ni and Spodoptera frugiperda. The mutant viruses were indistinguishable from wild type or control revertant virus in their ability to infect larvae of either species. Moreover, no effect was seen on the rate of replication or the overall amounts of budded or occluded virus produced in cultured cells. However, in co-infection experiments using TN-368 cells, it was consistently observed that mutants lacking a functional Ac-iap1 gene out-competed control viruses carrying Ac-iap1. Interestingly, this replication advantage was only evident in the TN-368 cell line, the cell line used for the original serial passage experiments, and not in SF-21 cells. (C) 2001 Elsevier Science B.V. All rights reserved.