DISSECTION OF A SYNTHESIZED QUANTITATIVE TRAIT TO CHARACTERIZE TRANSGENE INTERACTIONS

Six transgenic tobacco lines, each homozygous for the beta-glucuronida se (GUS) gene at a different locus, and wild type were selfed and inte rcrossed to evaluate GUS activity in all possible hemizygous, homozygo us and dihybrid combinations of GUS alleles. The transgenic lines are characterized by their GUS activity (two low, three intermediate, one high), T-DNA complexity (few single-copy, two more complex single-locu s) and the presence of the chicken lysozyme matrix-associated region ( MAR) around the full T-DNA (two lines). Gene action and interaction wa s analyzed by weighted linear regression with parameters for additivit y, dominance and epistasis. The analysis showed that each of the four single-copy lines acted fully additively. In contrast, the two complex single-locus lines showed classical single-locus overdominance and we re epistatic dominant over all other GUS alleles. The latter is manife sted in severe suppression of GUS activity in dihybrid lines, irrespec tive of the presence of MAR elements around the GUS gene. Such element s apparently do not protect against epistatic dominance. The quantitat ive data suggested that the epistatic dominance and overdominance are based on the same molecular mechanism. Our approach of a genetic analy sis of quantitative variation in well-characterized transgenic lines p rovides a powerful tool to gain insight into complex plant traits.