Hsp90 inhibition and the expression of phenotypic variability in the rainforest species Drosophila birchii

Recently a heat shock protein (Hsp90) has been implicated as controlling the expression of cryptic genetic variation through buffering developmental processes. The release of variability in canalized characters following Hsp90 inhibition has been established in model species including Drosophila melanogaster and Arabidopsis thaliana, but has not yet been examined in species with limited distributions. To test if Hsp90 has a role in releasing phenotypic variation in rainforest Drosophila species, developing larvae from a large (> 1000 individuals) outbred population of Drosophila birchii were treated with the Hsp90 inhibitors geldanamycin and radicicol, and morphological traits, desiccation resistance, and life-history traits were measured. The means of all traits were influenced by inhibition. Although only the phenotypic variances of two canalized bristle traits were affected consistently, variability for two of the continuously varying traits (fecundity and development time) were also affected, albeit inconsistently. There was also no effect of Hsp90 inhibition on the developmental stability of the morphological traits as measured by fluctuating asymmetry. Hsp90 inhibition did not increase phenotypic variability in desiccation resistance, a trait previously shown to represent an evolutionary limit in this species. These results question the extent to which Hsp90 buffers variation for both quantitative and discrete traits, and highlight the need for further empirical studies to determine the involvement of Hsp90 in canalization and developmental stability. Nevertheless the results demonstrated increased variability in canalized traits, consistent with observations in model systems.