Genus Ascovirus

Type Species	Spodoptera frugiperda ascovirus 1a	(SfAV-1a)

Virion Properties

Morphology

Virions of ascoviruses are either bacilliform, ovoid or allantoid in shape, depending on the species, have complex symmetry, and are large, measuring about 130  nm in diameter by 200-400  nm in length. The virion consists of an inner particle surrounded by an outer envelope. The inner particle typically measures 80 300  nm and contains a DNA/protein core bounded by an apparent lipid bilayer, the external surface of which bears a distinctive layer of protein subunits. The virion, therefore, appears to contain two lipid membranes, one associated with the inner particle, the other forming the envelope. In negatively stained preparations, virions have a distinctive reticulate appearance thought to result from superimposition of the protein subunits on the surface of the internal particle with those in the external envelope (Fig. 1).

Physicochemical and Physical Properties

Virions are sensitive to organic solvents and detergents. Other properties not known.

Nucleic Acid

The inner particle contains a single molecule of circular dsDNA ranging in size from 100-180  kbp. G+C ratio ranges from 42-60% depending on the species.

Proteins

Virions contain at least 12 polypeptides ranging in Mr from 6-200 10³.

Lipids

Ultrastructural evidence and detergent sensitivity indicate lipid in both the outer envelope and inner particle of the virion. Specific lipid composition is unknown.

Carbohydrates

Unknown.

Genome Organization and Replication

Only one ascovirus gene, encoding a DNA polymerase, has been cloned and sequenced. Therefore, the genomic organization is unknown. Ascoviruses initiate replication in the nucleus. The nucleus enlarges and ruptures, after which the plasmalemma invaginates forming internal membraneous folds that cleave the cell into a cluster of virion-containing vesicles. Virion assembly becomes apparent after the nucleus ruptures. The first recognizable structural component of the virion to form is the multilaminar layer of the inner particle. Based on its ultrastructure, this layer consists of a unit membrane and an exterior layer of protein subunits. As the multilaminar layer forms, the dense DNA/protein core assembles along the inner surface. This process continues and the allantoid, ovoid or bacilliform shape of the inner particle becomes apparent. After the inner particle is assembled, it is enveloped by membranes, apparently synthesized de novo, within the cell or vesicle. In some ascoviruses, the virions are occluded in an occlusion body composed of minivesicles and protein.

Antigenic Properties

Unknown.

Biological Properties

Ascoviruses cause disease in lepidopterous larvae, and have been reported most commonly from species of the family Noctuidae, including Trichoplusia ni, Heliothis virescens, Helicoverpa zea, Spodoptera frugiperda, and Autographa precationis. An ascovirus disease has also been reported in the lepidopteran Acrolepiopsis assectella (family Yponomeutidae). Ascoviruses are difficult to transmit per os, and experimental studies as well as field observations suggest most are mechanically vectored by endoparasitic wasps (Hymenoptera). The virus that causes disease in A. assectella is transmitted by the ichneumonid wasp, Diadromus pulchellus, and the viral genome is carried in wasp nuclei. Ascoviruses are known only from the United States and Europe, but as they have been discovered relatively recently, they likely occur worldwide. Ascoviruses vary in tissue tropism with some attacking most host tissues, whereas others replicate primarily in epidermal tissues are restricted to the fat body and reproductive tissues. Ascoviruses typically cause a chronic, fatal disease that greatly retards larval development, but which has few other gross signs. The most unique property of ascoviruses is their unusual cytopathology in which host cells are cleaved to form virion-containing vesicles. Infection results in nuclear hypertrophy followed by lysis. The anucleate cell enlarges 5- to 10-fold, and is then cleaved into 10-30 virion-containing vesicles. Membranes delimiting the vesicles form by invagination of the plasmalemma and de novo membrane synthesis. Vesicles accumulate in the hemolymph, turning it milky white. The formation of virion-containing vesicles is diagnostic for the virus family as well as for the disease.

List of Species Demarcation Criteria in the Genus

The following list of characters in combination is used to differentiate species of the genus.

	Virion morphology,
	Presence of occlusion bodies,
	Lack of DNA/DNA hybridization with other species under low stringency conditions,
	Restriction enzyme profiles of the DNA genome,
	Host of isolation and experimental host range,
	Tissue tropism,
	Association with specific hymenopteran parasites, if apparent.

Ascoviruses can have broad host ranges among the larvae of lepidopteran species, and the fat body tissue is a major site of replication for most species. In addition, virions of most isolates are similar in size and shape. The above characteristics are therefore used in combination to distinguish existing and new ascovirus species from one another. Hybridization studies have proven particularly useful and when combined with RFLPs can also be used to distinguish variants within a species.

For example, SfAV-1a, TnAV-2a, HvAV-3a, and DpAV-4a typically do not hybridize under conditions of either high or low stringency (see Federici, Vlak and Hamm, 1990 Federici and Govindarajan, 1990; for conditions). In addition to this important characteristic, TnAV-2a replicates in a range of larval tissues including the fat body, trachael matrix, and epidermis, but SfAV-1a and HvAV-3a appear to replicate primarily in the fat body tissue of most hosts. The DpAV-4a virion replicates primarily in the pupal stage, where the primary tissues attacked are the fat body and midgut. SfAV-1a virions are bacilliform and are occluded in vesiculate occlusion bodies, whereas TnAV-2a virions are allantoid and are not occluded in occlusion bodies. HvAV-3a virions are ovoidal to bacilliform and are not occluded in occlusion bodies. The DpAV-4a genome is carried as a circular molecule in nuclei of the wasp host, Diadromus pulchellus, and is transmitted vertically to wasp progeny. However, vertical transmission in a wasp host is not known to occur with other ascoviruses. TnAV-2a and HvAV-3a have been shown to have a broad experimental host range among larvae of the lepidopteran family Noctuidae, but SfAV-1a has a host range restricted primarily to species of Spodoptera and DpAV-4a to species of the lepidopteran family Yponomeutidae.

When hybridization occurs between a new isolate and an existing species, RFLPs can be useful in distinguishing variants. Numerous ascovirus isolates, for example, have been obtained from different noctuid larvae, including H. virescens, Helicoverpa zea, and Autographa precationis, S. frugiperda, and S. exigua. Many of the isolates from H. virescens, H. zea, and A. precationis show strong reciprocal hybridization with HvAV-3a under conditions of high stringency. RFL profiles of different isolates, however, often show variations from HvAV-3a that range from minor to major. Because these isolates cross-hybridize strongly with HvAV-3a, they are considered variants of this viral species. Moreover, experimentally these isolates have been shown to have host ranges that overlap, providing additional evidence that they are variants of the same species. A similar situation occurs with isolates of TnAV-2a.

Recent analyses of the DNA polymerase gene from the above species support the designation of the species described above and listed below (Fig. 2). It should be realized, however, that as the Ascoviridae is a newly recognized family, the species concept is subject to change once existing and new isolates are studied more thoroughly.

List of Species in the Genus

Official virus species names are in italics. Tentative virus species names, alternative names ( ), strains or serotypes are not italicized. Virus names and assigned abbreviations ( ) are:

Species in the Genus

Diadromus pulchellus ascovirus 4a	(DpAV-4a)
Heliothis virescens ascovirus 3a	(HvAV-3a)
Spodoptera frugiperda ascovirus 1a	(SfAV-1a)
Trichoplusia ni ascovirus 2a	(TnAV-2a)

Tentative Species in the Genus

None reported.

Phylogenetic Relationships within the Family

See Fig. 2.

Similarity with Other Taxa

Morphologically, the virions of ascoviruses bear some resemblance to the particles characterized as ichnoviruses of the family Polydnaviridae.

Derivation of Names

Asco: from the Greek for “sac”; referring to the virion-containing vesicles, characteristic for all known viruses of this family.