Identification by cryoEM of a densovirus causing mass mortality in mass-reared larval darkling beetles (Zophobas morio)

Abstract

A reared colony of larval superworms (Zophobas morio) experienced the swift and unexplained death of about 90% of its population. We isolated a high-abundance virus from dead larvae and, using cryoEM, identified it as a novel densovirus, which we name Zophobas morio black wasting virus (ZmBWV). Densoviruses (DVs) are small, ssDNA viruses of the family Parvoviridae that infect protostome and deuterostome invertebrates. The vast majority of DVs have been associated with severe pathology, especially in larval stage insects. By cryoEM we resolved the high-resolution capsid structure of this new DV at 2.9 [A] resolution for the genome-packaging, infectious particles and at a resolution of 3.3 [A] in case of the empty particles, both purified directly from the infected Z. morio larvae. The capsid structure of ZmBWV provides the first insights into the capsid morphology of a structurally previously-uncharacterized genus, Blattambidensovirus. Consequently, the ZmBWV capsid harbors a unique surface morphology within the family, yet shows the T=1 icosahedral symmetry, the eight-stranded jelly roll core, as well as general features of multimer interactions previously found typical of subfamily Densovirinae. Although we have not inoculated healthy larvae with ZmBWV, on the basis of its prodigious abundance in infected larvae and the prior probability of larval pathology with viruses of this subfamily, ZmBWV is the most probable cause of the observed mass mortality event.