Three maize streak virus (MSV) isolates were derived from an MSV population used to assess the response to infection of maize cultivars. Isolate SP1 was obtained from this population through short acquisition and inoculation periods (1 and 5 min, respectively), using a single Cicadulina mbila vector. Isolate SP2 was derived from SP1 after transmission to a wild perennial host (Coix lacryma-jobi), on which it was maintained for about 4 years without insect transmission. Isolate N2A, the most pathogenic isolate, was obtained from the initial population after serial passages on almost completely resistant inbred maize lines. The complexity of each isolate was analysed by RFLP analysis and sequencing based on 120 SP1 clones, 36 SP2 clones and 40 N2A clones. All three isolates were composed of different but related clones, consistent with a quasispecies structure. The mutations were distributed throughout the genome. Mutation frequencies, based on all available sequences, were 3.8 x 10(-4) for SP1, 10.5 x 10(-4) for SP2 and 6.9 x 10(-4) for N2A. As expected from the bottleneck selection step, the intra-isolate variability of SP1 was relatively low. Comparison between SP1 and SP2 showed that SP1 heterogeneity increased during maintenance on the wild host. Furthermore, the consensus sequences of SP1 and SP2 differed by two non-synonymous substitutions in the complementary sense gene repA. N2A had a relatively low degree of heterogeneity, but was composed of several sub-populations. The results reflect the influence of the mode of selection of MSV isolates on their quasispecies organization, i.e. distribution of variants, and master sequence.


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