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 to 47% in Tennessee (Simpson, 1954; Simpson and Dundirections 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 producences 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 polliMississippi, 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 C produces only 4% of the cotton in the in South Africa and Argentina, although greater PGF USA but exports 25% of the American crop and was detected in these locations because of the presence 5% of the seed (California Department of Agriculture, of different pollinators. In a recent study, PGF was 2002; USDA, 2003). In 2003, 73% of cotton in the USA measured on a field scale across Arizona, Mississippi, was transgenic, including almost 40% of cotton in Caliand Texas by means of herbicide-resistance assays and fornia. In the USA, transgenic (biotech) cultivars are not ELISAs for presence of insect-resistance genes from separated from nonbiotech cultivars once the introduced Bacillus thuringiensis (Bt) (Berkey et al., 2003). In Aritrait has been approved by government agencies. Pollenzona, a 55 m nonplanted area was enough to maintain mediated gene flow (PGF) or other sources of adventivarietal purity. In Mississippi, all samples had less than tious presence (e.g., seed contamination or mechanical 0.25% outcrossing, with distances beyond 3.7 m having mixtures) can therefore pose problems for export of cotno significant PGF. In Texas, PGF fell to nonsignificant tonseed into countries where the biotech trait may not levels from controls at distances greater than 14 m to the be approved or deregulated. Cotton is generally conwest and 25 m to the east of the pollen source. As cotton sidered to be a self-pollinating crop, but it is often crosspollen is not effectively carried by wind (McGregor, pollinated and the majority of cultivars are a mixture 1976), it is unlikely that the directional differences were of closely related pure lines. The flowers are visited by due to wind-blown pollen but could be due to the effects honeybees, bumblebees (Bombus spp.), and melissodes of wind on pollinator movement. As the advent of Bt (Melissodes spp.) bees. Studies document that provision cotton has reduced insecticide applications (Carpenter of honeybees can increase both seed and lint yield of and Gianessi, 2001), pollinator activity in fields may have cotton via improved pollination, and outcrossing rates increased in recent years (Betz et al., 2000). are affected by bee activity (McGregor 1976). The authors are not aware of any outcrossing studies Outcrossing rates reported for cotton vary depending for cotton in California, where both the environment and the varieties differ from those in the southeastern A.E. Van Deynze and K.J. Bradford, Seed Biotechnology Center, USA. The California crop is approximately one-third Pima and F.J. Sundstrom, California Crop Improvement Association, One (G. barbadense L./G. hirsutum) and two-thirds Acala Shields Ave., Univ. of California, Davis, CA 95616. This research was (G. hirsutum, upland) cottons, with 39% of the upland funded by a grant from the California Crop Improvement Association cotton being transgenic (primarily herbicide resistant) and by Bayer Crop Sciences, 2 Alexander Dr., P.O. Box 12014, Research Triangle Park, NC 27709. Received 29 July 2004. *Correspond(USDA 2003). Field inspectors for the California Crop ing author ([email protected]). Improvement Association have observed Acala/Pima cotPublished in Crop Sci. 45:1565–1570 (2005). Forage & Grazing Lands Abbreviations: Bt, Bacillus thuringiensis; ELISA, enzyme-linked immunosorbent assay; GUS, -glucuronidase; nptII, neomycin phosphodoi:10.2135/cropsci2004.0463 © Crop Science Society of America transferase II; PCR, polymerase chain reaction; PGF, pollen-mediated gene flow. 677 S. Segoe Rd., Madison, WI 53711 USA 1565 Published online June 24, 2005