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given f ield (sympatry) or a cross f ields t hrough l ong di stance di spersal of pol len can be
transmitted either through pollens or seeds from crops to wild relatives.
1.2.1 Seed dispersal
Seed banks of transgenic crops with dormant seeds can build up na turally in the soil. Seed
loss at harvest, shallow cultivation and ploughing timing have been identified as key factors to
prevent incorporation of seeds into the seedbank (Begg et al. 2006). Oilseed rape (B. napus)
possesses c haracteristics t hat f avour t he f ormation a nd p ersistence of a seed ba nk, i .e. pod
shattering and inducible secondary dormancy. Volunteer recruitment from oilseed rape field
was observed, suggesting seeds could be dormant in tilled as well as no-tilled fields (Simard
et a l. 2002) . B eside t he actual pr oduction a rea o f t ransgenic c rops a nd t he l ocations w here
crops gr own, hum ans c an a lso be a ve ctor f or unw anted gene flow dur ing t he t ransport,
processing, and exchange of transgenic seeds. Several GM oilseed rape plants were detected
at major ports and along roadsides in Japan, and these plants probably resulted from imported
GM oilseed rape seeds because these seeds was not commercially cultivated (Saji et al. 2005;
Aono et al. 2006). S imilarly, in Korea, a feral plant of GM maize (Zea mays) was detected
along a roadside near a Korean seaport (Kim et al. 2006). Volunteers arise from the seed bank
in subsequent years, which can serve as a source or bridge for pollen flow.
1.2.2 Pollen dispersal
Oilseed rape is self-fertile, with pollen movement by both wind and insects (Williams et al.
1987). Interplant outcrossing rates range from 12 to 47% (Becker et al. 1992; Lavigne et al.
1998). Pollen-mediated gene flow in oilseed rape is affected by a variety of factors, including
flowering t ime, ge notypes, w ind s peed a nd di rection, di stance be tween donor a nd r ecipient
populations. Pollens of oilseed rape were observed 1.5km from source field, and they were
sufficient in number (22 pollens grains m3) to allow seed set (Timmoms et al. 1995). Gene
flow for transgenic oilseed rape could occur at several kilometers from pollen resource (Table
1.1; Rieger et al. 2002; Cai et al. 2008). However, approximately half of the pollen produced
by an individual plant fell within 3 m , and dispersal from whole plots instead of individual
plants would have underestimated the proportion of pollen (Lavigne et al. 1998).
However, the majority of GM crossing fertilization occurs less than 10m (Husken and
Dietz-Pfeilstetter 2007), with a decline over 50m. Generally, the degree of gene flow between
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