FIELD TRIAL ANALYSIS OF NITRATE REDUCTASE CO-SUPPRESSION - A COMPARATIVE-STUDY OF 38 COMBINATIONS OF TRANSGENE LOCI

Co-suppression of host genes and 35S transgenes encoding nitrate reduc tase was previously reported in transgenic tobacco plants (Nicotiana t abacum cv. Paraguay or Burley) using either a full-length cDNA or frag ments devoid of the 3' and/or 5' UTR. Go-suppression was previously sh own to affect a limited fraction of the progeny of one transgenic toba cco line homozygous for a single transgene locus, and the phenomenon o ccurred at each generation. In this work, 38 combinations of transgene loci derived from 13 independent transgenic lines homozygous for a si ngle transgene locus were field-tested under two different conditions in an attempt to determine the corresponding frequencies of co-suppres sion, i.e. the percentage of plants showing co-suppression. Each of th e 13 homozygous lines exhibited a different frequency of co-suppressio n, ranging from 0% to 57%. High frequencies were found to be associate d with transgene loci carrying a high number of copy of the transgene, suggesting a transgene dose effect. Combinations carrying 2 non-allel ic transgene loci in a hemizygous state exhibited frequencies of co-su ppression between those of each of the 2 transgene loci in a homozygou s state, while combinations carrying 2 non-allelic transgene loci in a homozygous state exhibited frequencies of co-suppression higher than the sum of those of the 2 transgene loci alone in a homozygous state, clearly confirming a transgene dose effect. Co-suppression frequencies were increased when the plants were grown initially in vitro, suggest ing some environmental effect. The roles of transgene copy number, num ber of transgene loci and environmental factors are discussed in the l ight of a threshold hypothesis.