Co-infection by SeIV iflavirus and Spodoptera exigua multiple nucleopolyhedrovirus (SeMNPV): effects on occlusion body structure and conformation

Abstract

Lepidopteran populations frequently harbor cryptic infections involving one or more viruses. Three species have been repeatedly identified in covertly infected Spodoptera exigua : the S. exigua multiple nucleopolyhedrovirus (SeMNPV), the iflaviruses SeIV1 and SeIV2. Mixed infections have been detected in laboratory colonies and field collected insects but little is known about the interaction between these viruses and their effects on the insecticidal properties of SeMNPV occlusion bodies (OBs). When SeMNPVand SeIV were inoculated in the same host and a productive baculovirus infection occurred, PCR-based techniques and electron microcopy (TEM) of OBs previously indicated contamination of OBs by iflavirus particles (Jakubowska et al 2016, PeerJ e1687). In the present study OB populations generated by co-inoculation of SeM-NPV and one of three iflavirus treatments: SeIV1, SeIV2, and a 1:1 mixture of SeIV1+SeIV2 were examined for OB structure and iflavirus load, in comparison to OB populations from iflavirus-free insects. OB populations were purified using a continuous sucrose gradient. Interestingly, two OB bands were observed and collected for the three iflavirus treatments whereas only one band was present in the SeMNPV control. RNA was extracted from OBs and specific viral loads quantified by qPCR for the fragmented OB population corresponding to the upper and low bands collected. These resultsallowed us to correlate the different morphological OB conformation with high SeIV1 loads. We conclude that SeIV1 co-infection altered OB conformation that might explain the biological properties of these OBs. Results of ongoing experiments will be reported aimedat determining OB physical characteristics by TE