Genotype dependent interspecific hybridization of Sorghum bicolor

Abstract

Wild Australian Sorghum species are a tertiary gene pool to grain sorghum [Sorghum bicolor (L.) Moench], and they are of interest to breeders because of their resistance to important insects and pathogens. However, strong reproductive barriers have prevented hybridization between S. bicolor and these wild species. The purpose of this study was to determine if the recessive iap allele (dominant allele Iap = inhibition of alien pollen) would reduce or eliminate the pollen–pistil incompatibilities that prevent hybridization between S. bicolor and divergent Sorghum species. Cytoplasmic male-sterile S. bicolor plants, homozygous for the iap allele, were pollinated with three divergent Sorghum species, S. angustum Blake, S. nitidum (Vahl) Pers., and S. macrospermum Garber. The pollen of these three wild species readily germinated and the pollen tubes grew to the base of the S. bicolor ovary within 2 h after pollination. Hybrid embryos were detected in the S. bicolor florets 13 to 20 d post-pollination. Sorghum bicolor × S. angustum and S. bicolor × S. nitidum hybrids were obtained using embryo rescue followed by in vitro culture techniques and hybrids between S. bicolor and S. macrospermum were obtained by simply germinating the hybrid seed. These hybrids were confirmed by their morphological and cytological traits. These findings clearly demonstrate that the recessive iap allele circumvents pollen–pistil incompatibilities in the genus Sorghum and permits hybrids to be made between S. bicolor and species of the tertiary gene pool.