Sorghum has been improved by plant breeders for yield, biotic and abiotic stress resistance, as well as quality traits by using germplasm from within the species. Interspecific hybridization can greatly increase the amount of genetic variation available to plant breeders for improvement. Interspecific hybrids between sorghum and the 19 species in the tertiary gene pool have, until recently, not been successful. The Australian species, S. macrospermum, was recently successfully hybridized with sorghum by using germplasm homozygous for the iap allele, which eliminated reproductive isolation barriers. The objectives of this research were to evaluate the potential for use of S. macrospermum in an introgression breeding program, determine the map position of the Iap locus, and backcross the iap allele into elite Texas A&M germplasm. Interspecific hybrids between S. bicolor and S. macrospermum revealed moderate levels (2.6 II per PMC) of allosyndetic recombination, indicating that introgression through genetic recombination is possible. Genomic relationships were sufficient to assign S. macrospermum the genomic formula AAB1B1YYZZ, Y and Z remain unknown. In backcrosses to S. bicolor using the female interspecific hybrid gamete and embryo rescue, BC1F1 plants were recovered. They had high chromosome numbers (2n = 35-70) and were male-sterile but three plants set backcross seed. Ninety-five percent of BC2F1 plants were 2n = 20 chromosomes and 75% of them contained S. macrospermum introgression. BC2F1 plants carried between 0-18.5% S. macrospermum introgression; in total 26% of the S. macrospermum genome was detected in the BC2 generation. Three types of introgression germplasm were created: alien addition lines; alien substitution lines; and introgression lines. Recombinant chromosomes, containing S. macrospermum introgression sites, were identified in multiple introgression lines. The Iap locus was genetically mapped to sorghum chromosome 2 (SBI-02), flanking AFLP markers were 2.1 and 2.7cM away, one AFLP marker shared the same map position (0.0cM). A genetic stock, Tx3361, was created which has iap iap genotype and improved agronomic qualities such as short plant height, white seed color, non-pigmented testa, no awns, reduced lodging, early maturity, and backcross segregation of male-sterility (ms3). This research shows that S. macrospermum is now available to plant breeders for sorghum improvement.
