Pollen-Mediated Gene Flow in California Cotton Depends on Pollinator Activity

on the location, the time period, and how measurements are taken. In the 1950s comprehensive studies using visMany cotton (Gossypium hirsutum L.) pollination studies have ual phenotypic traits reported 10% outcrossing in Texas been performed in the southern USA, but no data exist for California. In this study, we measured pollen-mediated gene flow (PGF) in four to47%inTennessee(Simpson,1954;SimpsonandDundirections over 2 yr from herbicide-resistant source plots in upland can, 1956). These studies reported 28% outcrossing in cotton in the California cotton growing region and in a region with Mississippi, but a mean of 2% outcrossing was reported high pollinator activity. In addition, samples were taken from fields in similar locations in Mississippi 20 yr later (Meredith of conventional varieties at varying distances from fields planted with and Bridge, 1973). The authors suggested the differherbicide-resistant varieties to assess PGF under commercial produc- ences were due to a reduction in wooded areas and the tion conditions. A seedling herbicide bioassay confirmed by DNA tests heavy use of pesticides resulting in a decrease in bee was used to measure PGF. PGF was independent of direction from pollinators. Also in Mississippi, Umbeck et al. (1991) the source plot and declined exponentially with increasing distance measured pollen transfer to non-transgenic rows of cotfrom 7.65% at 0.3 m to less than 1% beyond 9 m when there was high ton planted up to 25 m from a 4 ha field of cotton carpollinator activity. In the absence of high pollinator (honeybee, Apis mellifera L.) populations, PGF was less than 1% beyond 1 m. Pollen rying the nptII gene. PGF dropped below 1% at disflow in commercial fields was consistent with the experimental plot tances beyond 7 m, but continued to be detectable at a data, with only 0.04% PGF detected at 1625 m (1 mile). This study distance of 25 m in solid-seeded cotton. Studies sumconfirms that PGF decreases exponentially with distance in cotton marizing data (15 000 samples) in Arizona, Arkansas, grown under California conditions and is low in the absence of polli- Mississippi, and North Carolina showed that PGF denators, although sporadic occurrence of PGF can be detected up to creased exponentially with increasing distance from the 1625 m. pollen source and was below 1% beyond 10 m at all locations, although PGF was detected at 20 m (Kareiva et al., 1994). The same authors observed similar trends