Genetic and cytological analyses of the male sterility mutation induced in a sorghum tissue culture with streptomycin

Treatment of sorghum callus cultures with 500-1000 mg/l streptomycin led to a high regeneration frequency of plants with complete or partial male sterility (MS), up to 100% of all green regenerants. The induced MS mutation (ms-str) was preserved in the F-1 and BC1 progenies and was genetically unstable: many families produced semisterile and fertile revertants, whose progenies again contained semisterile and sterile mutants. The ms-str mutation was maintained through eight generations via selection and self-pollination of semisterile plants. The mutation was inherited as a recessive nuclear mutation in test crosses of sterile plants segregated in the progenies of fertile and semisterile revertants and was expressed only in single cases in a test cross for ms-str transfer through pollen of hybrids with restored male fertility. Recessive nuclear mutations determining a low plant height (dwarfness) and the lack of waxy bloom on the stem and leaves (bloomless) were found in male-sterile plants with the ms-str mutation. Cytological analysis of sterile plants reveal multiple abnormalities at various pollen development stages and in tapetal cells: cytomyxis, defects of chromosome conjugation, distorted cytokinesis in meiotic division II, a lack of tetrad separation, a defective formation of the microspore coat, generation of microspores with two to four nuclei, and the formation of micronuclei and large vacuoles in tapetal cells. A possible transfer of the induced cytoplasmic MS mutation into the nuclear genome and the causes of the high genetic instability are discussed.