Although the Helianthus species most closely related to the cultivated sunflower is known to be the widespread Helianthus annuus, the geographic origin(s) of the cultivated sunflower is still unresolved. We have used nine microsatellite loci to investigate the relationships among 12 geographically diverse populations of H. annuus and 11 strains of cultivated sunflowers including both commercial lines and Native American-developed strains from Mexico and the United States. Our analyses of wild populations of H. annuus have revealed high levels of gene flow among populations throughout its geographic range and high levels of genetic diversity in all populations. In contrast, the cultivars exhibit a range of genetic diversity from almost monotypic at all loci examined to diversity indices nearly comparable to wild populations. A global Analysis of Molecular Variance (AMOVA) of all wild and cultivated populations revealed 72% of the variance was accounted for by differences within populations. Conversely, only 10% of the variance was explained by the differences between cultivated and wild H. annuus. There was considerably greater variance among individual cultivated populations than between cultivated and wild populations. This observation suggests multiple origins for the cultivated sunflower. Among the cultivars, only three strains showed strong genetic relatedness to each other: two Native American strains and the Mammoth commercial line. The latter of these was re-introduced to the United States from Russia in the late 19th century. Thus, it appears to have originated from cultivars developed by Native Americans in the east-central United States. Although some relationships were observed between cultivated strains and geographic regions, the exact genetic relationships between specific wild populations and specific cultivated strains were difficult to determine due to the high level of gene flow and genetic diversity in the wild populations.

Key words: domestication, gene flow, Helianthus annuus, microsatellite, sunflower