Complex dynamics of defective interfering baculoviruses during serial passage in insect cells

Abstract

Defective interfering (DI) viruses are thought to cause oscillations in virus levels, known as 3 the “Von Magnus effect”. Interference by DI viruses has been proposed to underlie these 4 dynamics, although experimental tests of this idea have not been forthcoming. For the 5 baculoviruses, insect viruses commonly used for the expression of heterologous proteins in 6 insect cells, the molecular mechanisms underlying DI generation have been investigated. 7 However, the dynamics of baculovirus populations harboring DIs have not been studied in 8 detail. In order to address this issue, we used quantitative real-time PCR to determine the 9 levels of helper and DI viruses during 50 serial passages of Autographa californica multiple 10 nucleopolyhedrovirus (AcMNPV) in Sf21 cells. Unexpectedly, the helper and DI viruses 11 changed levels largely in phase, and oscillations were highly irregular, suggesting the 12 presence of chaos. We therefore developed a simple mathematical model of baculovirus-DI 13 dynamics. This theoretical model reproduced patterns qualitatively similar to the 14 experimental data. Although we cannot exclude that experimental variation (noise) plays an 15 important role in generating the observed patterns, the presence of chaos in the model 16 dynamics was confirmed with the computation of the maximal Lyapunov exponent, and a 17 Ruelle-Takens-Newhouse route to chaos was identified at decreasing production of DI 18 viruses, using mutation as a control parameter. Our results contribute to a better 19 understanding of the dynamics of DI baculoviruses, and suggest that changes in virus levels 20 over passages may exhibit chaos.