Genomic sequence of Spodoptera frugiperda Ascovirus 1a, an enveloped, double-stranded DNA insect virus that manipulates apoptosis for viral reproduction

Abstract

Ascoviruses (family Ascoviridae) are double-stranded DNA viruses with circular genomes that attack lepidopterans, where they produce large, enveloped virions, 150 by 400 nm, and cause a chronic, fatal disease with a cytopathology resembling that of apoptosis. After infection, host cell DNA is degraded, the nucleus fragments, and the cell then cleaves into large virion-containing vesicles. These vesicles and virions circulate in the hemolymph, where they are acquired by parasitic wasps during oviposition and subsequently transmitted to new hosts. To develop a better understanding of ascovirus biology, we sequenced the genome of the type species Spodoptera frugiperda ascovirus 1a (SfAV-1a). The genome consisted of 156,922 bp, with a G+C ratio of 49.2%, and contained 123 putative open reading frames coding for a variety of enzymes and virion structural proteins, of which tentative functions were assigned to 44. Among the most interesting enzymes, due to their potential role in apoptosis and viral vesicle formation, were a caspase, a cathepsin B, several kinases, E3 ubiquitin ligases, and especially several enzymes involved in lipid metabolism, including a fatty acid elongase, a sphingomyelinase, a phosphate acyltransferase, and a patatin-like phospholipase. Comparison of SfAV-1a proteins with those of other viruses showed that 10% were orthologs of Chilo iridescent virus proteins, the highest correspondence with any virus, providing further evidence that ascoviruses evolved from a lepidopteran iridovirus. The SfAV-1a genome sequence will facilitate the determination of how ascoviruses manipulate apoptosis to generate the novel virion-containing vesicles characteristic of these viruses and enable study of their origin and evolution.

FIG. 1.

Schematic illustration of the organization of the Spodoptera frugiperda ascovirus 1a genome. Predicted ORFs are indicated by their locations, orientations, and putative sizes. White arrows represent ORFs in the forward strand, whereas gray arrows identify those in the complement strand. The relative positions of the 2.9-kbp fragments, organized as inverted repeats at one locus spanning the ∼80-kbp to 94-kbp positions and as a 5′- and 3′-truncated repeat at a second locus spanning the ∼147.5-kbp to 149.5-kbp positions, are also shown.

FIG. 2.

Variations in G+C content (vertical axis) along the Spodoptera frugiperda ascovirus 1a genome (horizontal axis). The graphic representation was calculated using the plot option in Vector NTI (Invitrogen) and a window of 675 nucleotides. G+C-poor intergenic regions are identified by vertical black arrows. Large light gray arrows and a gray box identify the 2.9-kbp repeats, organized as inverted repeats at one locus (spanning the ∼80-kbp to 94-kbp positions) and as a 5′- and 3′-truncated repeat at the second locus (spanning the ∼147.5-kbp to 149.5-kbp positions). The line located at a G+C content of 46.7% corresponds to the threshold used to differentiate intergenic regions from those putatively transcribed as mono- and polycistronic mRNA. G+C peaks lower than 46.7% along the SfAV-1a genome sequence are indicated by arrows.