The Drosophila heat shock hsr-omega gene: An allele frequency cline detected by quantitative PCR

The hsr-omega gene of Drosophila melanogaster produces RNA products both co nstitutively and at elevated levels in response to heat stress. A single-nu cleotide difference in this gene that has been detected using denaturing gr adient gel electrophoresis (DGGE) is responsible for an hsr-omega(a/b) poly morphism, and selection experiments have indicated an association between t he hsr-omega(a) allele and susceptibility to heat stress. Since allele freq uency estimates for population surveys using PCR and DGGE for single flies would be relatively time-consuming and expensive, we here develop a quantit ative competitive-PCR method using mass-grind genomic DNA preparations for this purpose. Geographical and temporal variation of allele frequency at th e hsr-omega locus in Australian populations of D. melanogaster are examined . Regular samples from a southern population through a summer season sugges ted stability of hsr-omega(a) frequency. Field populations sampled from a s imilar to 2,250 km north-south transect along eastern Australia revealed a strong positive association between the frequency of hsr-omega(a) and latit ude, and marked spatial autocorrelation. Using appropriate analyses, strong associations between population differences in hsr-omega(a) frequencies an d differences in temperature and rainfall measures, after controlling for l atitudinal differences, support the idea that the dine in hsr-omega(a) freq uency may be attributable to some form of climatic selection.