Mapping genes on an integrated sorghum genetic and physical map using cDNA selection technology

Sorghum is an important target of plant genomics. This cereal has unusual tolerance to adverse environments, a small genome (750 Mbp) relative to most other grasses, a diverse germplasm, and utility for comparative genomics with rice, maize and other grasses. In this study, a modified cDNA selection protocol was developed to aid the discovery and mapping of genes across an integrated genetic and physical map of the sorghum genome. BAC DNA from the sorghum genome map was isolated and covalently bound in arrayed tubes for efficient liquid handling. Amplifiable cDNA sequence tags were isolated by hybridization to individual sorghum BACs, cloned and sequenced. Analysis of a fully sequenced sorghum BAC indicated that about 80% of known or predicted genes were detected in the sequence tags, including multiple tags from different regions of individual genes. Data from cDNA selection using the fully sequenced BAC indicate that the occurrence of mislocated cDNA tags is very low. Analysis of 35 BACs (5.25 Mb) from sorghum linkage group B revealed (and therefore mapped) two sorghum genes and 58 sorghum ESTs. Additionally, 31 cDNA tags that had significant homologies to genes from other species were also isolated. The modified cDNA selection procedure described here will be useful for genome-wide gene discovery and EST mapping in sorghum, and for comparative genomics of sorghum, rice, maize and other grasses.