Submitted to: Crop Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/8/2004
Publication Date: 5/27/2005
Citation: Hodnett, G.L., Burson, B.L., Rooney, W.L., Dillon, S.L., Price, H.J. 2005. Pollen-pistil interactions result in reproductive isolation between Sorghum bicolor and divergent Sorghum species. Crop Science. 45:1403-1409. Interpretive Summary: Sorghum is grown throughout the warmer regions of the world as a cereal crop and a forage grass. Besides the grain and forage sorghums, there are a number of wild relatives that grow in the tropical regions of Australia, Southeast Asia, and Africa. Most of these are grassy types that are resistant to several diseases and insects that frequently attack cultivated grain and forage sorghums. Therefore, to improve the disease and insect resistance in cultivated sorghum, it would be desirable to produce hybrids between cultivated sorghum and the wild species. Unfortunately, when plant breeders cross the wild species with sorghum, no hybrids are produced. This study was undertaken to find out why hybrids are not produced. Fourteen different wild sorghum species were crossed onto the grain sorghum line ATx623. After pollination, pollen germination and pollen tube growth in the flowers of ATx623 were monitored using a microscope to determine what happened. The pollen germinated in all of the crosses, but then in most crosses the pollen tubes stopped growing and fertilization did not occur. In crosses with three of the wild species, fertilization occurred, but after a few days the endosperm (food) in the developing seed broke down and the seed died. So the reason hybrids cannot be produced between cultivated sorghum and the wild sorghum species is the failure of: 1) the pollen tubes to fertilize the egg; and 2) the endosperm to develop in the seed. Research is under way to solve these problems.
Technical Abstract: Sorghum [Sorghum bicolor (L.) Moench] breeders have long recognized the importance of exotic germplasm and non-cultivated 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 prior to maturation, apparently because of breakdown of the endosperm.