Molecular variation in vector-borne plant viruses: epidemiological significance

Abstract

Patterns of variation are examined in four groups of plant viruses, with special reference to their particle proteins and to changes in vector transmissibility and specificity. In the nepoviruses and potyviruses, non-circulative transmission, by nematodes and aphids respectively, seems dependent on structural features on the surface of the virus particles. The N-terminal part of the particle protein may play the key role in potyviruses. Similarly in the luteoviruses, and possibly in the geminiviruses, specificity of circulative transmission by aphids, whiteflies and leafhoppers is linked to the antigenic specificity of the virus particles. Among naturally occurring isolates of the same virus, variation seems often to be discontinuous, and is predominantly of two sorts. Minor variations, characterized by loss of an epitope or substitutions of a few amino acids, can be associated with loss of transmissibility in luteoviruses and potyviruses, or have no effect. Major variations are associated with differences in vector specificity and seem likely to involve radical genetic changes that have evolved over long periods. The adaptation of virus particle proteins for transmission by vectors probably results in conservation of the genes that encode them, and in greater conservation of some parts of these genes than of others.

Evidence that the 75K readthrough protein of beet necrotic yellow vein virus RNA-2 is essential for transmission by the fungus Polymyxa betae

“…Thus, it is becoming clear that in several virus groups vector transmission is governed by interactions between virus particle proteins and virus-encoded proteins, which are not needed for the integrity of the virus particles. The extensive evidence that coat protein plays an important role in transmission (Harrison & Murant, 1984;Harrison & Robinson, 1988) is therefore only part of the story.…”

Complete nucleotide sequence and host range of South African cassava mosaic virus: further evidence for recombination amongst begomoviruses

“…Three distinct groups of species of cassava-infecting whitefly-transmitted geminiviruses (WTGs, genus Begomovirus) have been distinguished based on their nucleotide sequences (Stanley & Gay, 1983 ;Hong et al, 1993 ;Zhou et al, 1997) and by their reactions with a panel of monoclonal antibodies (Harrison & Robinson, 1988 ;Swanson & Harrison, 1994). Group A, African cassava mosaic virus (ACMV), is found in West Africa, including Burundi, the western parts of Kenya, Chad, Uganda and recently, Cameroon (Fondong et al, 1998).…”

Evidence that DNA-A of a geminivirus associated with severe cassava mosaic disease in Uganda has arisen by interspecific recombination.

“…The probable role of recombination in geminivirus evolution in the short to medium term is discussed. Harrison & Robinson, 1988 ;Swanson & Harrison, 1994). Comparisons of the nucleotide sequences of the complete genomes (DNA-A and DNA-B) of isolates representing two of these categories (Stanley & Gay, 1983 ;Hong et al, 1993), and of part of DNA-A of an isolate in the third category, indicated that the isolates are not closely related and should be regarded as three separate species, which were named African cassava mosaic virus (ACMV), East African cassava mosaic virus (EACMV) and Indian cassava mosaic virus (ICMV) (Hong et al, 1993).…”