SYNTHESIS OF HEXAPLOID (AABBCC) SOMATIC HYBRIDS - A BRIDGING MATERIALFOR TRANSFER OF TOUR CYTOPLASMIC MALE-STERILITY TO DIFFERENT BRASSICASPECIES

Most of the alloplasmic cytoplasmic male sterility (CMS) systems are k nown to be associated with a number of floral abnormalities that resul t from nuclear-cytoplasmic incompatibilities. One such system, 'tour', which is derived from Brassica tournefortii, induces additional flora l abnormalities and causes chlorosis in Brassica spp. While the restor er for this CMS has been reported to be present in B. napus, in B. jun cea, where the abnormalities are more pronounced, no restorer has yet been identified. Rectification of these floral abnormalities through m itochondrial recombinations and chloroplast replacement might result i n the improvement of this CMS system. As organelle recombinations can possibly be achieved only by somatic cell hybridization, fusion experi ments were carried out between hygromycin-resistant B. juncea AABB car rying 'tour' cytoplasm and phosphinotricin-resistant, normal B. olerac ea CC to generate AABBCC hexaploid somatic hybrids. The presence of se lectable marker genes facilitated the selection of hybrids in large nu mbers. The resulting hybrids showed wide variation in floral morpholog y and organelle composition. Regenerants with normal, male-sterile flo wers having recombinant 'tour' or 'oleracea'-type mitochondria and 'ol eracea'-type chloroplasts were obtained. Hybrids with male-fertile flo wers were also obtained that had recombined 'tour' mitochondria. The A ABBCC hexaploid hybrids synthesized in the present study were successf ully utilized as a bridging material for transferring variability in t he organelle genome simultaneously to all the digenomic Brassica speci es, and all of these hybrids are now being stabilized through repeated backcrosses to the allo-polyploid crop brassicas.