Per os infectivity factors:a complicated and evolutionarily conserved entry machinery of baculovirus

Baculoviruses are a family of arthropod-specific large DNA viruses that infect insect species belonging to the orders Lepidoptera,Hymenoptera and Diptera.In nature,occlusion-derived viruses(ODVs) initiate baculovirus primary infection in the midgut epithelium of insect hosts,and this process is largely dependent on a number of ODV envelope proteins designated as per os infectivity factors(PIFs).Interestingly,PIF homologs are also present in other invertebrate large DNA viruses,which is indicative that per os infection is an ancient and phylogenetically conserved entry mechanism shared by these viruses.Here,we review the advances in the knowledge of the functions of individual PIFs and recent discoveries about the PIF complex,and discuss the evolutionary implications of PIF homologs in invertebrate DNA viruses.Furthermore,future research highlights on the per os infection mechanism are also prospected.

HearNPV Pseudotyped with PIF1,2,and 3 from MabrNPV：Infectivity and Complex Stability

Effective oral infection is set off by interaction of a group of conserved per os infectivity factors(PIFs) with larval midgut columnar epithelial cells. We constructed pseudotyped viruses by substituting pif1, pif2 or pif3 genes of Helicoverpa armigera nucleopolyhedrovirus(Hear NPV) with their homologs from Mamestra bracissae multiple nucleopolyhedrovirus and tested their infectivity to tissue culture cells and to larvae. Transfection and infection assays revealed that all recombinant viruses generated infectious budded virus in both cell culture and in larvae. Electron microscopy showed synthesized occlusion body and occlusion derived virus(ODV) were morphologically indistinguishable from those of the parental virus. By contrast, feeding assays revealed that pseudotyped viruses could not rescue oral infectivity except for pif3 pseudotyped virus that only partially rescued oral infectivity but at a mortality rate much lower than that of the parental Hear NPV. Consistent with the bioassay result, PIF complex was detected in ODVs of pif3 pseudotyped virus only but not in pif1 or pif2 pseudotyped viruses. Our results suggest that PIF complex is essential for oral infectivity, and in the formation of the PIF complex, PIF1, 2 are virus-specific while PIF3 does not appear to be as specific and can function in heterologous environment, albeit to a much more limited extent.

Novel strategies for the biocontrol of noctuid pests(Lepidoptera)based on improving ascovirus infectivity using Bacillus thuringiensis

ldentifying novel biocontrol agents and developing new strategies are urgent goals in insect pest biocontrol.Ascoviruses are potential competent insect viruses that may be developed into bioinsecticides,but this aim is impeded by their poor oral infectivity.To improve the per os infectivity of ascovirus,Bacillus thuringiensis kurstaki(Btk)was employed as a helper to damage the midgut of lepidopteran larvae(Helicoverpa armigera,Mythimna separata,Spodoptera frugiperda,and S.litura)in formulations with Heliothis virescens ascovirus isolates(HvAV-3h and HvAV-3j).Btk and ascovirus mixtures(Btk/HvAV-3h and B1k/HvAV-3j)were fed to insect larvae(3rd instar).With the exception of S.frugiperda larvae,which exhibited low mortality after ingesting Btk,the larvae of the other tested species showed three types of response to feeding on the formulas:type I,the tested larvae(H.armigera)were killed by Btk infection so quickly that insufficient time and resources remained for ascoviral invasion;type II,both Btk and the ascovirus were depleted by their competition,such that neither was successfully released or colonized the tissue;type II,Btk was eliminated by the ascovirus,and the ascovirus achieved systemic infection in the tested larvae.The feeding of Btk/ascovirus formulas led to a great reduction in larval diet consumption and resulted in a significant decrease in the emergence rate of H.armigera,M.separata,and S.litura larvae,which suggested that the formulas exerted marked oral control effects on both the contemporary individuals and the next generation of these tested pest species.

Genomics and proteomics of Apis mellifera filamentous virus isolated from honeybees in China

Apis mellifera filamentous virus(Am FV)is a large DNA virus that is endemic in honeybee colonies.The genome sequence of the Am FV Swiss isolate(Am FV CH–C05)has been reported,but so far very few molecular studies have been conducted on this virus.In this study,we isolated and purified Am FV(Am FV CN)from Chinese honeybee(Apis mellifera)colonies and elucidated its genomics and proteomics.Electron microscopy showed ovoid purified virions with dimensions of 300–500×210–285 nm,wrapping a 3165×40 nm filamentous nucleocapsid in three figure-eight loops.Unlike Am FV CH–C05,which was reported to have a circular genome,our data suggest that Am FV CN has a linear genome of approximately 493 kb.A total of 197 ORFs were identified,among which36 putative genes including 18 baculoviral homologs were annotated.The overall nucleotide similarity between the CN and CH–C05 isolates was 96.9%.Several ORFs were newly annotated in Am FV CN,including homologs of per os infectivity factor 4(PIF4)and a putative integrase.Phylogenomic analysis placed Am FVs on a separate branch within the newly proposed virus class Naldaviricetes.Proteomic analysis revealed 47 Am FV virionassociated proteins,of which 14 had over 50%sequence coverage,suggesting that they are likely to be main structural proteins.In addition,all six of the annotated PIFs(PIF-0–5)were identified by proteomics,suggesting that they may function as entry factors in Am FV infection.This study provides fundamental information regarding the molecular biology of Am FV.