Evolution of Viruses - Zhdanov V. M. 1990

Special Section
Iridoviruses

Iridoviruses form a distinct group of Viruses that infect a wide range of animals, including insects, marine invertebrates, fish, and amphibians [Matthews R., 1982]. The only similar virus known to date, the African swine fever virus [Ortin J. et al., 1979], has been classified into its own independent family [Brown F., 1986]. Iridoviruses are among the largest Introduction/6.html">DNA-containing viruses. Their genome—a linear double-stranded DNA molecule, or possibly two—has a Molecular Weight of 100x10⁶—250x10⁶, which accounts for 12—30% of the virion mass. The virion is an icosahedron consisting of 180 copies of a capsid protein with a molecular weight of 29,000. A protein is covalently attached to its 5'-end, and there is no poly(A) sequence at the 3'-end. During reproduction, RNA with a molecular weight of 1.4x10⁶ encodes the synthesis of 4 Proteins: two related proteins with molecular weights of 105,000 and 75,000, a capsid protein (29,000), and a protein with a molecular weight of 14,000 [Salermo-Rife Т. et al., 1980]. Virions contain 13—25 structural proteins with molecular weights ranging from 10,000 to 25,000, including protein kinase and other virion-associated Enzymes.

Morphologically, iridoviruses are icosahedrons with a diameter of 125—200 nm, whose Structure also incorporates Lipids. However, this is not a lipid bilayer envelope, and iridoviruses are classified as non-enveloped viruses, representing, along with Tailed Phages, the largest members of this group. Their Replication mechanism is complex. Virions enter Cells via endocytosis; uncoating of the nucleoprotein (release from the lipid-protein shell, The Mechanism of which remains unclear) takes place within endocytic vacuoles. METABOLISM/31.html">Transcription and Replication of viral DNA require intact Cell nuclei; however, a significant portion of DNA replication occurs with the participation of virus-induced enzymes localized in the Cytoplasm, where paracrystalline clusters of virions are also found. The release of virions from The Cell occurs via cytolysis or budding, in which case they may acquire an envelope derived from The cell membrane, although its presence or absence does not affect infectivity.

Studies on iridoviruses (frog virus V-3) have demonstrated that virus-induced syntheses initially take place in the nuclei, where the viral genome enters and ultra-early transcription of some genes occurs with the help of cellular polymerase II. Subsequently, the viral genome moves into the cytoplasm, where transcription of late genes is driven by a newly synthesized virus-specific transcriptase that is absent in virions [Willis D. et al., 1984].

The family is divided into 4 genera. Viruses of two genera infect insects and differ in virion size. The genus Iridovirus infects Tipula larvae and other insects, possessing a fairly wide host range. The genus comprises over 30 viruses, some of which share immunological relationships. In addition, Chiromonas plumosus and Octopus vulgaris viruses are tentatively assigned to this genus. The virions have a diameter of 120 nm and contain lipids, yet infectivity is not lost following ether Treatment. Infected larvae and purified virus exhibit a blue glow (iridescence). The genus Chloridovirus encompasses 20 viruses (with the mosquito iridescence virus as the type species) that cause a yellow-green glow (iridescence). Virions have a diameter of 180 nm, and correspondingly, their Genome Size is approximately twice as large as that of the members of the first genus.

The genus Ranavirus consists of more than 40 viruses that infect amphibians (frogs). In their natural hosts—adult Rana pipiens frogs—they do not cause disease, but they are fatal to tadpoles of this and other frog species. In addition to amphibian Tissues, these viruses replicate in fish, bird, and mammalian cell cultures at relatively low temperatures (12—32°C). The single genus Lymphocystisvirus comprises the fish disease agents of the same name.

The African swine fever virus has been classified into a separate family, although it shares certain characteristics with iridoviruses. The virions contain a DNA-dependent RNA polymerase, yet the virus replicates only in cells with undamaged nuclei. In addition to porcine cells, it can replicate in cells of other warm-blooded animal species.

The scant data on iridoviruses and the African swine fever virus are clearly insufficient for any well-founded Conclusions regarding their evolution, let alone their potential origin.

Iridoviruses are characterized by a complex mechanism of virus-induced syntheses (ultra-early, early, and late transcription) involving both the Cell Nucleus and virus-induced nucleic acid synthesis enzymes. Although this information is important in itself to support the hypothesis of a common origin for all the virus groups under consideration, it is not conclusive, since the ecological niches of each group are biologically isolated (insects, amphibians, fish, mammals), and even within the same or closely related ecological niches (two genera of insect iridoviruses), there are clear generic differences in genome size. However, the latter should not be absolutized, as similar properties are observed in other groups of related viruses (e.g., papillomaviruses and polyomaviruses, whose common origin is unquestionable) and even within the same taxonomic group (e.g., in Herpesviruses or individual genera thereof).

It is of interest to determine whether the 4 genera of iridoviruses and the African swine fever virus are currently existing groups with extinct connecting links, or whether we simply have yet to discover many representatives of this evolutionary Lineage. Hopefully, this gap will be filled in the coming years.