Submitted to: Crop Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/30/2006
Publication Date: 12/4/2006
Citation: Price, H.J., Hodnett, G.L., Burson, B.L., Dillon, S.L., Stelly, D.M., Rooney, W.L. 2006. Genotype dependent interspecific hybridization of Sorghum bicolor (Poaceae). Crop Science. 46:2617-2622.
Interpretive Summary: For years, sorghum breeders have known that wild grassy relatives of cultivated sorghum are resistant to diseases and insects that often attack and reduce yields in grain sorghum. In order to transfer these desirable traits from the wild relatives to cultivated sorghum, hybrids have to be made. Unfortunately, efforts to produce hybrids between grain sorghum and these wild grassy relatives have not been successful. However, recently we produced a hybrid between cultivated sorghum and one of the wild relatives. This was done by removing young embryos from the developing seeds and growing them on a nutrient media. Only one hybrid was recovered, but when it had grown to maturity, it had desirable characteristics as a forage grass and potential as a biofuel crop. Because of the disease and insect resistance and these desirable forage/biofuel characteristics, we are interested in producing more hybrids between cultivated sorghum and the grassy wild relatives. A grain sorghum line with a gene that allows the pollen tubes of distantly related species to grow into and fertilize the grain sorghum flower was recently obtained. Crosses were made between this sorghum line and three of the wild relatives and the pollen tubes of the wild relatives readily grew into and fertilized the sorghum flower. This discovery is very important because it will allow for the production of many more hybrids between cultivated sorghum and the wild grassy relatives. In fact it should result in enough hybrids that a breeding program can be developed. This is of importance in the development of sorghum germplasm with improved insect and disease resistance and the development of forage sorghums with higher yields and a potential as a biofuel crop.
Technical Abstract: Wild Australian Sorghum species which are a tertiary gene pool to grain sorghum [Sorghum bicolor (L.) Moench.] are of interest to sorghum breeders because they are resistant to important insects and pathogens. However, strong reproductive barriers have prevented hybridization between sorghum 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 and permit hybridization between sorghum and divergent Sorghum species. Cytoplasmic male-sterile sorghum 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 sorghum ovary within 2 h after pollination. Hybrid embryos were detected in the sorghum florets 13 to 20 d post-pollination. Sorghum x S. angustum and sorghum x S. nitidum hybrids were obtained using embryo rescue and in vitro culture techniques and hybrids between sorghum and S. macrospermum were obtained from germinated seeds. These hybrids were confirmed by their morphological and cytological traits as well as by chromosome number. These findings clearly demonstrate that the recessive iap allele circumvents pollen-pistil incompatibilities in the genus Sorghum and hybrids can be made between sorghum and members of the tertiary gene pool.