Structure and temporal dynamics of populations within wheat streak mosaic virus isolates

Variation within the Type and Sidney 81 strains of wheat streak mosaic viru s was assessed by single-strand conformation polymorphism (SSCP) analysis a nd confirmed by nucleotide sequencing. Limiting-dilution subisolates (LDSIs ) of each strain were evaluated for polymorphism in the P1, P3, NIa, and CP cistrons. Different SSCP patterns among LDSI of a strain were associated w ith single-nucleotide substitutions. Sidney 81 LDSI-S10 was used as foundin g inoculum to establish three lineages each in wheat, corn, and barley. The P1, HC-Pro, P3, CI, NIa NIb and CP cistrons of LDSI-S10 and each lineage a t passages 1, 3, 6, and 9 were evaluated for polymorphism. By passage 9, ea ch lineage differed in consensus sequence from LDSI-S10. The majority of su bstitutions occurred within NIa and CP, although at least one change occurr ed in each cistron except HC-Pro and P3. Most consensus sequence changes am ong lineages were independent, with substitutions accumulating over time. H owever, LDSI-S10 bore a variant nucleotide (G(6016)) in NIa that was restor ed to A(6016) in eight of nine lineages by passage 6. This near-global reve rsion is most easily explained by selection. Examination of nonconsensus va riation revealed a pool of unique substitutions (singletons) that remained constant in frequency during passage, regardless of the host species examin ed. These results suggest that mutations arising by viral polymerase error are generated at a constant rate but that most newly generated mutants are sequestered in virions and do not serve as replication templates. Thus, a s ubstantial fraction of variation generated is static and has yet to be test ed for relative fitness. In contrast, nonsingleton variation increased upon passage, suggesting that some mutants do serve as replication templates an d may become established in a population. Replicated mutants may or may not rise to prominence to become the consensus sequence in a lineage, with the fate of any particular mutant subject to selection and stochastic processe s such as genetic drift and population growth factors.