Evolution of Viruses - Zhdanov, V. M. 1990

Special Section
Polydnaviruses

A completely unique group is represented by polydnaviruses (Polydnavirus), which develop in the larvae of the tobacco budworm, Heliothis virescens. Their genome consists of at least 28 molecules of double-stranded supercoiled circular DNA, ranging from 6,000 to 21,000 Base Pairs in non-equimolar ratios [Krell P. et al., 1982]. Since none of these fragments hybridized with one another, the total Genome Size is estimated at 190,000 to 240,000 base pairs.

The virus replicates in the epithelial Cells of the wasp's oviduct calyx and is secreted into the calyx fluid, from which it is injected along with the eggs into the host—the moth larva. This reliable mechanism presumably ensures the delivery of all components of such a multipartite virus. Interestingly, in the absence of the virus, the wasp eggs fail to develop and are encapsulated by the host; however, when the virus is present, encapsulation does not occur, and The Development of the host larva is drastically slowed down [Edson K. et al., 1981]. The latter effect is also observed when the virus is injected without the parasite's eggs [Vinson S. et al., 1979].

When complementary DNA was used as a probe, viral mRNA was detected in the larva as early as 2 hours post-infection and persisted throughout the entire period of parasitism [Fleming J. et al., 1983]. Analysis of viral mRNA at various stages of infection identified at least 12 distinct mRNA species. Hybrids of this RNA with certain TYPES OF VIRAL DNA were obtained, and several Proteins with molecular weights ranging from 19,000 to 45,000 were identified via in vitro Translation [Blissard G. et al., 1986].

Studying the distribution of viral DNA across various wasp Tissues using dot-blot Hybridization revealed DNA in the HEAD and thoracic tissues of both sexes, as well as in the abdominal tissues of males. Furthermore, the viral DNA (specifically its supercoiled fragments B and Q) was found in both free and integrated states [Fleming J., Summer M., 1986].

Naturally, there is currently no sufficient basis to speculate definitively on the ORIGIN AND EVOLUTION of this remarkable virus. By describing it, we merely intended to emphasize how natural Selection could facilitate the survival of such a multipartite virus within this triple host-parasite-virus system. The aggregation of particles comprising the complete virus in the wasp's oviducts, combined with their injection into the parasite's host along with the egg, largely guarantees the Relay transmission of the virus to the next Generation of the parasitic wasp. An additional guarantee is provided by the Integration of the virus into the wasp Chromosomes, which ensures vertical transmission. Evidently, this dual safeguard has made possible the existence of a virus that parasitizes a parasite while simultaneously altering the PHYSIOLOGY OF THE parasite's host.