Pollen–Pistil Interactions Result in Reproductive Isolation between Sorghum bicolor and Divergent Sorghum Species

Sorghum [Sorghum bicolor (L.) Moench] breeders have long recognized the importance of exotic germplasm and noncultivated sorghum races as sources of valuable genes for genetic improvement. The genus Sorghum consists of 25 species classified as five sections: Eu-sorghum, Chaetosorghum, Heterosorghum, Para-sorghum, and Stiposorghum Species outside the Eu-sorghum section are sources of important genes for sorghum improvement, including those for insect and disease resistance, but these have not been used because of the failure of these species to cross with sorghum. An understanding of the biological nature of the incompatibility system(s) that prevent hybridization and/or seed development is necessary for the successful hybridization and introgression between sorghum and divergent Sorghum species. The objectives of this study were to determine the reason(s) for reproductive isolation between Sorghum species. The current study utilized 14 alien Sorghum species and established that pollen–pistil incompatibilities are the primary reasons that hybrids with sorghum are not obtained. The alien pollen tubes showed major inhibition of growth in sorghum pistils and seldom grew beyond the stigma. Pollen tubes of only three species grew into the ovary of sorghum. Fertilization and subsequent embryo development were not common. Seeds with developing embryos aborted before maturation, apparently because of breakdown of the endosperm.