TRANSGENIC POTATO PLANTS CAN BE USED TO EVALUATE STABILITY OF FOREIGNGENES - REVERSIONS AND CHIMERAS IN MULTIPLE COPIES OF ROLC HARBORING CLONES

35S-rolC and rbcS-rolC transgenic potato plants were used to evaluate the stability of the phenotypic marker gene rolC during the life cycle of transgenic potato plants and in subsequent vegetative generations. Transgenics carrying one or two copies of the gene revealed a high st ability of the gene during growth and vegetative propagation. Two 35S- rolC transgenic plants harboring four and five copies of the rolC gene showed a strong phenotypic role expression and were viable only when grown in vitro. Under these conditions, spontaneous reversions were ob served to a less expressed rolC phenotype. In Southern experiments, th ese transgenics still carried four and five copies of the rolC gene, w hereas the northern blot signals were comparable to those of transgeni cs carrying a lower copy number of rolC. From the same two clones, chi meric shoots were observed during vegetative growth in a greenhouse th at showed light and dark green normal sectors on the same leaf. The pr esence of four or five copies of rolC was confirmed in the dark green normal sectors, although no signal was obtained in northern blots for this phenotypically normal tissue. The results are discussed with resp ect to possible gene inactivation.