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Is canalization more than just a beautiful idea?

DOI: 10.1186/gb-2010-11-3-109

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In the 1940s, the developmental biologist and geneticist CH Waddington coined the concept of 'developmental stability', or the robustness of the phenotype against genetic and environmental perturbations [1,2]. It has been claimed that this robustness, termed 'canalization', has evolved under natural selection to stabilize phenotypes and decrease their variability. This is achieved by buffering the expression of traits, holding them near their optimal states despite genetic and environmental perturbations. Canalization also allows the accumulation of 'cryptic genetic variation' caused by mutations that do not affect the phenotype. Canalized traits are phenotypically expressed only in particular environments or genetic backgrounds and become available for natural selection, a mechanism that can lead to the assimilation of novel traits.It was found some years ago that reduction in the function of the Hsp90 protein in Drosophila (whether by mutation or by specific inhibitors) apparently uncovered previously silent genetic variation, which led to an increase in morphological variation [3]. Hsp90 is a chaperone and heat-shock protein, which in Drosophila is encoded by the Hsp83 gene. The morphological changes could become fixed and stably transmitted even if wild-type Hsp90 function were restored in subsequent generations. These findings implied that functional Hsp90 is a capacitor (that accumulates cryptic genetic variation and releases it under certain circumstances) that masks the effect of hidden or pre-existing genetic variation (Figure 1).The Hsp90 story in flies has become very complicated, however. Recent studies have shown that the buffering by Hsp90 is limited to specific morphological traits and does not affect others. This supports the idea that numerous mechanisms are involved in developmental buffering, and that Hsp90 is just one of many capacitors for genetic variation [1,2]. In addition, Hsp90 is a very abundant protein, in some cells accounting for up to


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