Chilo iridescent virus (CIV) or Invertebrate irides-cent virus 6 (IIV-6) was originally isolated from diseased larvae of the rice stem borer, Chilo suppres-salis (Lepidoptera; Pyralidae) in Japan (29), and can infect many insect species. The term iridescent in the virus name is due to the turquoise colour of heavily infected larvae. The massive proliferation of CIV particles and their paracrystalline arrangement in the cytoplasm of host cells cause this turquoise color, which is indicative of a patent infection that is usually fatal. The colours include lavender and blue. However patent CIV infections are usually rare. Concentrated samples of purified virus from sublethally infected insects may also iridescence (62).
CIV has been most studied in the respect of host range and has been shown to cause patent infections in numerous species from the major insect orders, in-cluding species of agricultural and medical impor-tance (28, 52) and a number of arthropods (53).
The large DNA genome of CIV has been totally sequenced, and several genes have been characterized at transcriptional levels (39, 22, 23). Recently the molecular aspects of CIV have been the subject of a number of studies (16, 36, 49, 50, 56,).
The family Iridoviridae comprises icosahedral particles with an internal lipid membrane and a large double stranded DNA genome (14). There are cur-rently five genera recognized in this family (Table 1). Invertebrate iridescent virus 6 (IIV-6), often referred to as Chilo iridescent virus (CIV) is the type species of the Iridovirus genus which comprises one additional species: Invertebrate iridescent virus 1 (IIV-1).
Table 1. Taxonomy of iridoviruses
CIV has been used as the standard model for studies on invertebrate iridoviruses. Besides CIV and IIV-1, there are additional 11 tentative species recognized in the genus. However currently, there is not enough information available to determine their species status (63). Williams and Cory (64) noted that at least two strains of CIV being used in laboratories in different parts of the world. The isolate that has been com-pletely sequenced by Jakob et al. (39) in Germany differs from the isolates used in New Zealand, Australia, and the USA.
CIV is a large non occluded virus with icosahedral symmetry (Fig. 1). The virion is composed of three concentric domains; an outer proteinaceous capsid, an intermediate lipid membrane with associated poly-peptides, and a central core DNA-protein complex containing the genome (14, 62, 63). Viral particles can be either enveloped or nonenveloped, depending on whether they are released from the cell by lysis or bud from the plasma membrane.
CIV particle structure has been defined by cryo-electron microscopy and three-dimensional image reconstruction. Image reconstruction shows that the CIV capsid (185 nm maximum diameter) consists of 12 pentamers 1460 doughnut-like trimers (each trimer has a fiber that emanates from the center) arranged in a T=147 icosahedral lattice. A lipid bilayer follows the inner side of the capsid. Also the CIV capsid consists of 20 trisymetrons and 12 pentasymetrons and each trisymetrons contain 55 trimers (67). In a recent study the structure of CIV was determined to high resolution (13 Å ) by means of cryo-electron microscopy [cryo-EM] and three-dimensional image reconstruction. These studies revealed that there are at least three different types of minor capsid proteins associated with the capsomers outside the lipid membrane of CIV (68).
The structure of the internal lipid membrane has been investigated and noted to be synthesized de novo in the infected cell cytoplasm without any visible continuity with the cell membrane (58). Another group investigated the lipid composition of the CIV. They found that the lipid composition of the viral membrane was unchanged whether the virus was propagated in vivo in larvae or in vitro in invertebrate cell cultures and was clearly different from that of hosts. A high abundance of phosphatidylinositol and diglycerides was detected in compositional analyses, indicating that CIV preferentially incorporates these lipids into its internal membrane (1).
The host range of CIV in the class Insecta has been investigated by intrahemocoelic or per os inoculation and found to include more than 100 insect species belonging to six orders (Lepidoptera, Coleoptera, Diptera, Hymenoptera, Hemiptera and Orthoptera) (28, 32, 33, 47, 48, 52, 53). Fukuda (28) succeeded in perorally infecting 13 species of mosquitoes with CIV. Ohba (52) infected 65, 8 and 2 species of Lepidoptera, Coleoptera and Hymenoptera, respectively.
CIV replicates in many insect cell lines (13, 18, 41, 52) including cells from Bemisia tabaci (30), Diaprepes root weevil (35), and the boll weevil (24), and can even infect reptile cells (46). However, CIV does not replicate in vivo in frogs (54). Following the in-traperitoeal injection into Rana limnocharis, the titer of CIV declined 100 fold over a period of four days. In a recent study, the cell line AFKM-On-H, from hemocytes of the European corn borer Ostrinia nubilalis (Hübner) (Lepidoptera, Pyralidae) was also studies for its ability to productively sustain CIV infection (5). Several ultrathin sections demonstrated internalization of virus particles in cytoplasm of cells without clear signs of CIV replication. However, suspicions were raised concerning the toxicity of the virus because cells were inoculated with high con-centrations of CIV and since CIV readily infects O. nubilalis and replicates abundantly in different tissues and cells of larvae including the fat tissue and hemocytes (4).
In spite of its wide host range CIV has attracted little attention as a potential biopesticide, because of the limited mortality effect on its hosts (63). However, non-lethal inapparent infections may be common in certain insect populations (60, 61) and such infections may seriously reduce the reproductive capacity, body size and longevity of infected individuals. Little is known about the factors that determine the virulence of these viruses, but it is assumed that covert infections open the way to vertical transmission of the virus from parent to offspring.
Invertebrate iridescent viruses (IIV) infect inverte-brates, especially insect species that live in damp or aquatic habitats. Soil is believed to be an important environmental reservoir for IIVs. The effect of soil moisture and the presence of microorganisms on the persistence of CIV have been studied (57). The loss of activity of CIV in dry soil (6.4% moisture, −1000 kPa metric potential) was very rapid and was not studied beyond 24 h. However, soil moisture did not affect the rate of inactivation of virus in damp (17% moisture, −114 kPa metric potential) or wet soil (37% moisture, −9.0 kPa metric potential). In contrast, soil sterili-zation significantly improved the persistence of CIV activity, both in damp and wet soil. These figures represent half lives of 4.9 days for CIV in non-sterile soil, 6.3 days in sterilized soil, and 12.9 days for the control virus suspension. This study also concluded that extra-host persistence in soil habitats may be an important aspect of the ecology of IIVs.
Temperature affects the rate of deactivation as iridescent viruses are thermolabile and are inactivated within minutes at temperatures over 55℃ (19, 44). Aqueous suspensions of CIV showed a 10 fold reduction in titer after 50 days either at 4℃ or 25℃ (43).
Ultraviolet radiation has been used to deactivate CIV in laboratory studies (54), and exposure to solar UV light also resulted in a very rapid inactivation of CIV in water, with infectivity dropping by approximately nine logarithms after 24 hour exposure to sunlight (63). The rate of loss of activity of CIV was also confirmed by Hernandez et al (34). They tested the UV effect on CIV in aquatic habitats and found that direct sunlight causes rapid loss of activity (99.99% reduction).
The sensitivity of CIV to a selection of organic solvents, detergents, enzymes and heat treatment was assayed in Spodoptera frugiperda (Sf9) cells and by injection of inocula into larvae of Galleria mellonella. CIV was found to be sensitive to chloroform. Sen-sitivity (defined as a reduction of at least 1 log activity) was detected following treatment by 1.0 and 0.1% SDS, 1% Triton-X100, 70% ethanol, 70% methanol, 1% sodium deoxycholate, pH 11.1 and 3.0. No sensitivity was detected to 1% Tween 80, 1 M MgCl2, 100 mM EDTA, lipase, phospholipase A2, proteinase K, or trypsin at the concentrations tested. Viral activity was reduced by approximately 4 logs following heating to 70℃ for 60 min or 80℃ for 30 min (45).