Evaluating the Risks of Engineered Viruses: Modeling Pathogen Competition

Abstract

Recently there has been a great deal of interest in the potential use of genetically engineered baculoviruses as environmentally benign insecticides. Because baculoviruses often have a significant impact on the population dynamics of their hosts, any effort to assess the environmental impact of releasing engineered viruses must confront the question: Will genetically engineered baculoviruses outcompete wild‐type strains, thereby altering the natural population dynamics of the host? To begin to answer this question, we develop a mathematical model of competitive interactions between genetically engineered and wild‐type baculoviruses. We find that the interactions between these viruses are characterized mostly by dominance of one strain or the other, and that the chance that an engineered strain will outcompete a wild‐type strain depends on its particular combination of speed of kill and infectiousness. That is, baculoviruses must kill their host to become infectious, so the faster speed of kill of most recombinant viruses confers a competitive advantage. Most such strains, however, also produce fewer infectious particles and so are less infectious. Our model shows that the extent of this decrease in infectiousness must be rather small for an engineered strain to become dominant. Nevertheless, even engineered strains that are at a substantial competitive disadvantage relative to the wild type may take decades to go extinct. An additional complicating factor is that the outcome of competition depends on the overwinter survival of these viruses, about which little is known even for wild‐type viruses. Caution is therefore necessary in predicting the outcome of competitive interactions involving introduced baculoviruses, and further work is needed in understanding pathogen overwinter survival rates.

Microparasite manipulation of an insect: the influence of theegtgene on the interaction between a baculovirus and its lepidopteran host

“…By introducing these data into a mathematical model of Gypsy Moth-NPV dynamics, Dwyer et al (2002) also showed that this enhanced transmission gave the wild-type virus a significant fitness advantage over the egt-minus virus. This analysis has been extended to examining the potential competition between genetically modified baculoviruses with enhanced speed of kill in general (Dushoff & Dwyer 2001). They concluded that faster-acting viruses were unlikely to become dominant unless the reduction in yield was small.…”

Adaptation in an insect host–plant pathogen interaction

“…Thus a faster speed of kill means that virus is released from the host more rapidly, and this creates the potential for earlier secondary transmission of disease and more cycles of virus amplification within the single generation of the host. This has clear implications for increased virus fitness (Dushoff & Dwyer 2001). As predicted, the phenotype of the isolate collected from the non‐persistent M. c. pluviale population at Montague on Galiano Island, did not follow this trend.…”

The Ecology and Evolution of Insect Baculoviruses

“…Theoretical exploration of the dynamics of egt plus and minus viruses indicates that the fitness of viruses lacking the egt gene would be less than that of the parent wild-type (Dwyer et al 2002). Extending this analysis to competition between viruses that differ in speed of kill and transmissibility indicates that the faster killing virus requires relatively high levels of transmission to dominate (Dushoff & Dwyer 2001). However, overwinter survival of virus, about which little is known, is crucial in these simulations.…”