Model for biological control of canalization and evolvability. In any individual the strength of signaling through Hsp90 target pathways is directly proportional to the level of Hsp90 function (top panel), but the phenotypic effects of changes in signaling strength are often highly nonlinear, creating thresholds (the inflection point, second from top).

<div><p>A simple consequence of steep non-linearity is that sensitivity to all kinds of perturbation and stochastic effects is expected to be highest at the inflection point (second panel from bottom).</p> <p>This simple relationship between sensitivity to variation and the steepness of the signaling-phenotype relationship at the inflection point, the inverse of canalization (bottom panel), suggests why all variation increases in concert for individuals and/or populations near trait thresholds.</p></div

Hsp90 controlled environmental variation specific to previously invariant or canalized traits.

<div><p>Comparison of mean-normalized components of purely-environmental variation (z-axis) across the <i>RI</i> line backgrounds (x-axis).</p> <p>Developmental stability was calculated as the averaged (unsigned) deviations of left and right from the mean, i.e. (L+R)/2, within each individual (<i>V_e within</i>) or on the averaged (unsigned) deviations of each individual (L+R) from their clone means for each <i>RI</i> line genotype (<i>V_e among</i>).</p> <p>There was generally no effect of Hsp90 allele (<i>Sami</i>, blue or <i>P582i</i>, yellow) on the variable traits, and a highly significant effect on either measure of <i>V_e</i> for canalized bristle traits but not wing area, which was highly canalized independent of Hsp90 (<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0000075#pone-0000075-t004" target="_blank">Table 4</a>).</p></div

Hsp90 controlled phenotypic variation of most invariant quantitative traits.

<div><p>(<b>A</b>) Effects of <i>P582i</i> (yellow) and <i>Sami</i> (blue) alleles of Hsp90 in the isogenic <i>Sam</i> background on <i>V_P</i> of mutant and control sets of 450 males across the 9 <i>RI</i> line backgrounds.</p> <p>(<b>B and C</b>) Effects of 3^rd chromosomes carrying null (<i>P582</i>), and dominant-negative (<i>9J1</i>) Hsp90 mutations or wild-type Sam alleles introgressed into the <i>Sam</i> background in mutant and control sets of 225 male or 225 female sibs.</p> <p> <i>TM6B</i> contains the dominant <i>Humeral</i> (<i>Hu</i>) mutation <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0000075#pone.0000075-FlyBase1" target="_blank">[32]</a>, which increases humeral TH bristle numbers.</p> <p>Therefore, for comparison of TH bristles between TM6B and the other genotypes we used TH-HU, indicating that TH was scored only for the remaining 18 non-humeral TH bristle types.</p> <p>All experiments were conducted under temperature, density and humidity controlled conditions.</p> <p>Environmental effects were further controlled by direct comparisons between flies from the same vial and maternal environments.</p> <p>Coefficients of variation (<i>CV</i> = standard deviation/mean) are shown to enable between-trait comparisons. Statistical tests of the significance of Hsp90 effects on phenotypic variability and <i>P</i>-values are found in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0000075#pone-0000075-t001" target="_blank">Table 1</a> (for A) and <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0000075#pone-0000075-t002" target="_blank">Table 2</a> (for B and C).</p></div

Hsp90-buffered variation contributed to predicted selection responses.

<p>Fold-increase in the predicted response to truncation, directional or stabilizing selection of the ‘populations’ of 9 <i>RI</i> line genotypes in the <i>P582i</i> mutant relative to equivalent ‘populations’ of genotypes in the <i>Sami</i> control flies (<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0000075#pone-0000075-t005" target="_blank">Table 5</a>).</p

Experimental design partitions genetic and purely-environmental components of variation.

<p>Females heterozygous for <i>P582i</i> (<i>Sam1;Sam2;P582i/Sami</i>; left) were crossed to males from each of 9 highly inbred (95% homozygous) <i>RI</i> line backgrounds (different colors) to create matched “clones” of Hsp90 mutant and control male progeny from the same vial and maternal environments and hybrid for <i>Sam</i> and each <i>RI</i> line background.</p