Molecular thermal telemetry of free-ranging adult Drosophila melanogaster

The expression of two temperaturte-sensitive reporter genes, hsp70 and an h sp70-LacZ fusion, in free-ranging adult Drosophila melanogaster indicates t hat natural thermal stress experienced by such small and mobile insects may be either infrequent or not severe. Levels of the heat-shock protein Hsp70 , the major inducible Hsp of Drosophila, were similar in most wild Drosophi la captured after warm days to levels previously reported for unstressed fl ies in the laboratory. In a transgenic strain transformed with an hsp70-lac Z fusion (i.e., the structural gene encoding bacterial beta-galactosidase u nder control of a heat shock promoter), exposure to temperatures greater th an or equal to 32 degrees C in the laboratory typically resulted in beta-ga lactosidase activities exceeding 140 mOD(450) h(-1) mu g(-1) soluble protei n. Flies caged in sun frequently had beta-galactosidase activities in exces s of this level, whereas flies caged in shade and flies released and recapt ured on cool days did not. Most flies (>80%) released on warm, sunny days h ad low beta-galactosidase activities upon recapture. Although the balance o f recaptured flies had elevated beta-galactosidase activities on these days , their beta-galactosidase activities were <50% of levels for flies caged i n direct sunlight or exposed to laboratory heat shock. These data suggest t hat even on warm days most flies may avoid thermal stress, presumably throu gh microhabitat selection, but that a minority of adult D. melanogaster und ergo mild thermal stress in nature. Both temperature-sensitive reporter gen es, however, are limited in their ability to infer thermal stress and demon strate its absence.