ROBERTS, S. P.*; FEDER, M.E.; POSLUSZNY, J.A: Developmental and fitness consequences of natural thermal stress and hsp70 copy number in Drosophila melanogaster

Natural heat exposure of larval Drosophila melanogaster results in high rates of larval mortality and severe developmental defects in eclosing adults. Transgenic increases in copy number of the gene encoding the major inducible heat-shock protein (Hsp) of D. melanogaster, Hsp70, mitigate these effects yet increase mortality during benign thermal conditions. In two wild D. melanogaster populations studied, approximately 10% of adult flies reared from field-collected fruit had severe developmental anomalies of wing and abdominal morphology. The frequency of developmental abnormalities varied along a natural thermal gradient, reaching 4% in adults eclosing from larvae developing in cool, shaded fruit and exceeding 12% for flies from warm, sunlit fruit. Heat-induced wing deformities dramatically reduced mating success in male flies. When exposed to strong natural heat stress, D. melanogaster with the wild-type number of hsp70 genes (10) had higher rates of larval mortality and developmental disruption than a transgenic sister strain with additional (22) copies of the hsp70 gene. However, under conditions of little or moderate natural thermal stress, larvae possessing 22 hsp70 copies had higher mortality rates than larvae possessing 10 hsp70 copies. These results support the hypothesis of an evolutionary trade-off of the benefits and adverse consequences of Hsp70 expression.