Genotypic Diversity of Beet Curly Top Virus Populations in the Western United States

The genotypic diversity of beet curly top virus (BCTV) present in the western United States has been examined by the analysis of 58 field isolates and eight laboratory or nursery isolates of the virus. Full-length clones for each isolate have been characterized for genotype by restriction endonuclease mapping. The results indicate that most of the genotypes examined may be classified as variants of the CFH, Worland, or Cal/Logan strains of BCTV. Two genotypes were recovered that appear to share certain genotypic markers of both Worland and CFH strains. Genotypic variants of the CFH and Worland strains and the two genotypes sharing markers of both strains were recovered from field isolates collected during 1994 and 1995. In contrast, the Cal/Logan strain was recovered only from isolates maintained in laboratories or nurseries. Comparisons of restriction endonuclease maps of cloned BCTV genomes revealed considerable variability both within and between strains. Although a total of 43 distinct genotypes of BCTV were identified, only 36 (84%) were recovered from field isolates. Of 37 field isolates for which more than a single clone was recovered, 16 (43%) contained more than a single genotype of one strain, whereas 4 (11%) harbored mixed infections of the CFH and Worland strains. A phylogenetic analysis using 43 characters derived from restriction endonuclease mapping data supported the grouping of 41 genotypes into three taxa consistent with the three currently recognized strains of BCTV. The relationships of the two genotypes sharing genotypic markers of both the Worland and CFH strains to other BCTV genotypes was unresolved in the phylogenetic analysis. Based on the mild symptom phenotype of the isolates from which these two genotypes were recovered and the presence of Worland genotypic markers in portions of the genome containing both cis- and trans-acting elements determining replication specificity, these two genotypes were tentatively considered as variants of the Worland strain.