CROP/WEED MICROGAMETOPHYTE COMPETITION IN CUCURBITA-PEPO (CUCURBITACEAE)

Prior studies of crop/weed gene flow in Cucurbita pepo have demonstrated that pollinators can deposit mixed pollen loads. The fate of microgametophytes emerging from these pollen mixtures is unknown. In an effort to define the relationship between pollen mixture composition and progeny composition, experimental plants representing a zucchini cultivar and a cross-compatible free-living type were genotyped with an isozyme marker and grown under greenhouse conditions. Weighed mixtures of weed/crop pollen were applied to stigmas of both types in differing ratios. Subsequent fertilizations were tracked by an electrophoretic screen of over 1,600 progeny from 39 fruits. Equal mixtures of weed/crop pollen do not produce a corresponding suite of progeny. Pollen of the pistillate parent is favored, and this advantage increases as the proportion of pollen from the pistillate parent increases. However, increasing the proportion of zucchini pollen does not produce a corresponding increase in fertilization success when the free-living type is used as pistillate parent. These results indicate that microgametophyte competition in both domesticated and free-living Cucurbita gynoecia could be a significant component of gene flow dynamics. The nature of this competition can only be defined by experiments that central numerous variables, both paternal and maternal, that might be involved. The tendency for crop/weed hybrids to occur at different portions of the ovary suggests that interactions involving two factors-microgametophyte growth rate and gynoecium structure-might play a significant role.