Forced recombination between distinct strains of Maize streak virus.

Recombination between divergent virus genomes is believed to be a major mechanism for generation of novel virus genotypes. We have examined the recombination process in geminiviruses by forcing recombination between two distinct isolates of Maize streak virus (MSV), MSV-Kom and MSV-Set. Heterodimeric agroinfectious constructs containing tandemly cloned mixtures of complete or partial MSV-Set and MSV-Kom genomes were used to simulate a circular dimeric form similar to that which would be expected to occur following a single intermolecular crossing-over event between MSV-Set and MSV-Kom replicative form DNAs at the long intergenic region (LIR)-movement protein gene (MP) interface. We isolated, analysed and biologically characterized many of the recombinant MSV genomes that were generated from the constructs in planta. Apart from having the same simulated breakpoint at the LIR-MP interface, all the genomes examined had a second breakpoint that had been generated through either intramolecular homologous recombination or a replicational release mechanism. The pathogenicities of six predominantly MSV-Kom-like recombinants were tested in maize. While all were capable of producing a symptomatic infection in this host, none was more virulent than MSV-Kom and only two were more virulent than MSV-Set. The two most virulent recombinants were leafhopper transmitted to a range of differentially MSV-resistant maize, wheat and barley genotypes and both were found to have unique biological properties.

The relative infectivities and genomic characterisation of three distinct mastreviruses from South Africa.

The genomic nucleotide sequences of the cloned agroinfectious genomes of three South African mastreviruses obtained from Zea mays, a Setaria sp., and Panicum maximum (designated MSV-Kom, MSV-Set, and PanSV-Kar respectively), were determined. Additionally, their relative infectivities and virulence were analysed in a range of differentially susceptible wheat, maize, and barley genotypes. MSV-Kom produced moderate to severe streak symptoms in all maize genotypes tested, but only moderate to very mild symptoms in the wheat and barley genotypes. MSV-Set infected only the susceptible to tolerant maize genotypes, but was generally more severe in the barley and wheat genotypes than MSV-Kom. PanSV-Kar was incapable of infecting any of the wheat and barley genotypes and only produced very mild symptoms on the three most sensitive maize genotypes. Genomic characteristics in common with related mastreviruses were identified. Phylogenetic analysis indicated that while MSV-Kom was closely related to previously sequenced MSV isolates, MSV-Set and PanSV-Kar represented distinctly novel strains of MSV and PanSV respectively. In the case of MSV-Set, this is the most distantly related MSV strain yet characterised.