Nudivirus

A nudivirus (family Nudiviridae) is a large, rod-shaped virus with a circular, double stranded DNA genome of 96–231 kb. The genome encodes 98 to 154 open reading frames. Virions are rod-shaped and when enveloped are up to 382×77 nm.

The word "nudivirus" comes from the Latin nudus, which means naked and virus, poison. Naked refers to the fact that, unlike baculoviruses, they are not occluded by cryoproteins.

Taxonomy

In 2007, the genus Nudivirus was proposed to include viruses similar to the Oryctes rhinoceros virus (Wang et al., 2007b^[1]). The nudiviruses were classified as the family Nudiviridae in 2013.^[2]

Phylogeny

All 3 sequenced nudivirus have 33 open reading frames in common. Gene content comparaison and phylogenetic analyses show that Nudivirus share 20 cores genes with baculovirus and form a monophyletic sister group with them. Fossil calibration estimate this association arose 100 million years ago(Mya), while the last common ancestor of BVs, nudivirus, and baculovirus existed approximately 312 Mya. Baculoviruses and nudiviruses differ in gene content, genome organization, cytopathology, infection of adults and most likely in host range.^[3] The 20 cores genes common in both Baculovirus and Nudivirus are involved in RNA transcription, DNA replication, virion structural components and many other functions.^[4] Gene content and sequence similarity suggest that the nudiviruses GbNV, HzNV-1, and OrNV form a monophyletic group of nonoccluded double-stranded DNA viruses, which separated from the baculovirus lineage before this radiated into dipteran-, hymenopteran-, and lepidopteran-specific clades of occluded nucleopolyhedroviruses and granuloviruses.^[3]

Virology

This genus is characterized by rod-shaped and enveloped nucleocapsids and its members replicate in the nucleus of infected host cells. The virus genome, in proviral form, is integrated on wasp genome. However, the viruses are not embedded in inclusion bodies (crystallized proteins) like the baculoviruses.^[5] Nudivirus are known to infect preferentially insects and marine arthropods.^[1] The transmission occurred generally by feeding and/or mating. Infections can be lethal for the larvae and can possibly reduce the fitness of the host by reducing offspring production and survival among adults.^[6]

Host-virus relations

• Gryllus bimaculatus nudivirus – black cricket (Teleogryllus commodus)
• Helicoverpa zea nudivirus 1 – cotton bollworm
• Helicoverpa zea nudivirus 2 – cotton bollworm
• Penaeus monodon nudivirus – a shrimp
• Oryctes rhinoceros nudivirus – rhinoceros beetle (Dynastinae species)
• Drosophila innubila Nudivirus – Drosophila innubila (Diptera)

Defense Mechanisms

In many organisms, apoptosis can be regarded as an early cellular defense mechanism against a strong viral infection. Although, Heliothis zea Nudivirus 1 (HzNV-1 or Hz-1 virus), a relatively broad host range nudivirus, has been shown to block an induced-apoptosis gene (hhi1), and so, prevent his own death. A functional antiapoptosis gene, (Hz-iap2), has been highlighted to suppress the hhi1 gene which can either activate the expression of prodeath gene or alter the cell cycle within the host, resulting in the death of the cell. A second inihibitor gene (Ac-iap2) to the hhi1 gene has been also discovered, interestingly, Ac-iap2 was speculated to be a nonfunctional iap, raising many questions regarding its functionnality.^[7]

Relation to polydnaviruses in parasitic wasps

The parts of the cycle that relate to the transmission of wasp genomic DNA and therefore to proviral DNA are shown by the claret arrows. Those parts of the cycle that apply only to virus replication and assembly/transmission are shown by the green arrows. Polydnavirus (PDVs) are associated to endoparasitic wasps from the Ichneumoninae family (Hymenoptera). There are two genera of PDVs: The ichnoviruses occur in ichneumonid wasps species and bracoviruses in braconid wasps. The polydnaviridae appear to replicate and accumulate in the ovarian calyx epithelial cells and calyx fluid of the wasps and do not replicate in the lepidopteran hosts of the wasps, although their gene products are expressed there. The packaged Virus is injected along with the wasp egg into the body cavity of a lepidopteran host caterpillar and infects cells of the caterpillar (Whitfield, 1990^[8]). The infection does not lead to replication of new viruses, rather it affects the caterpillar's Immune system. For example, M. R. Strand(Wisconsis University) showed that 24h after egg laying, granulocytes had almost disapered from hemolymph.

Without the virus infection, Phagocyte Hemocyte (blood cells) will encapsulate and kill the wasp egg but the immune suppression caused by the virus allows for survival of the wasp egg, leading to hatching and complete development of the immature wasp in the caterpillar. Additionally, genes expressed from the polydnavirus in the parasitised host alter host development and metabolism to be beneficial for the growth and survival of the parasitoid larva (Fleming et Summers, 1991^[9]). Thus the virus and wasp have a Symbiosis (mutualistic) relationship. It appears in most cases that the PDVs DNA is required for successful Parasitism of Lepidoptera by the wasps. So, the full genome of the virus is integrated into the genome of the wasp and the virus only replicates in specific cells in the female wasp's reproductive system.. As a result, the polydnaviral DNA has experienced spectacular reproductive success; it is now thought to be represented in species of many ichneumonoid wasps.

The conservation of this viral machinery in different braconid wasp lineages sharing polydnaviruses suggests that parasitoid wasps incorporated a nudivirus-related genome into their own genetic material (Bézier et al., 2009^[10]).

References