Drosophila melanogaster is polymorphic for a specific repeated (CATA) sequence in the regulatory region of hsp23.

To identify sequence variation associated with a selection response for heat tolerance in Drosophila melanogaster, we sequenced 1400bp of the heat shock protein 23 gene (hsp23) promoter region in four heat-selected and two control lines. The region was found to be variable for a specific (CATA) repeated sequence, and the sequence CTT seems to be a hot spot for mutation. The repeated tetranucleotide sequence was located in several short repeats scattered throughout the entire region. Similar variable repeats are also located downstream the of hsp23 gene in the intergenic region between hsp23 and hsp27. We detected nine different hsp23 alleles. Their frequencies in the selection and control lines seemed to be mainly determined by genetic drift. The function of the CATA repeats is not yet known, though these regions have homology to SAR elements located in the intergenic region between two hsp70 genes, suggesting a similar function.