Lh. Rieseberg et al., Introgression between the cultivated sunflower and a sympatric wild relative, Helianthus petiolaris (Asteraceae), INT J PL SC, 160(1), 1999, pp. 102-108
High rates of hybridization and introgression have been reported between th
e cultivated sunflower and its wild progenitor (both Helianthus annuus), ra
ising concerns that neutral or favorable transgenes might escape and persis
t in wild H. annuus populations. However, little consideration has been giv
en to the possibility that other wild sunflower species may hybridize with
the cultivated sunflower. Here, we estimate the frequency of hybridization
and introgression between the cultivated sunflower and Helianthus petiolari
s, a closely related wild species whose range almost completely overlaps th
at of the cultivated sunflower in the central and northern plains and which
hybridizes extensively with wild H. annuus. Assays of 21 cultivar-specific
AFLP (amplified fragment length polymorphism) markers in 159 individuals f
rom four sympatric populations of H. petiolaris revealed low rates of intro
gression, with average marker frequencies per population ranging from 0.006
to 0.026. Twenty-seven individuals had a hybrid ancestry, and all hybrids
appeared to represent later-generation backcrosses. There was significant h
eterogeneity in rates of introgression for different markers, and eight of
the 21 markers failed to introgress into any of the four populations. This
and other evidence indicate that the H. petiolaris genome may be differenti
ally permeable to introgression. These data indicate that (1) beneficial an
d neutral transgenes will take much longer to move into H. petiolaris popul
ations than into wild H. annuus; (2) the risk of escape may depend on the g
enomic location of the transgene; and (3) escape is likely to be sporadic,
occurring in some populations and not others and at different times. Thus,
risk assessment in wild H. annuus is of more immediate concern than in H. p
etiolaris.