Seed size variation and predation of seeds produced by wild and crop-wild sunflowers

The movement of pollen between crop and wild sunflowers (both Helianthus an nuus) hits led to concerns about the possible introduction of crop transgen es into wild population, The persistence of crop traits in wild populations will depend in part on the relative fitness of crop-wild hybrid vs. wild p lants. Using seeds from two large experimental field plots, we found that s eeds produced by crop-wild plants were twice the size of wild seeds and dif fered in coloration. Head diameter, date of flowering. identity of mother p lant, and levels of predispersal predation explained some variation in mean seed size. We hypothesized that postdispersal vertebrate seed predation wo uld be affected by seed size, with hybrid seeds preferentially eaten. In ea ch of three field trials, significantly more hybrid seeds were eaten (61% o f hybrid seed; 42% of wild seed). Within the category of wild seeds, larger seeds were preferentially eaten; however among hybrid seeds, predation was not significantly related to seed size. In this study, differential predat ion thus reduces hybrid fitness and would presumably slow the spread of tra nsgenes into wild populations.