Fine Mapping and Characterization of the iap Gene in Sorghum [Sorghum bicolor (L.) Moench]

The production of interspecific or intergeneric progeny using sorghum [Sorghum bicolor (L.) Moench] is greatly enhanced by the presence of the iap (Inhibition of Alien Pollen) allele. Hybridization between S. bicolor homozygous for iap and divergent species of sorghum and sugarcane (Saccharum spp.) has been demonstrated with introgression occurring in hybrids between S. bicolor and S. macrospermum. The intergeneric F1 hybrids between sorghum and sugarcane were male and female sterile, so the plants were doubled with colchicine in an attempt to restore fertility. The objectives of this research were to determine the viability of the sorghum ? sugarcane amphidiploids as a bioenergy crop, determine the optimum humidity for maize (Zea mays L.) pollen tube growth on the pistils of sorghum homozygous for iap, and fine map the Iap locus and identify candidate genes. Sorghum ? sugarcane amphidiploids were evaluated in a yield trial in Weslaco, TX. Although the amphidiploid genotypes were inferior to the intergeneric F1?s for most agronomic traits, genotypic variation among families indicates that continued selection and breeding could produce more desirable genotypes. Somatic chromosome counts on selected genotypes revealed that chromosome transmission between generations appears to be occurring normally. Knowing the ideal environmental conditions could lead to greater success rates in future wide crosses. The greatest maize pollen adhesion and germination on the pistils of Tx3361 (iap iap) was observed at 45% humidity. Multiple maize pollen tubes were observed in the sorghum style and ovary at 45% humidity but not at higher humidity levels. Future interspecific and intergeneric crosses with iap sorghum should be performed at low humidity levels. Knowledge of the identity and function of the iap allele could lead to discovery of additional genes that regulate barriers to intergeneric hybridization. Fine mapping resolved the location of the Iap gene to a 48 kb region on the short arm of chromosome 2. There are three putative genes present in this region, however none of them are easily identified as Iap. This research provides valuable insight to proper and efficient utilization of the iap allele and a foundation for continued mapping and characterization of Iap.