Basic Information - Genetics, Genomics and Breeding of Crop Plants - page 42

year. Burke et al. (2002a) indicate that the opportunity for crop-wild
hybridization exists throughout the range of sunflower cultivation where
approximately two-thirds of all cultivated fields surveyed occurred in close
proximity to, and flowered coincidentally with, common sunflower
populations. In these populations, the phenological overlap was extensive,
with 52-96% of all wilds flowering coincidentally with the adjacent cultivar
field. These findings indicate that crop-wild hybridization is likely in all
areas where sunflowers are cultivated in the USA.
Crop-to-wild gene flow with species other than H . annuus is far less
likely due to infertility barriers and non-overlapping ranges. Although
transgenes will often escape from cultivation, their rate of spread will be
mainly governed by their fitness effects, not the migration rate. Thus, only
highly advantageous transgenes will spread rapidly enough to have
substantial ecological impact. Therefore, research on the risks associated
with transgene escape should focus on the fitness effects of the genes in
question and consequences of “escaped” traits. Snow et al. (2003) concluded
that wild sunflower plants containing a Bt-toxin gene ( cry1Ac ) specific to
lepidopterans exhibited decreased lepidopteran herbivory and produced
on average 55% more achenes than non-transgenic controls. Burke and
Rieseberg (2003) examined the fitness effects of a transgene, an oxalate
oxidase ( OXO ) gene, conferring resistance to Sclerotinia white mold. They
concluded that the OXO transgene will do little more than diffuse neutrally
after its escape.
Wild-crop hybridization has the potential to influence the evolutionary
ecology of related wild/weedy taxa such as sunflower, but little is known
about the persistence or ecological effects of crop genes that enter wild
populations via pollen movement. Snow et al. (1998) studied F 1 wild-crop
hybrids of sunflower and observed that F 1 wild-crop hybrids had lower
fitness than wild genotypes, especially when grown under favorable crop
conditions, but the F 1 barrier to the introgression of crop genes is quite
permeable.
High rates of hybridization and introgression have been reported
between the cultivated sunflower and its wild progenitor, H . annuus .
However, little consideration has been given to the possibility that other
wild sunflower species may hybridize with cultivated sunflower. A closely
related wild progenitor, H . petiolaris, was studied by Rieseberg et al. (1999)
using selectable amplified fragment length polymorphism (AFLP) markers.
They examined four sympatric populations of H . petiolaris and found a low
rate of introgression ranging from 0.006 to 0.026, indicating that the H.
petiolaris genome is differentially permeable to introgression and that escape
is likely to be sporadic, occurring in some populations and not others and at
different times. Thus, the risk assessment of wild H . annuus is of more
immediate concern than that of H . petiolaris .