Kenneth M. Smith

The Arthropod Viruses

Publisher Summary This chapter discusses the arthropod viruses. The virus diseases of insects have been known for many years, but it is only during the last decade that intensive research has been carried on. The invention of the electron microscope enlarged the scope of virus research in general and enabled several noninclusion diseases, comparable to the virus diseases of plants and the higher animals, to be discovered in the arthropods. Cross-transmission experiments have shown that virus diseases can be induced artificially in insect orders in which no naturally occurring virus diseases have yet been found. The new methods of “staining” for electron microscopy, particularly the negative staining with phosphotungstic acid (PTA), have added greatly to the morphology and ultrastructure of viruses. The method of replication of the insect viruses is more in line with the process of assembly characteristic of viruses from other groups rather than the “life cycle” with developmental stages. The methods of isolation and purification have improved and already two or three of the cytoplasmic viruses, affecting insects, have been obtained in crystalline form. Some attempt has been made to apply the binomial system to the insect viruses and a few genera have been erected, such as Borrelinu, Bergoldia, and Smithia . The real difficulty in developing a classification of the viruses attacking the arthropods is the lack of knowledge concerning the fundamental properties of the viruses. Since morphology of the particles seems unlikely to be of much help, many of the viruses from the cytoplasmic polyhedroses appear identical under the electron microscope.

Morphology and development of insect viruses.

Publisher Summary From the intensive study of insect viruses by several workers during the past decade, it is becoming increasingly clear that insects, like plants and the higher animals, are susceptible to a multiplicity of viruses, of which the polyhedral viruses are only a part. Before the advent of the electron microscope, there existed much confusion of thought regarding the polyhedral diseases. The actual polyhedra were variously considered to be organisms, to contain the virus, and to be crystalline aggregates of the virus itself. It was shown by Komarek and Breindl with the optical microscope and by Bergold with the electron microscope that the second theory was the correct one and that the virus particles were contained within the polyhedral crystals. Weak alkalis dissolve the polyhedra, leaving the virus particles behind inside the membrane that enclosed the crystal. This procedure is, in effect, a microdissection technique but the pH has to be carefully adjusted, otherwise the virus particles, which are themselves susceptible to the effects of alkalis, would also be dissolved. This fact is shown by the difficulty experienced in dissolving the cytoplasmic polyhedra, which have no protective membrane, without also dissolving the virus particles. The polyhedral bodies from the different types of polyhedroses appear to be genuine crystals. They are not, however, nucleoprotein and the arrangement of the virus particles inside the crystal is not regular but haphazard according to the manner, in which they are drawn into the crystal at the time of its formation. These facts differentiate the polyhedra sharply from the plant virus crystals that are nucleoprotein and are composed only of the virus particles themselves. With the knowledge of the insect virus diseases at present, which is admittedly scanty, it can be put in four arbitrary groups as follows; Group I: The polyhedral virus diseases. These are subdivided into (a) nuclear polyhedroses, and (b) cytoplasmic polyhedroses. Group II: The granuloses or capsular virus diseases. Group III: Viruses without intracellular inclusion. Group IV: A miscellaneous collection of apparent viruses that require further study.