Introgression between the cultivated sunflower and a sympatric wild relative, Helianthus petiolaris (Asteraceae)

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.