BOTH ALLELIC VARIATION AND EXPRESSION OF NUCLEAR AND CYTOPLASMIC TRANSCRIPTS OF HSR-OMEGA ARE CLOSELY ASSOCIATED WITH THERMAL PHENOTYPE IN DROSOPHILA

Inducible heat shock genes are considered a major component of the mol ecular mechanisms that confer cellular protection against a variety of environmental stresses, in particular high temperature extremes. We h ave tested the association between expression of the heat shock RNA ge ne hsr-omega and thermoresistance by generating thermoresistant lines of Drosophila melanogaster after application of two distinct regimes o f laboratory selection. One set of lines was selected for resistance t o knockdown by heat stress and the other was similarly selected but be fore selection a mild heat exposure known to increase resistance (heat hardening) was applied. A cross between resistant and susceptible lin es confirmed our earlier observation that increased thermal tolerance cosegregates with allelic variation in the hsr-omega gene. This cosegr egating variation is attributed largely to two haplotype groups. Using quantitative reverse transcription-PCR, we find evidence for divergen t phenotypic responses in the two selection regimes, involving both st ructural and regulatory changes in hsr-omega. Lines selected after har dening showed increased levels of the cytoplasmic transcript but decre ased levels of the nuclear transcript. Lines selected without hardenin g showed decreased levels of the cytoplasmic transcript. The allelic f requency changes at hsr-omega could not by themselves account for the altered transcription patterns. Our results support the idea that the functional RNA molecules transcribed from hsr-omega are an important a nd polymorphic regulatory component of an insect thermoresistance phen otype.