Historical Background |
Maize streak disease was first described - as "mealie variegation" - in the Natal Province of South Africa at the turn of the century (Fuller, 1901).
The disease occurs only in Africa and adjacent Indian Ocean islands, where it is one of the worst occurring in maize. The causal agent was discovered to be a virus by Storey (1924, 1932a), who termed it maize streak virus (MSV). The virus was found to be obligately transmitted by the leafhopper Cicadulina mbila (Naude) (Homoptera: Cicadellidae) (Storey, 1924, 1925a). Subsequently, several other Cicadulina species were reported as being able to transmit MSV (see review by Rose, 1978; Dabrowski, 1987). In 1978, MSV was designated the type virus of the newly described group taxon Geminivirus (Harrison et al., 1977; Matthews, 1979).
Early studies indicated that there were several distinctly different African streak viruses adapted to different host ranges (Storey & McClean, 1930; McClean, 1947). These studies were based on the transmission of virus isolates between different host species and symptomatology.
In a subsequent study of streak virus transmission between maize, sugarcane, and Panicum maximum, the relatively new technique of immunodiffusion was employed, using antiserum to the maize isolate. From the results it was concluded that the maize, sugarcane, and Panicum isolates were strains of the same virus, MSV (Bock et al., 1974). The maize isolate was given as the type strain. The virus was only properly physically characterised in 1974 (Bock et al., 1974), and only found to be a single-stranded circular DNA virus in 1977 (Harrison et al., 1977).
The first isolates of MSV were sequenced in 1984 (Kenya, Howell, 1984; Nigeria, Mullineaux et al., 1984), and the virus was found to have a single component of single-stranded circular DNA (sscDNA), and to be about 2700 bases in size. The two isolates were about 98% identical in sequence. A major advance in the field of MSVirology occurred in 1987, when Grimsley et al. (1987) showed that a tandem dimer clone of MSV-Nig in an Agrobacterium tumefaciens Ti plasmid-derived cloning vector, was infectious when the bacterium was injected into maize seedlings. Subsequently, Lazarowitz (1988) obtained the sequence of an infectious clone of a South African isolate (from Potchefstroom) - MSV-SA - and showed that it also shared about 98% identity with the first two sequences.
Since the early days other transmission tests (Pinner et al., 1988) and more sophisticated serological assays (Pinner et al., 1988; Dekker et al., 1988; Pinner & Markham, 1990) were performed on a wide range of streak isolates from different hosts and locales, and it was claimed that all forms of streak disease in the Gramineae in Africa were caused by strains of the same virus, MSV (Damsteegt, 1983; Pinner et al., 1988; Dekker et al., 1988; Pinner & Markham, 1990b). This view changed as more and more viruses were characterised, however, and it became obvious that there were distinctly separate groupings of viruses that constituted different species: these were sugarcane streak viruses (SSV, see Hughes et al., 1993), the panicum streak viruses (PanSV, see Briddon et al., 1992), and the maize streak viruses. Together these viruses constituted an African streak virus group (see Hughes et al., 1992; Rybicki and Hughes, 1990), distinct from an Australasian striate mosaic virus group (Pinner et al., 1992), and other more distantly related viruses (see Mastreviruses and MSV Diversity).
Copyright Ed Rybicki, November 1997, August 1998, February 1999
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