Alternative cytoplasmic male sterility systems in sorghum and their utilization

In sorghum, Stephen and Holland (1954) discovered cytoplasmic male sterility (CMS) designated as milo cytoplasm in the progenies of a cross between two cultivars, milo and combine kafi r, with milo as the female and kafi r as the male. Male-sterile plants to the extent of 25% were observed in the F2 generation of the above cross when milo was used as female and not as male. The male-sterile segregants from this cross produced male-sterile hybrids when crossed with the kafi r parent and fully fertile hybrids when crossed with the milo parent. Thus, it was recognized that kafi r could be used as a maintainer of this source of CMS. Since the progeny received the cytoplasm from the female, it was hypothesized that the milo parent had a male sterility-inducing cytoplasm and dominant genes for pollen fertility, whereas the combine kafi r parent contained a normal (fertile) cytoplasm but the recessive male-sterile genes. All progenies of the milo×combine kafi r cross contained milo (sterility-inducing) cytoplasm, but those that also inherited the homozygous recessive genes from the kafi r parent were male-sterile. The malesterile plants in the milo×combine kafi r cross were used as females in repeated backcrossing with kafi r as the male parent. At the end of seven backcrosses, the entire genome of kafi r was transferred into the milo cytoplasm. This resulted in two morphologically similar versions of the combine kafi r (CK 60) parent: a malesterile combine kafi r (CK 60A) and a male-fertile combine kafi r (CK 60B). The male-sterile lines are designated as A-lines and their maintainer lines as B-lines. Subsequently, several sources of CMS systems (A1, A2, A3, A4, A5 and A6) (Table 13) were discovered