Modularity and intrinsic evolvability of Hsp90-buffered change

Area of Science:

• Evolutionary biology
• Developmental genetics

Background:

• Heat shock protein 90 (Hsp90) influences phenotypic plasticity and is hypothesized to act as an evolutionary capacitor.
• The role of Hsp90 in evolution is debated, particularly regarding whether Hsp90-buffered traits are unconditionally deleterious.

Purpose of the Study:

• To investigate the fitness costs associated with Hsp90-buffered morphological abnormalities in Drosophila.
• To determine if Hsp90-buffered traits, such as a deformed eye (dfe), incur correlated fitness deficits.

Main Methods:

• Selection experiments were conducted on Drosophila lines with a dfe trait, varying its penetrance.
• Fitness was assessed through viability, lifespan, and competitive fitness tests.
• Quantitative Trait Loci (QTL) analysis was performed on recombinant lines to identify genes associated with dfe and fitness.

Main Results:

• Despite significant variation in dfe penetrance (up to 120-fold), no measurable differences in viability, lifespan, or competitive fitness were observed.
• Seventeen dfe QTL did not correlate with fitness effects in over 1,400 recombinant lines.
• The study detected fitness effects of inbreeding, environment, and the white mutation independently of dfe penetrance.

Conclusions:

• Large morphological changes controlled by Hsp90 can be selected for without significant, unconditional fitness costs.
• Hsp90 buffers polygenic variation, allowing the substitution of deleterious alleles with less harmful ones that maintain similar phenotypes.
• This supports the Hsp90 capacitor hypothesis, linking environmental stress to developmental variation and morphological plasticity.