Baculoviruses up-regulate genes controlling visual perception in their caterpillar hosts, thus inducing phototaxis and promoting their own dispersal

Baculoviruses can induce climbing behavior in their caterpillar hosts to ensure they die at elevated positions, providing an excellent model to study parasite manipulation of host behavior. Here, we confirm that Helicoverpa armigera single nucleopolyhedrovirus (HaSNPV) induces phototaxis in host larvae to produce climbing behavior, resulting in their death at elevated heights. Increased height at death improved wind-assisted dispersal of the virus, and the infectivity of crude extracts of cadavers appeared to be protected from direct exposure to sunlight or UV light relative to purified virus. Climbing behavior occurs mostly during daylight hours, and phototaxis is strongest toward green-blue wavelengths. Three opsin genes (HaBL, HaLW, HaUV), all integral to the visual perception pathway, were significantly up-regulated after NPV inoculation. RNAi and CRISPR/Cas9 systems were employed to demonstrate that expression of both HaBL and HaLW genes were required for NPV-induced climbing behavior. TheTRPL gene, which codes for a critical ion channel protein downstream of the opsins in the visual signalling pathway, was also up-regulated post NPV-inoculation, and its knockdown also inhibited phototaxic behavior. These results reveal that HaSNPV employs genetic manipulations to alter host visual perception at its most fundamental levels – photoreception and phototransduction – in order to change host behavior and benefit its own transmission.