Basic Information - Genetics, Genomics and Breeding of Crop Plants - page 14

monomorphism in the cultivated lines for both isozyme and cpDNA
phenotypes further suggest a single origin of the domesticated sunflower
from a very limited gene pool. In addition, Native American varieties of the
domesticated sunflower were genetically more variable than other cultivated
lines, possibly indicating that they gave rise to the other cultivated stocks.
Molecular evidence did not, however, allow conclusions as to the exact
geographic origin of the domesticated sunflower (Rieseberg and Seiler 1990).
Arias and Rieseberg (1995) used randomly amplified polymorphic DNA
(RAPD) loci to investigate the origin and genetic relationships of
domesticated sunflower and its wild relatives. RAPD data supported the
origin of the domesticated sunflower from wild H. annuus ; however, because
of the high level of identity between the two species, little information was
provided regarding the geographic origin of the domesticated sunflower.
Cronn et al. (1997), using allozyme variation, concluded that domesticated
sunflowers form a genetically coherent group and that wild sunflowers
from the Great Plains may include the most likely progenitor of domesticated
sunflower. Systematic data does support the southwestern USA as a site of
origin for annual sunflowers, including H. annuus (Heiser et al. 1969;
Rieseberg et al. 1991).
Quantitative trait loci (QTL) controlling phenotypic differences between
cultivated sunflower and its wild progenitor were investigated by Burke
et al. (2002b). They concluded on the basis of the directionality of QTLs that
strong directional selection for increased achene size appears to have played
a central role in sunflower domestication. None of the other traits showed
similar evidence of selection. The occurrence of the numerous wild alleles
with cultivated-like effects, combined with the lack of major QTLs, suggest
that sunflower was readily domesticated.

1.3.2 Dispersion

The original geographic range of wild H. annuus or the identity of the form
from which H. annuus is derived is not clear (Asch 1993). Asch (1993) suggests
that H . annuus originated as a colonizer of natural disturbances, and that
bison ( Bison bison Skinner and Kaiser) created extensively disturbed habitats
suitable for colonization by sunflower. Bison may also have served as a
dispersal agent for sunflower, by transporting sunflower achenes trapped
in matted hair. He also suggested that pre-domestication events generated a
wide distribution for wild H. annuus throughout the Midwest prior to the
arrival of mankind on the scene, and that it was this midwestern form of H.
annuus that actually gave rise to the domesticated sunflower. Unfortunately,
no archaeological evidence exists to either refute or support such hypotheses
and archaeological records tell us nothing about the pre-human geographic
distribution of wild H. annuus .