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.The family Ascoviridae was erected recently to accommodate several new species of large double-stranded DNA (dsDNA) viruses with circular genomes that attack insects of the order Lepidoptera at the larval and pupal stages, causing a chronic, fatal disease (38). Viruses of this family are characterized by large, enveloped virions with a distinctive reticulate surface pattern. Depending on the species, virions are either bacilliform or allantoid (sausage shaped), contain an internal lipid membrane surrounding the DNA/protein core, and are composed of at least 12 structural proteins, ranging in mass from 10 to 200 kDa (40).These structural characteristics of the virions are sufficient to distinguish ascoviruses from all other large dsDNA viruses. However, the most novel feature of ascoviruses is not their virion structure, but rather their unusual cellular pathology and transmission. Unlike for all other viruses, a variety of evidence suggests that ascoviruses induce apoptosis as part of a mechanism that enhances their reproduction and transmission. A typical pattern of cytopathology, as exemplified by Spodoptera frugiperda ascovirus 1a (SfAV-1a), the type species, begins with nuclear hypertrophy and cleavage of host DNA, followed by lysis of the nucleus and fragmentation of the nuclear memb...

The Spodoptera frugiperda ascovirus, a DNA virus that attacks lepidopterans, codes for an executioner caspase synthesized by 9 h after infection of Sf21 cells. This caspase alone induces apoptosis in insect cells and, during viral replication in vivo, contributes to a novel cell cleavage process in which developing apoptotic bodies are rescued by the virus and differentiate to form large vesicles in which virions assemble. These viral vesicles disseminate to the blood, where they are acquired during egg-laying by parasitic wasps that transmit the virus. No other viruses encode caspases or form such modified apoptotic bodies, suggesting this caspase plays a direct role in cell partitioning that facilitates viral reproduction and transmission.Supplemental material is available at http://www.genesdev.org. Ascoviruses are complex double-stranded DNA viruses that attack lepidopterans, most commonly species of the family Noctuidae. Virions of these viruses are enveloped, bacilliform to allantoid in shape and 400 nm in length, and contain circular genomes, which, depending on the species, range from 116 to 180 kbp (Federici et al. 2000). An unusual feature of ascoviruses in comparison to most other viruses that attack invertebrates is that they are poorly infectious per os . Instead, laboratory experiments and ecological studies have shown that these viruses are transmitted mechanically by female parasitic wasps during oviposition. The ovipositor of the female wasp becomes contaminated with virions and virion-containing vesicles when inserted into an infected host to lay eggs. Subsequent oviposition in other caterpillars transfers virions to new hosts, resulting in their infection and death as well as the death of wasp progeny (Hamm et al. 1985;Govindarajan and Federici 1990;Tillman et al. 2004). An interesting exception to this transmission pattern is the ascovirus that attacks pupae of the leak moth, Acrolepiosis assectella. The genome of this ascovirus is carried unintegrated in the nuclei of males and females of its wasp vector, Diadromus pulchellus, and virions are transmitted vertically during oviposition by the wasp, which is highly dependent on this virus for parasitic success (Bigot et al. 1997).While the transmission of ascoviruses is unusual for an insect virus, the most novel feature of these viruses compared with all others is their reproductive cellular biology. After infecting a cell, these viruses initiate a cytopathology that resembles apoptosis, but in vivo differs significantly in several features. Within a few hours of infection, the nuclear membrane invaginates and then fragments (Federici 1983;Federici and Govindarajan 1990). Subsequently, rather than shrink and form characteristic apoptotic bodies by blebbing as cell death proceeds, the cell hypertrophies, enlarging as much as 10 times the size of healthy cells . As the cell enlarges, the plasma membrane invaginates as part of process that cleaves the cell into numerous virion-containing vesicles, 5-10 µm in diameter, which subsequently disperse ...

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We recently identified 21 structural proteins in the virion of Spodoptera frugiperda ascovirus 1a (SfAV1a), a virus with a large, double-stranded DNA genome of 157 kbp, which attacks species of the lepidopteran family Noctuidae. The two most abundant virion proteins were the major capsid protein and a novel protein (P64) of 64 kDa that contained two distinct domains not known previously to occur together. The amino-terminal half of P64 (residues 1 to 263) contained four repeats (a recently recognized motif with an unknown function) of a virus-specific two-cysteine adaptor. Adjoined to this, the carboxy-terminal half of P64 (residues 279 to 455) contained 14 copies of a highly basic, tandemly repeated motif rich in arginine and serine, having an 11-to 13-amino-acid consensus sequence, SPSQRRSTS(V/K)(A/S)RR, yielding a predicted isoelectric point of 12.2 for this protein. In the present study, we demonstrate by Southwestern analysis that SfAV1a P64 was the only virion structural protein that bound DNA. Additional electrophoretic mobility shift assays showed that P64 bound SfAV1a as well as non-SfAV1a DNA. Furthermore, we show through immunogold labeling of ultrathin sections that P64 is a component of virogenic stroma and appears to be progressively incorporated into the SfAV1a DNA core during virion assembly. As no other virion structural protein bound DNA and no basic DNA-binding proteins of lower mass are encoded by the SfAV1a genome or were identified by proteomic analysis, our results suggest that P64's function is to condense the large genome of this virus and assist in packaging this genome into its virion.