Non Drosophila viruses

These are viruses not so far isolated from natural populations or routine laboratory cultures of flies, which can be used as research tools to investigate the basic mechanisms of virus-host-cell interactions in Drosophila.

4.2.2.1 Flock House virus (FHV)

FHV belongs to the Nodaviridae family. These are small («30 nm diameter), non-enveloped ribo-viruses with a genome composed of two single-stranded, positive-sense RNAs. Both RNAs are capped at the 5' end, but not polyadenylated. This bipartite genome is packaged in an icosahedral capsid assembled from 180 copies of a single type of coat protein. The genome organization and replication strategy of the nodaviruses is among the simplest of known viruses (reviewed in Venter and Schneemann, 2008). RNA1 (3.1 kb) encodes the 112 kDa replicase, whereas RNA2 (1.4 kb) encodes the 43 kDa capsid-protein precursor. In infected cells, the subgenomic RNA3 (0.4 kb) is produced from RNA1. It encodes the protein B2, a potent suppressor of RNA interference (Li et al., 2002).

FHV was originally isolated from the grass grub Costelytra zealandica near the Flock House Agricultural research station in New Zealand in 1983. Although isolated from a Coleopteran insect, FHV replicates efficiently in cultured Drosophila cells and in vivo, providing a valuable model to study host-virus interactions (Li et al, 2002; Galiana-Arnoux et al., 2006; Wang et al., 2006).

4.2.2.2 Sindbis virus (SINV)

SINV is the type-specific member of alphaviruses, a widely distributed group of significant human and animal pathogens that belong to the family of Togaviridae. Among them, Venezuelan equine encephalitis and O'nyong nyong viruses are an important public health threat. Alphaviruses circulate in nature by continuous transmission between mosquitoes and susceptible vertebrate hosts. In insect vectors, they cause lifelong chronic infections, whereas vertebrate hosts show acute disease characterized by a high viraemia. SINV was originally collected in 1952 from the mosquito Culex univittatus near the town of Sindbis in Egypt. SINV is one of the least pathogenic alphaviruses, but its study provided highly valuable information about the mechanism of RNA replication and virus interaction with host cells. SINV has a single-stranded RNA genome of 11 703 nt. The RNA is capped at the 5' end, and polyadenylated at the 3' end. It is packaged in an icosahedral nucleocapsid surrounded by a lipid envelope embedded with glycoprotein components. Virions are about 70 nm in diameter. The 5' two-thirds of the genomic RNA encode four non-structural proteins, nsP1 to nsP4, that form, together with cellular factors, an RNA-dependent RNA polymerase complex required for transcription and replication of the RNA. The 3' one-third codes for the structural proteins C, E1, E2, E3, and 6K. In nature, the basic maintenance cycle of SINV is between mosquitoes and birds, although other vertebrates, including humans, may also be infected, causing only subclinical disease fever. SINV can also replicate in Drosophila, but does not cause overt pathology upon infection in wild-type flies (Galiana-Arnoux et al., 2006).

4.2.2.3 Cricket paralysis virus (CrPV)

CrPV was isolated from laboratory colonies of Australian field crickets (Teleogryllus oceanicus and Teleogryllus commodus) containing some early-instar nymphs which developed a paralysis of the hind legs, became uncoordinated, and died. Electron microscopic sections of paralysed insects revealed many virus-like particles in crystalline arrays reminiscent of those observed in picornavirus-infected cells. CrPV belongs to the Dicistroviridae family and is closely related to DCV (Figure 4.1). Although originally isolated from crickets, CrPV has a wide host range, infecting insects belonging to the orders Diptera, Lepidoptera, Orthoptera, and Heteroptera. Importantly, it also replicates efficiently in Drosophila SL2 cells and is pathogenic when injected into flies (Wang et al, 2006).

4.2.2.4 Vesicular stomatitis virus (VSV)

VSV is a member of the family Rhabdoviridae, genus Vesiculovirus. Virions are 70 nm in diameter and 170 nm long, and consist of an envelope within which is a helically coiled cylindrical nucleocapsid. The precise cylindrical form of the nucleocapsid is what gives the viruses their distinctive bullet or conical shape. The genome is a single, linear, negative-sense 11.2 kb molecule of single-stranded RNA. Virus replication is restricted to the cytoplasm of the host cell. VSV infects biting insects (e.g. sand flies, mosquitoes) and can be transmitted to mammalian hosts, in particular cattle, horses, and swine, by these insects. In cultured Drosophila cells, VSV establishes a persistent, non-cytopathic infection (Dezelee et al., 1987).

4.2.2.5 Insect DNA viruses

Insects can also be infected by DNA viruses. These include baculoviruses, which are large rod-shaped (250-300 nm long, 30-60 nm in diameter) enveloped viruses (Friesen and Miller, 2001). The baculovirus genome is a large covalently closed circle of dsDNA. The Autographa californica nucle-opolyhedrovirus (AcMNPV) genome («134 kb) encodes more than 300 ORF. Baculoviruses have been isolated from many insect species. In general they have distinct and relatively narrow host ranges. However, certain baculoviruses, including AcMNPV, have broader host ranges. Interestingly, AcMNPV can efficiently enter Drosophila cells, but viral entry or early gene expression triggers apoptosis and as a consequence, Drosophila is not permissive for this DNA virus (Lannan et al., 2007). Iridoviridae, Ascoviridae, Poxviridae, and Parvoviridae are also found in insects. Curiously however, no DNA viruses have been isolated from Drosophila so far.

4.2.2.6 Infectious retrotransposons and endogenous retrovirus

Retrotransposons are transposable elements that replicate by reverse transcription of an RNA intermediate, followed by integration of the resulting DNA into the genome of host cells. Retrotransposons are widespread in Drosophila, and belong to different classes (Kaminker et al., 2002). Some Drosophila long terminal repeat (LTR) retrotransposons contain an env gene and are classified in the family Errantiviridae. The env gene encodes a transmembrane glycoprotein, which can mediate binding to a host-cell receptor. However, whereas vertebrate retroviruses are predominantly transmitted horizontally by cell-to-cell infection, Errantiviridae, also known as endogenous retroviruses, are mainly transmitted vertically from mother to offspring as integrated copies in the host-cell genome. The genome of D. melanogaster contains a large number of LTR retrotransposable elements (up to 304, belonging to 49 families), of which only a few contain an env-like gene. The endogenous retroviruses from Drosophila include gypsy, the best-characterized Errantivirus, which has a genetic organization reminiscent of that from classical vertebrate gammaretroviruses. Importantly, gypsy is the only endogenous errantivirus that has been demonstrated to be capable of exogenous infection in Drosophila (Bucheton, 1995).

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