In this article we will discuss about the viruses that infect insects, fungi and cyanobacteria.
Insect Viruses:
We know that some of the animal viruses can infect insects. More than 200 different viruses are known to infect various groups of insects. They belong to different viral families of which the three more important are Baculoviridae, Iridoviridae, and Reoviridae.
Baculoviridae are ds-DNA viruses with enveloped cylindrical virions measuring 40-110 nm in breadth and 200-400 nm in length. The viruses of this family produce polyhedral protein inclusions in i he nucleus of the infected cells (nuclear polyhedrosis), or granular inclusion-bodies in the cytoplasm (cytoplasmic granulosis). Nuclear polyhedrosis viruses infect mainly the larvae of Lepidoptera, Hymenoptera and Diptera.
Iridoviridae have non-enveloped, icosahedral capsids with linear ds-DNA genome. The virions contain lipids in addition to protein and measure 130-180 nm in diameter. They mainly infect arthropods, like certain flies and beetles, and some members of Coleoptera, Diptera and Lepidoptera. Crystalline virions accumulate in the fat bodies of larvae of the infected insects producing iridescens.
Reoviridae are ds-RNA viruses with non-enveloped icosahedral virions measuring 45-80 nm in diameter. A representative genus is Cypovirus causing cytoplasmic polyhedrosis in infected insects, mainly the larvae of Lepidoptera.
Some of these insect viruses may cause considerable economic loss when they infect and destroy valuable insects like silkworms or honey-bees. Another economic importance of the insect viruses is that they can be profitably used for developing eco-friendly bio-insecticides.
The specificity of host- virus relationship is a special advantage in their possible use as destructive agents. The polyhedrosis and granulosis viruses — the so-called occluded insect viruses — are specially useful for this purpose. Nuclear polyhedrosis virus has been given official approval for control of boll worm (Heliothis zed) of cotton.
A commercial product, called ‘Elcar’ is available for this purpose. A cytoplasmic polyhedrosis virus has also been used in controlling caterpillar infection (Dendrolimus spectabilis) of pine trees. These viruses can be grown in large scale in the larvae of susceptible insects to reach a high concentration and can be isolated from the larvae for preparing the commercial products.
Viruses Growing in Fungi (Mycoviruses):
Fungal viruses are much less known than animal, plant or bacterial viruses, though several viruses or more precisely virus-like particles have been observed in the higher fungi viz. Basidiomycetes, Ascomycetes and Deuteromycetes.
Many species of Penicillium and Aspergillus are known to contain these agents. In most cases, the virus or virus-like particles do not produce any cytopathic effect on the hosts i.e. the virus is latent. In some hosts, however, the virus infection causes damage to the host, e.g. the infection of the cultivated mushroom, Agaricus bisporus results in loss of yield and mycelial degeneration.
The viruses affecting higher fungi are mostly polyhedral particles measuring 25-50 nm in diameter and they contain double-stranded RNA. However, the fungal viruses do not appear to be taxonomically related to the ds-RNA viruses of other groups of biological organisms. Some myco-viruses are also known to have rod-shaped particles, resembling tobacco mosaic virus.
Much less known are the viruses affecting the lower fungi. But they appear to differ from those of the higher fungi. Some of them have ds-DNA as genetic material in isometric hexagonal capsids measuring 40-200 nm in diameter. Infection by these agents generally leads to destruction of the host cells by lysis.
An interesting aspect that once aroused much enthusiasm in myco-viruses was their possible use as interferon-inducers. Interferon’s are proteins produced by animals including humans and by cultured cells in response to viral infection. These proteins can inhibit viral replication and thus they can be used as potential agents for combating viral diseases.
Apart from naturally occurring viral infections, interferon’s can be induced in cell cultures by artificial agents. The best interferon-inducing agent is double-stranded RNA. The ds-RNA extracted from myco-viruses could be used as an effective interferon inducer. However, interferon’s produced through the myco-viral RNAs were found to be unsuitable for human application, because they had serious side-effects.
Viruses Infecting Cyanobacteria (Cyanophages):
Viruses infecting cyanobacteria (formerly called the blue-green algae or cyanophyceae) are generally known as cyanophages. They resemble the bacteriophages in both structure and in the mode of replication within hosts.
The cyanophages are commonly named after the hosts they infect. Some of the more well-known cyanophages are LPP, N1, SM etc. The LPP group of cyanophages infect Lyngbya, Plectonema and Phormidium. Similarly, N1 was isolated from Nostoc muscorum and the cyanophage SM was first recorded in Synechococcus and Microcystis.
In general, the cyanophages are ds-DNA viruses with a non-enveloped virion having a binary symmetry consisting of an icosahedral hexagonal head and a helical tail which may or may not be contractile. The head may be elongated or isometric. The tail may vary in length or may be absent.
Morphological characteristics of LPP, N1 and SM are summarized in Table 6.5:
So far, as it is known, the infection cycle of cyanophages follows the same sequences as that of ds-DNA bacteriophages. Cyanophage LPP1 is known to infect host cells by injecting its DNA while the empty phage capsid remains outside, just as in E. coli T-phages.
The time taken for completing the life-cycle is, however, much longer in cyanophages varying between 12 to 48 hours. After maturation and assembly, the progeny viruses are released by lysis of the host cells. Like bacteriophages, cyanophages also form plaques on a lawn of the host organism growing on agar plates.