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Generation and maintenance of genetic variation within wheat streak mosaic virus populations
Diversity within wheat streak mosaic virus (WSMV) field populations can be as much as the variability found between different geographical regions. To understand how WSMV diversity originates and is maintained, I coinoculated wheat plants with several isolates of WSMV. I showed that there are at least three mechanisms promoting genetic isolation of WSMV genotypes within infected plants. Cross-protection reliably prevents infection of a closely related challenge virus but to lesser degrees with more divergent genotypes. Isolates are spatially separated within plants as shown by populations in lateral tillers and within individual leaves. Transmission by the natural vector also imposes a bottleneck whereby mixed infections are resolved. Thus WSMV genotypes become genetically isolated from other populations, fostering their independent divergence. ^ Viable populations within isolates were examined by infecting plants with limiting dilutions of inocula. Limiting dilution sub-isolates (LDSI) were screened for genetic changes in four genes by single-strand conformation polymorphism analysis and changes were verified by nucleotide sequencing. The LDSI haplotype diversity was low, consistent with linear population growth. Population genetic analysis showed that the effective population sizes also were quite small and that very few progeny genomes contribute to further rounds of replication.^ Genetic changes were monitored in a founding genome (an LDSI) serially passaged in different host species. While no host specific selection was uncovered, changes accumulated in all genomes over nine passages. Diversity within populations also increased with several layers of mutations: older, more frequent alleles, and newer alleles that occurred as singletons. Stochastic forces such as genetic drift were shown to play significant roles in the evolution of WSMV. These results provide compelling evidence that plant virus populations do not conform to a quasispecies model. Thus, animal virus models of populations and virus evolution are not appropriate for studies of plant virus populations and evolution. The concepts presented here provide a new and more accurate basis for understanding how plant virus populations diverge, evolve, and maintain their diversity. ^
Biology, Genetics|Biology, Microbiology|Agriculture, Plant Pathology
Hall, Jeffrey Scott, "Generation and maintenance of genetic variation within wheat streak mosaic virus populations" (2001). ETD collection for University of Nebraska - Lincoln. AAI3009720.