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Backcross versus Forward Breeding in the Development of Transgenic Maize Hybrids: Theory and Practice

GeneMax Services, 85410 Dudley, Chapel Hill, NC 27517

^* Corresponding author (rita.mumm{at}genemaxservices.com).

Key factors affecting the choice of breeding methods employed in the development of transgenic maize (Zea mays L.) hybrids are identified and evaluated, particularly as these pertain to the use and balance of backcross and forward breeding in the overall design of a breeding program. These factors are type of trait, population(s) under selection, the predicted response to selection, stewardship of transgenic events, costs, and risks. Analysis suggested that simultaneous hybrid improvement and event integration via a forward breeding approach is not practical, given that some level of backcrossing is basic to event integration and the need to contain events not fully authorized by the local government for cultivation, food, and feed. Considering event integration conducted in a stream separate from hybrid improvement, a forward breeding component to event integration may be warranted, particularly if a threshold level of expression of the transgenic trait of interest is dependent on endogenous alleles that interact with the event, some of which may be influenced by the environment. For this situation, directives are given for determining the number of generations of backcrossing to be conducted to create partial conversions from which to select for favorable endogenous alleles via forward breeding. In addition, ways to maximize the advantages of a backcross-only approach to event integration are discussed.

Received for publication April 7, 2007.