Isolation of maize from pollen-mediated gene flow by time and distance

Development of improved genetic traits in maize (Zea mays L.) requires robust measures to prevent pollen-mediated gene flow (PNIGF) and assure isolation of new traits, whether these traits are the result of conventional breeding or of modern genetic techniques. Studies were conducted in California and Washington to evaluate the relationship of distance and temporal separation for isolation from PNIGF. Kernel color was used to detect outcrossing from source plots of 0.4 to 1.2 ha in size to receptor plots planted at distances up to 750 m and planting intervals of up to 3 wk from the pollen source. Outcrossing from source to receptor plots was observable to 0.0002% (1 kernel in approximate to 500 000 kernels). Increasing temporal separation reduced the distance required to achieve genetic isolation. Outcrossing was < 0.01% at 500 m when source and receptors flowered at the same time, whereas this level of confinement was achieved at 62 m or less when 2 wk of temporal separation was used. No outcrossing was detected at 750 m and 2 wk of temporal separation. This is the first practical evaluation of time and distance acting together to achieve genetic purity in maize.