In this study, we characterize a recessive X-linked temperature-sensitive mutation of the gene CG32711. The mutation, named wiser ( tsl ) (wings scalloped-eyes rough), was isolated from a dysgenic cross and is due to a natural P element insertion within the 5' regulatory region of the gene. Mutant (wiser ( tsl )) individuals exhibit wing notching, rough eyes, tarsal malformations and reduced life-span. At 29 degrees C they die at larval and late pharate stages or during eclosion. The CG32711 (wiser) gene is mainly expressed in the ventral midline cells, the peripheral neural system, the hemocytes and the tracheal system of embryos. It is also expressed in nurse cells of adult female ovaries. Our results show that the wiser gene is alternatively spliced generating two mRNAs, which share the same open reading frame, while western analysis identified two protein isoforms. Their expression pattern depends on the stage of development and the culture temperature. wiser ( tsl ) and wild-type individuals display different expression patterns of the two isoforms and this difference most probably accounts for the mutant phenotype. Our results indicate that wiser is a vital gene for the development of Drosophila melanogaster which has no orthologs outside the Drosophilidae.