Department of Plant Pathology, P.O. Box 7616, North Carolina State University, Raleigh 27695.
Exploring the genetic diversity and evolutionary history of plant viruses is critical to understanding their ecology and epidemiology. In this study, maximum-likelihood and population genetics-based methods were used to investigate the population structure, genetic diversity, and sources of genetic variation in field isolates of Tomato spotted wilt virus (TSWV) from peanut in North Carolina and Virginia. Selected regions of the nucleocapsid, movement, and RNA-dependent RNA polymerase genes were amplified and sequenced to identify haplotypes and infer genetic relationships between isolates of TSWV with heuristic methods. The haplotype structure of each locus consisted of 1 or 2 predominant haplotypes and >100 haplotypes represented by a single isolate. No specific haplotypes were associated with geographic area, peanut cultivar, or year of isolation. The population was panmictic at the regional level and high levels of genetic diversity were observed among isolates. There was evidence for positive selection on single amino acids in each gene on a background of predominant purifying selection acting upon each locus. The results of compatibility analyses and the persistence of specific gene sequences in isolates collected over three field seasons suggest that recombination was occurring in the population. Estimates of the population mutation rate suggest that mutation has had a significant effect on the shaping of this population and, together with purifying selection, these forces have been the predominant evolutionary forces influencing the TSWV population in peanut in North Carolina and Virginia.