Accumulation, functional annotation, and comparative analysis of expressed sequence tags in eggplant (Solanum melongena L.), the third pole of the genus Solanum species after tomato and potato

Eggplant (Solanum melongena L.) is a widely grown vegetable crop that belongs to the genus Solanum, which is comprised of more than 1000 species of wide genetic and phenotypic variation. Unlike tomato and potato, Solanum crops that belong to subgenus Potatoe and have been targets for comprehensive genomic studies, eggplant is endemic to the Old World and belongs to a different subgenus, Leptostemonum, and therefore, would be a unique member for comparative molecular biology in Solanum. In this study, more than 60,000 eggplant cDNA clones from various tissues and treatments were sequenced from both the 5′- and 3′-ends, and a unigene set consisting of 16,245 unique sequences was constructed. Functional annotations based on sequence similarity to known plant reference datasets revealed a distribution of functional categories almost similar to that of tomato, while 1316 unigenes were suggested to be eggplant-specific. Sequence-based comparative analysis using putative orthologous gene groups setup by reciprocal sequence comparison among six solanaceous species suggested that eggplant and its wild ally Solanum torvum were clustered separately from subgenus Potatoe species, and then, all Solanum species were clustered separately from the genus Capsicum. Microsatellite motif distribution was different among species and likely to be coincident with the phylogenetic relationships. Furthermore, the eggplant unigene dataset exhibited its utility in transcriptome analysis by the SAGE strategy where a considerable number of short tag sequences of interest were successfully assigned to unigenes and their functional annotations. The eggplant ESTs and 16k unigene set developed in this study would be a useful resource not only for molecular genetics and breeding in eggplant itself, but for expanding the scope of comparative biology in Solanum species.