Do persistent NPV infections in Spodoptera exigua affect host fitness?

Abstract

Vertical transmission of covert NPV infections is a common phenomenon in field populations of the beet armyworm (Spodoptera exigua, Huber) in the horticultural region of Almería (Spain). The genotypic diversity of the Spodoptera exigua nucleopolyhedrovirus population in this region has proved to be extremely high. A recent study suggests that some of these genotypic variants are involved in vertically-transmitted infections whereas other genotypes are involed in horizontally-transmitted infections. These findings led us to examine whether host-pathogen interactions are associated with a range of fitness correlates or tradeoffs in either the insect or the virus. First, we demonstrated that S. exigua adults that survived a virus challenge (9 x 103 OB/ml) presented a persistent sublethal infection detected by RT-PCR (85 - 100% of insects were positive for viral transcripts of the DNA-polymerase gene) following inoculation by vertically transmitted genotypes. A vertically transmitted genotype was then selected to examine the effects of larval age at inoculation and the dose of virus ingested on the prevalence of persistent virus infections in adults. Larvae of four different larval instars (L2, L3, L4, and L5) were dosed with four different concentrations that resulted in 20 to 80% of NPV-mortality. A similar trend was observed for each larval stage, the higher the dose of inoculum, the higher the proportion of adults that proved positive for viral transcripts. Finally, we observed that persistent infections reduce the fitness of individuals which survive the infection in terms of host developmental parameters (larval developmental time, pupal weight, adult fecundity, fertility, and longevity) in an insect colony in which a persistent infection had been induced compared to mocked-infected insects. On-going experiments are being performed to examine the biological costs of harbouring covert infections.