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.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...

Proteomic analysis of the Spodoptera frugiperda ascovirus 1a virion reveals 21 proteins

“…Apparent homologues and orthologues are present in HvAV-3e, TnAV-6a and iridoviruses (Table 1; Bideshi et al, 2006). However, at present, their putative roles in SfAV-1a virion structure and virogenesis could not be assigned due to a lack of information regarding the function(s) of related proteins in their respective hosts.…”

“…SfAV-1a ORFs 002, 003,027,033,035,036,038,043,054,060, 064 and 084 do not have conserved domains or motifs, although ORF084 and ORF064 share low levels of sequence similarities with dynein-like beta chain and serine-threonine kinase, respectively (Bideshi et al, 2006). Apparent homologues and orthologues are present in HvAV-3e, TnAV-6a and iridoviruses (Table 1; Bideshi et al, 2006).…”

“…Indeed, analysis of genomic sequences of the SfAV-1a, HvAV-3e and TnAV-6a showed that these viruses contained an abundance of gene orthologues in iridoviruses compared with other virus families (Asgari et al, 2007;Bideshi et al, 2006;Wang et al, 2006). The results reported here showed that 10 of the 21 SfAV-1a virion structural proteins (ORFs 009, 035, 041, 048, 054, 061, 064, 084, 091 and 109) had orthologues in iridoviruses, whereas only one orthologue (ORF015) was found in baculoviruses.…”

“…Collectively, these proteins are known to function as DNA/RNA helicases, and participate in DNA replication, recombination and repair, transcription and translation. They are also known to be involved with pre-mRNA splicing, ribosome *Functional assignments were described previously (Bideshi et al, 2006). DApparent (observed) molecular masses are shown in parentheses.…”

“…The family Ascoviridae comprises large double-stranded DNA viruses with circular genomes ranging from 100 to 186 kb (Asgari et al, 2007;Bideshi et al, 2006;Bigot et al, 1997a, b;Wang et al, 2006). Four species of ascoviruses (AVs) are recognized, Spodoptera frugiperda AV (SfAV-1), the type species, Trichoplusia ni AV (TnAV-2), Heliothis virescens (HvAV-3) and Diadromus pulchellus AV (DpAV-4).…”