A Sorghum bicolor × S. macrospermum hybrid recovered by embryo rescue and culture

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Abstract
Although exotic germplasm is extensively used in sorghum improvement programs, Sorghum species classified in sections other than Eu-sorghum have not been utilised as germplasm because of strong reproductive barriers involving pollen–pistil incompatibilities. S. macrospermum is of particular interest to sorghum breeders because of its close phylogenetic relationship and cytogenetic similarities to S. bicolor and its resistance to important sorghum pests and pathogens, such as sorghum midge and sorghum downy mildew. A vegetatively vigorous interspecific hybrid was obtained from a cross between a cytoplasmic male-sterile S. bicolor plant and S. macrospermum by using embryo rescue and in vitro culture techniques. The hybrid was morphologically intermediate to S. bicolor and S. macrospermum in leaf width, leaf pubescence, plant height, inflorescence morphology, chromosome number and nuclear DNA content. It was male-sterile like its ATx623 parent. The hybrid produced no offspring when used as the female parent in a backcross with S. bicolor. This is the first confirmed hybrid between S. bicolor and S. macrospermum, and to our knowledge, it is the first reported hybrid between S. bicolor and any Sorghum species outside the Eu-sorghum section.

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