Evolution of Viruses - Zhdanov, V. M. 1990

Special Section
Baculoviruses

The extensive group of baculoviruses (family Baculoviridae) includes insect nuclear polyhedrosis and granulosis Viruses. While biologically distinct from other viruses, this group is not ecologically isolated. Although the majority of viruses have been discovered in insects, similar agents have been found in mites, spiders, crustaceans, and even Fungi. Furthermore, despite the similarities among members of this conditionally designated family, significant differences exist between subgroups (genera), not to mention variations in Genome Size [Matthews R., 1982].

The Genome is represented by a double-stranded circular supercoiled DNA with a Molecular Weight of 58×10⁶—100×10⁶, accounting for 8—15% of the virion mass. The G+C content ranges broadly from 28% to 59%. Physical restriction maps have been obtained that indicate genomic similarities among several studied viruses [Smith G. et al., 1979; Vlak J., Smith G., 1982].

Up to 15—25 Proteins have been detected in virions, including Enzymes with molecular weights ranging from 10,000 to 160,000. At least 8 proteins have been identified in nucleocapsids, and no fewer than 5 in the outer envelopes [Tweeten К. et al., 1980]. The major nucleocapsid protein VP12 is similar in polyhedrosis and granulosis viruses and forms a DNA-protein complex comparable to Histones [Tweeten К. et al., 1980]. A highly characteristic protein is polyhedrin or its corresponding granulin, with a molecular weight of 25,000—33,000, which serves as the primary component of inclusion bodies. Three additional proteins participate in inclusion formation, and virions contain a protease capable of degrading polyhedrin [Paune C., Ralmakoff J., 1978]. Antigenic relationships exist among many members of the family [Summers М. et al., 1980; Smith G., Summers M., 1981].

Virions consist of one or more rod-shaped nucleocapsids enclosed within a common envelope. Nucleocapsid dimensions range within 40—60×200—400 nm. Baculoviruses have been described in whose particles the DNA content varies widely [Krell Р., Stoltz D., 1979], and particles with linear DNA have also been reported [Odindo М. et al., 1986]. Possibly, these instances involve defective virions or viruses outside the group under consideration. Viruses replicate in the nuclei (subgroups A and C), while some replicate partially in the Cytoplasm (subgroup B). Virion occlusion, so characteristic of viruses in this group, occurs at the terminal stage, though it may be absent (subgroups C and D). Polydisperse DNA may also indicate the multiparte nature of certain baculoviruses.

The sole currently recognized genus, Baculovirus, is divided into 4 subgroups, a Classification that must be considered provisional, as certain differences between subgroups could warrant their elevation to separate genera or even families. Subgroup A comprises nuclear polyhedrosis viruses, where a single envelope may enclose several nucleocapsids (MNPV — multiple nucleocapsid particle virions) or a single nucleocapsid (SNPV — single, etc.). The polyhedrosis viruses of Autographa californica and Bombyx mori represent these two subtypes, infecting insects and crustaceans (encompassing over 10 viruses). Subgroup B includes granulosis viruses. Their segregation is relatively provisional, since the major proteins (polyhedrin and granulin) share common antigenic determinants. Over 50 viruses of this subgroup have been described in lepidopterans. Subgroup C encompasses viruses possessing enveloped nucleocapsids. Subgroup D includes viruses with a polydisperse genome, with type D1 represented by virions bearing two outer envelopes, and type D2 by multiple nucleocapsids enclosed within a single envelope.

There is probably no need to reiterate the general considerations expressed in the description of Iridoviruses. We merely note that morphological primacy can hardly be decisive for analyzing this rather variable family, in which type D2 approaches the polydnavirus family. Nevertheless, attempts have been made within this group to trace the evolution of individual members. To this end, several parameters were compared among multiple Representatives of the virus group in question: the Amino Acid Composition and sequence of polyhedrin, individual codon usage frequencies, degrees of Homology, and the hydrophilicity and Structure/106.html">Hydrophobicity of polypeptide chains.

Based on these data, a Phylogenetic Tree was constructed for 10 viruses infecting lepidopterans, dipterans, and hymenopterans (Fig. 55). The presented data indicate the possible existence of a common ancestor or a few common ancestors (at least within the baculovirus group). If their emergence is dated to the heyday of insects, it could point to the Devonian period (300—400 million years ago). In reality, however, their appearance should be dated to much later periods—perhaps even tens of millions of years ago, when hymenopterans and dipterans close to modern forms emerged. The relatively late emergence of baculoviruses is evidenced by the insufficiently pronounced divergence of polyhedrin and granulin genes, as well as the preservation of antigenic relationships among certain baculoviruses. It should also be borne in mind that the aforementioned insect groups are rapidly progressing, and therefore even co-Evolution of the virus with its "host" could have served as a powerful stimulus for speciation, not to mention the colonization of new ecological niches.

Upon infection of Autographa californica and Trichopulsia ni larvae with baculovirus, a small RNA-containing virus was isolated that resembles the Nudaurelia capensis virus as well as Caliciviruses. The major virion protein has a molecular weight of 67,000 [Morris Т. et al., 1979].

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Fig. 55. Molecular phylogeny of baculovirus occlusion proteins. Numbers indicate the percentage difference in amino acid composition; the dashed line shows The formation of hymenopteran baculoviruses; A, B, C, and D represent divergence points.