CONSTRUCTION OF A DESIGNER CHROMOSOME IN TOBACCO

The tobacco (Nicotiana tabacum L.) breeding line NC 152 is a doubled h aploid that possesses an addition chromosome from N. africana [Merxm. and Buttler]. A gene on this chromosome confers potyvirus resistance ( Poty(R)). Our objective was to use the addition chromosome as a base o n which to construct a designer chromosome containing a foreign gene l inkage package. A mutant dhfr gene conferring resistance to methotrexa te (Mtx) was inserted into NC 152-haploid (n = 25) leaf tissue via Agr obacterium tumefaciens-mediated transformation. After chromosome doubl ing, 135 NC 152dhfr transformants (2n = 50) were pollinated with the p otyvirus-susceptible (Poty(S)) cultivar 'McNair 944' (2n = 48). Linkag e analysis was performed in the BC, generation. Two transformants, NC 152dhfr-996 and NC 152dhfr-1517, exhibited complete linkage between Mt x resistance (Mtx(R)) and Poty(R). Segregants from these two transform ants which were Mtx(R) and Poty(R) possessed 49 chromosomes, while Mtx sensitive (Mtx(S)) and Poty(S) progeny possessed 48 chromosomes. Eigh ty percent of the NC 152dhfr transformants transmitted the dhfr gene a s one locus. Other foreign genes can be directed to the addition chrom osome through transformation followed by selection for single loci wit h linkage to Poty(R) or Mtx(R). The integrity of both the foreign-gene linkage package and the rest of the genome will be maintained because recombination between the N. africiana and the N. tabacum chromosomes has not been observed.