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

The ease of integrative transformation with foreign genes and the exte nt of their expression and stability in successive generations determi ne the applicability of direct gene transfer. In Nicotiana plumbaginif olia, one to ten copies of foreign DNA were integrated into the plant genome, resulting in simple to complex patterns of integration. Geneti c analysis showed that in more than 50% of the cases, this DNA inserte d at two or more loci in the genome. Of the 156 crosses performed betw een F-1 monogenic transformants, only eight combinations showed linkag e of the inserted neomycin phosphotransferase genes (npt). The followi ng instability events were registered: physical loss, alterations in t he initial segregation rates in successive meiotic generations observe d in either selfing or crossing (reduction or increase in number of se gregating loci) and genomic disorders in crosses between transformants . Among them of particular interest were the ''discordant'' segregatio n values observed between corresponding R(1) and F-1 progenies in up t o 9% of the evaluated transformants. In addition, 5% of the transforma nts showed a phenotypic loss of resistance. In the F-3 generation, 5 o ut of 15 transformants exhibited instability, which was transmitted to the F-4 generation. Further increases in instability rates were obser ved with higher numbers of insertion loci and in crosses between indep endent transgenic plants, reaching 100% when a trigenic partner was in volved. 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-su ppression and genomic imbalance.