EXPRESSION INSTABILITY AND GENETIC-DISORDERS IN TRANSGENIC NICOTIANA-PLUMBAGINIFOLIA L PLANTS

The ease of integrative transformation with foreign genes and the extent of their expression and stability in successive generations determine the applicability of direct gene transfer. In Nicotiana plumbaginifolia, one to ten copies of foreign DNA were integrated into the plant genome, resulting in simple to complex patterns of integration. Genetic analysis showed that in more than 50% of the cases, this DNA inserted at two or more loci in the genome. Of the 156 crosses performed between F-1 monogenic transformants, only eight combinations showed linkage of the inserted neomycin phosphotransferase genes (npt). The following instability events were registered: physical loss, alterations in the initial segregation rates in successive meiotic generations observed in either selfing or crossing (reduction or increase in number of segregating loci) and genomic disorders in crosses between transformants. Among them of particular interest were the ''discordant'' segregation values observed between corresponding R(1) and F-1 progenies in up to 9% of the evaluated transformants. In addition, 5% of the transformants showed a phenotypic loss of resistance. In the F-3 generation, 5 out of 15 transformants exhibited instability, which was transmitted to the F-4 generation. Further increases in instability rates were observed with higher numbers of insertion loci and in crosses between independent transgenic plants, reaching 100% when a trigenic partner was involved. N. plumbaginifolia exhibited more instability than N. tabacum under equivalent experimental conditions. The molecular bases of such instability events are discussed in relation to DNA methylation, co-suppression and genomic imbalance.