OCCASIONAL LOSS OF EXPRESSION OF PHOSPHINOTHRICIN TOLERANCE IN SEXUALOFFSPRING OF TRANSGENIC OILSEED RAPE (BRASSICA-NAPUS L.)

The commercial and economic value of genetically modified crops is det ermined by a predictable, consistent and stable transmission and expre ssion of the transgenes in successive generations. No gene inactivatio n is expected after selfings or crosses with non-transformed plants of homozygous transgenic oilseed rape plants if the expression of the tr ansgene in homozygous or hemizygous nature in such plants is stable. T he segregation ratios of phosphinothricin (PPT) tolerance in successiv e generations of selfings and mutual crosses of a few independent tran sgenic PPT-tolerant oilseed rape plants indicated a dominant, monogeni c inheritance. In within-variety and between-variety crosses no transg ene inactivation was observed. However, after selfings and backcrosses with non-transgenic oilseed rape infrequent loss of the expression of the PPT tolerance transgene was observed independent from its homozyg ous or hemizygous nature. Molecular analysis of PPT-susceptible plants showed that the loss of expression was due to gene inactivation and n ot to the absence of the transgene. Methylation and co-suppression are mechanisms that might cause reduced or even loss of expression of the transgene in later generations. The implications of this observation for seed multiplication of varieties and breeding activities with tran sgenic oilseed rape are discussed.