INHERITANCE AND EXPRESSION OF A TRANSGENE INSERT IN AN ANEUPLOID TOBACCO LINE

A T-DNA locus comprising nptII, uidA and nos genes - all under the con trol of the nos promoter (this locus was designated K because it encod es resistance to Kanamycin)- was found to be inherited erratically in a transgenic tobacco line. This anomalous behavior was partially expla ined following a karyotype analysis of plants representing several gen erations: these plants were aneuploids, presumably for the K-containin g chromosome. During four generations of sexual propagation, transgeni c plants that were either trisomic or tetrasomic for the K-containing chromosome (i.e. 2n=49 or 2n=50, respectively) were obtained. The tris omic plants (2n=48+1) were virtually indistinguishable phenotypically from normal euploids (2n=4x=48), whereas the tetrasomic plants (2n=482) were smaller, had somewhat misshapen leaves and exhibited reduced f ertility. Although the amount of NPTII protein in different trisomic ( K-, KK-, KKK) and tetrasomic (KK-, KKK-) plants was generally consiste nt with a K dosage effect, the genetic behavior of each trisomic - wit h respect to segregation of Kan(R) and marker gene activity in progeny - was unique and not completely explicable by invoking aneuploidy. Sp ecifically, unexpected gains or losses of K could occur, suggesting th e formation of double reductional gametes and/or frequent gene convers ion at this locus. The susceptibility of K locus marker genes to trans -inactivation in the trisomic and tetrasomic lines was tested by cross ing in partially homologous silencing loci. In all transgenotypes test ed, the three K marker genes were sensitive to trans-silencing, which was accompanied by methylation in all copies of the nos promoter. In a ddition to this directed inactivation/methylation, the K locus could a lso undergo infrequent, spontaneous partial methylation, which produce d stable epialleles. In most plants, however, the multiple copies of t he nos promoter at this locus remained unmethylated and active through four generations in all transgenotypes examined. The significance of these results for irregular inheritance patterns, aneuploid syndromes and homology-dependent gene silencing is discussed.