Abstract
Many parasites enhance their dispersal and transmission by manipulating host behaviour. One intriguing example concerns baculoviruses that induce hyperactivity and tree-top disease (i.e., climbing to elevated positions prior to death) in their caterpillar hosts. Little is known about the underlying mechanisms of such parasite-induced behavioural changes. Here, we studied the role of the ecdysteroid UDP-glucosyltransferase (egt) gene of Spodoptera exigua multiple nucleopolyhedrovirus (SeMNPV) in tree-top disease in S. exigua larvae. Larvae infected with a mutant virus lacking the egt gene exhibited a shorter time to death and died before the induction of tree-top disease. Moreover, deletion of either the open reading frame or the ATG start codon of the egt gene prevented tree-top disease, indicating that the EGT protein is involved in this process. We hypothesize that SeMNPV EGT facilitates tree-top disease in S. exigua larvae by prolonging the larval time to death. Additionally, we discuss the role of egt in baculovirus-induced tree-top disease.
Highlights
A wide range of parasites are able to modify the behaviour of their hosts upon infection.Intriguing examples include the suicidal behaviour of the cricket Nemobius sylvestris when parasitized by the hairworm Paragordius tricuspidatus [1] and the long-time hypokinetic stage of the cockroachPeriplaneta americana when stung by the parasitoid jewel wasp Ampulex compressa [2]
We show that WT-infected S. exigua larvae climbed up prior to death and died at elevated positions, while larvae infected with an Spodoptera exigua multiple nucleopolyhedrovirus (SeMNPV) mutant lacking the egt gene did not climb up prior to death
To study whether deletion of the viral egt gene or of the egt start codon from the SeMNPV genome affected viral infectivity, we performed a logistic regression on the mortality data obtained after infection of S. exigua 3rd instars with G25 WT, SeBac10 WT, ∆egt-open reading frame (ORF), ∆egt-ATG or egt-repair viruses, for each replicate separately
Summary
A wide range of parasites are able to modify the behaviour of their hosts upon infection.Intriguing examples include the suicidal behaviour of the cricket Nemobius sylvestris when parasitized by the hairworm Paragordius tricuspidatus [1] and the long-time hypokinetic stage of the cockroachPeriplaneta americana when stung by the parasitoid jewel wasp Ampulex compressa [2]. A wide range of parasites are able to modify the behaviour of their hosts upon infection. Many parasite-induced behavioural changes are thought to enhance parasite reproduction, transmission and/or survival [3,4,5]. Baculoviruses are large, double-stranded DNA viruses that induce behavioural changes in caterpillars. Infected host caterpillars show hyperactivity, which may spread viral progeny over larger areas [6,7,8]. Infected larvae present atypical climbing behaviour prior to death, leading to their migration to the top of plants. This pre-death climbing behaviour is described as “Wipfelkrankheit” or “tree-top disease” [8,9,10,11]. The resulting increased visibility of infected insects to predators may enhance long-distance virus dispersal [12] and the final liquefaction of larval cadavers at elevated positions promotes virus dissemination over the foliage [8]
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