Quasispecies nature of three maize streak virus isolates obtained through different modes of selection from a population used to assess response to infection of maize cultivars

Three maize streak virus (MSV) isolates were derived from an MSV population used to assess the response to infection of maize cultivars, Isolate SP1 w as obtained from this population through short acquisition and inoculation periods (1 and 5 min, respectively), using a single Cicadulina mbila vector . Isolate SF2 was derived from SP1 after transmission to a wild perennial h ost (Coix lacryma-jobi), on which it was maintained for about 4 years witho ut insect transmission. Isolate N2A, the most pathogenic isolate, was obtai ned from the initial population after serial passages on almost completely resistant inbred maize lines. The complexity of each isolate was analysed b y RFLP analysis and sequencing based on 120 SP1 clones, 36 SF2 clones and 4 0 N2A clones. All three isolates were composed of different but related clo nes, consistent with a quasispecies structure. The mutations were distribut ed throughout the genome. Mutation frequencies, based on all available sequ ences, were 3.8 x 10(-4) for SP1, 10.5 x 10(-4) for SP2 and 6.9 x 10(-4) fo r N2A. AS expected from the bottleneck selection step, the intra-isolate va riability of SP1 was relatively low. Comparison between SP1 and SP2 showed that SP1 heterogeneity increased during maintenance on the wild host. Furth ermore, the consensus sequences of SP1 and SP2 differed by two non-synonymo us 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 mast er sequence.