Reprogramming of larval locomotory pattern during baculovirus-induced host manipulation
Many parasites manipulate host behaviour to enhance their transmission. Baculovirus induces enhanced locomotory activity (ELA) combined with subsequent climbing behaviour in lepidopteran larvae, which facilitates viral dispersal. Previously, larval locomotion during ELA was summarized as the distance travelled for a few minutes at several time points. However, as ELA continues for several hours, these methods are unlikely to evaluate larval locomotion precisely during ELA. We developed a novel method to continuously trace the locomotion of Bombyx mori larvae using time-lapse imaging. This method successfully quantified the locomotory activities of larvae infected with Bombyx mori nucleopolyhedrovirus (BmNPV) for 24 h. We found that both mock- and BmNPV-infected larvae periodically repeated moving and pausing with a similar frequency. In contrast, BmNPV-infected larvae showed fast and long-lasting locomotion compared with mock-infected larvae, which resulted in longer locomotory distances in infected larvae. Moreover, BmNPV-infected larvae exhibited biphasic behaviour. Initially, BmNPV-infected larvae showed longer locomotory distances, but the locomotory pattern was similar to mock-infected larvae. However, during the second phase, the locomotory pattern was drastically altered, with an extremely larger locomotory area. These results indicate that BmNPV reprograms host locomotory pattern, which is a turning point for the process of BmNPV-induced host manipulation.