Comparative mapping between model plant species for which the complete genome sequence is known and crop species has been suggested as a new strategy for the isolation of agronomically valuable genes. In this study, we tested whether comparative mapping between Arabidopsis and maize of a small region (754 kb) surrounding the DREB1A gene in Arabidopsis could lead to the identification of an orthologous region in maize containing the DREB1A homologue. The genomic sequence information available for Arabidopsis allowed for the selection of conserved, low-copy genes that were used for the identification of maize homologues in a large EST database. In total, 17 maize homologues were mapped. A second BLAST comparison of these genes to the recently completed Arabidopsis sequence revealed that 15 homologues are likely to be orthologous as the highest similarity score was obtained either with the original Arabidopsis gene or with a highly similar Arabidopsis gene localized on a duplication of the investigated region on chromosome 5. The map position of these genes showed a significant degree of orthology with the Arabidopsis region. Nevertheless, extensive duplications and rearrangements in the Arabidopsis and maize genomes as well as the evolutionary distance between Arabidopsis and maize make it unlikely that orthology and collinearity between these two species are sufficient to aid gene prediction and cloning in maize.
