Cytoplasmic male sterile lines of a tobacco variety, Tsukuba 1, developed by asymmetric protoplast fusion.

Protoplasts of a normal fertile tobacco variety, Tsukuba 1, were fused with X-irradiated protoplasts of a male sterile (MS) tobacco line, MS Burley 21. Regenerated plants which resembled Tsukuba 1 but showed male sterility were selected, and backcrossed to Tsukuba 1, as the male parent. In the BC1 generation, 31 MS Iines were examined in a field experiment both for their agronomic characters and the presence of male sterility, and two lines with a better performance were selected. These lines again were evaluated in the three subsequent backcross generations. Male sterility was transmitted throughout these four generations and plants showing male fertility were not observed. The selected lines were not significantly different from Tsukuba 1 in their agronomic characters, and were considered to be comparable to Tsukuba 1 in all the generations. It was concluded that the male sterile lines developed by protoplast fusion were genetically stable even in later backcross generations and therefore could be used for practical breeding of F1 hybrids in tobacco.