116 G.P. WAGNER ET AL.JOURNAL OF EXPERIMENTAL ZOOLOGY (MOL DEV EVOL) 285:116–118 (1999)

© 1999 WILEY-LISS, INC.

Is Hsp90 a Regulator of Evolvability?GÜNTER P. WAGNER,* CHI-HUA CHIU, AND THOMAS F. HANSENDepartment of Ecology and Evolutionary Biology, Yale University, NewHaven, Connecticut 06520-8106

ABSTRACT In a recent paper, Rutherford and Lindquist (1998. Nature 396:336–342) identi-fied mutations in the Hsp90 protein that act to unmask hidden genetic variation with a variety ofphenotypic effects. The Hsp90 protein has a number of properties that suggest a role in regulatingthe expression of genetic variation and therefore in adjusting the evolvability of the organism. Inthis paper we reflect upon the evolutionary feasibility of such mechanisms and suggest some pos-sible ways of testing the adaptation-for-evolvability hypothesis in more detail. We conclude thatHsp90 holds promise as a molecular model system for the evolution of evolvability. J. Exp. Zool.(Mol. Dev. Evol.) 285:116–118, 1999. © 1999 Wiley-Liss, Inc.

It has long been known that the sensitivity ofan organism’s phenotype to environmental and ge-netic factors (phenotypic variability sensu Wagnerand Altenberg, ’96) is genetically regulated. In-deed, mutations with major phenotypic effectssuch as Scute in Drosophila and Tabby in miceunmask hidden genetic variation, which indicatesthe increased sensitivity of the mutant phenotypeto genetic variation (Scharloo, ’91). Similarly, ex-treme environmental conditions can also unmaskgenetic variation for traits (Hoffmann and Par-sons, ’97). In a recent paper, Rutherford andLindquist (’98) have provided one of the firstglimpses into the molecular mechanisms that un-derlie the genetic regulation of phenotypic vari-ability. They have shown that loss of functionmutations at the Drosophila Hsp83 locus (encod-ing the heat-shock protein Hsp90) lead to an in-crease in the genetic variation of a number ofphenotypic traits.

Hsp90 is a highly conserved molecular chaper-one that, in addition to participating in the cellstress-response system by helping to refold dena-tured proteins, also has a more specific role in sig-nal transduction. Through repeated low-affinityinteractions, Hsp90 keeps inherently unstable pro-teins involved in signal transduction pathwayspoised for action. Presumably, the rampant devel-opmental disturbances observed in Hsp83 loss-of-function mutants, or by chemically inhibitingHsp90 protein function, result from destabilizedsignal-transduction pathways that are less robustagainst genetic and environmental perturbations.Rutherford and Lindquist have shown that someof the exposed developmental changes are heritableand that it is possible to reinforce these variants

by selection to the point that they are expressedeven when wildtype Hsp90 function is restored.

Hsp90’s dual involvement in stress response andsignal transduction raises the intriguing possibil-ity that it serves to link the expression of geneticvariation to environmental stress. Rutherford andLindquist hypothesize that during stress Hsp90becomes diverted from its function in signal trans-duction through its affinity for denatured proteins.The result is the expression of normally crypticgenetic variation in Hsp90-dependent transduc-tion pathways. If this variation was purely del-eterious one might expect that the regulatoryproteins would have evolved to become indepen-dent of Hsp90, which seems plausible given thatproteins vary in their dependence on Hsp90, orthat Hsp90 through gene duplication and diver-gence evolved to decouple its two functions. Thesepossibilities present Hsp90 as the first experimen-tally accessible model system for investigating theevolution of evolvability. Rutherford and Lindquistsuggest that evolvability is increased by the un-masking of normally hidden genetic variationwhich might be useful in the adaptation of a spe-cies to a new environment. In this note we wantto discuss some of the population genetic under-pinnings and difficulties with this hypothesis, aswell as make some suggestions for empirical andexperimental approaches to testing it.

What then, is the relationship between geneticvariability of phenotypic characters and evolv-

*Correspondence to: Günter P. Wagner, Department of Ecology andEvolutionary Biology, Yale University, 165 Prospect St., PO Box 208106,New Haven, CT 06520-8106. E-mail: Gunter.Wagner@yale.edu

Received 5 May 1999; Accepted 5 May 1999.

IS HSP90 A REGULATOR OF EVOLVABILITY? 117

ability? The answer to this question depends on anumber of factors. Clearly, heritable variation ofadaptive characters increases evolvability, sincethe amount of heritable phenotypic variation lim-its the rate of response to natural selection. Buton the other hand, there are more subtle effectsby which phenotypic variability can decreaseevolvability. All of them have to do with deleteri-ous pleiotropic effects of mutations. The fitnessof a mutant usually results from the balance offitness enhancing effects on the phenotype anddeleterious side effects. The chance of a pheno-typic character to improve by natural selectiondepends on average amount of deleterious side ef-fects of mutations (“redundant variation” sensuRiedl, ’78). A phenotype that is too sensitive togenetic variation may lead to many deleteriousside effects of any mutation and thus be less evolv-able than one with lower variability. In fact,Gerhart and Kirschner (’97) have argued thatmany features of the molecular biology of a cellcan be considered as means to improve evolv-ability by preventing deleterious effects of geneticand environmental variation. This intuition is sup-ported by the mathematical analysis of popula-tion genetic models (Wagner, ’84, ’88a; Bürger, ’86).The models predict that potentially adaptive ge-netic variation is lost to selection if it is associ-ated with unconditionally deleterious effects onso-called “core characters.” Unconditionally delete-rious variation decreases evolvability, i.e., it de-creases the covariance of the adaptive characterswith fitness. It has been argued that this form offunctional dependency between adaptive charac-ters and core characters is general, rather thanexceptional, for biological systems (Wagner, ’88b).For instance, the adaptive value of variation in alocomotory organ (e.g., a bird wing) is conditionalon the performance of many other characters suchas the nervous, respiratory, and circulatory sys-tems. Deleterious variation in any of these char-acters thus makes the variation in the locomotoryorgans adaptively irrelevant. Increased variationin core characters thus removes some of the po-tentially adaptive variation of the phenotype as awhole, and consequently decreases evolvability.That is to say that the effect of phenotypic varia-tion on evolvability depends on the balance be-tween the evolvability enhancing and inhibitingeffects of phenotypic variation. In general, it ispredicted that the more complex an organism isthe more severe the negative effects of phenotypicvariation (Wagner, ’84, ’88a,b). Phenotypic stabil-ity of core characters rather than their variabil-

ity is a prerequisite for the evolvability of com-plex organisms.

The results cited above raise the question ofwhether the kind of phenotypic variation, causedby changes in the level of Hsp90 protein, in bal-ance increases or decreases evolvability. The varia-tions in Drosophila wing vein, eye and headmorphologies (Rutherford and Lindquist, ’98) re-semble unconditionally deleterious variations thatare predicted to decrease evolvability. Whether,or under what conditions, Hsp90 is an enhancerof evolvability is a question that can be investi-gated experimentally. We propose the followingexperiments to address this problem. First, thequestion of whether evolvability is enhanced canbe investigated by comparing the response ofHsp90 geldanamycin-inhibited and Hsp90-wild-type control fly strains to artificial selection onlife-history traits. Second, the question of whetherenhanced evolvability can convey a competitiveadvantage could be investigated by allowing aHsp90-inhibited fly strain and a control strain (la-beled with some neutral marker) to each adapt toa novel environment and subsequently compete.The outcome of this experiment can be directlycompared to the result of first subjecting the samestrains to competition before they adapt to thenew environment. It may even be possible to di-rectly estimate the heterozygote fitness of Hsp83loss-of-function mutants relative to wild-type al-leles when they compete in a novel environment.

Hsp90 may also influence evolvability by struc-turing the co-variability of traits. The evolvabilityof a complex organism depends on how its vari-ability is structured into semi-independent mod-ules (Bonner, ’88; Raff, ’96; Wagner, ’96; Wagnerand Altenberg, ’96). If functionally related traitsmust change in unison to preserve functionalitythen co-variation of the characters is advanta-geous to evolvability. A striking aspect of Hsp90is that it affects many seemingly unrelated pro-teins simultaneously. Do these proteins have any-thing in common? The involvement of Hsp90 inthe cellular response to temperature may providea clue to this question. Adaptation to changes intemperature is a recurrent challenge on both eco-logical and evolutionary time scales, and clearly,adjusting the temperature optima of a large num-ber of enzymes simultaneously is a daunting task.Could it be that Hsp90 serves to correlate the tem-perature adaptation of many proteins simulta-neously? And, in particular, does it serve tocorrelate the temperature adaptation of regula-tory proteins that need to be adjusted in concert

118 G.P. WAGNER ET AL.

to preserve functionality? Hsp90 may achieve thisrole by facilitating the accumulation and subse-quent release of genetic variation in target inter-acting proteins, thereby increasing the chances ofsimultaneously exposing mutations that work to-gether in the new temperature regime. Alterna-tively, evolutionary changes in Hsp90 regulationor activity may directly and simultaneously ad-just the temperature optima of its targets. Thesehypotheses could be tested by comparing Hsp90function in closely related species living under dif-ferent temperature regimes. Indeed, there is evi-dence that the regulation of another heat-shockprotein, Hsp68, differs among lizard species withdifferent temperature requirements (Ulmazov etal., ’92). Similar studies of the Hsp90 system areurgently needed.

Even if it can be shown that the Hsp90 systemis able to enhance evolvability, the question re-mains, “What selective forces shaped it?” WhileRutherford and Lindquist do not explicitly pro-pose a scenario for the evolutionary origin of theHsp90 system, one could certainly think of a sce-nario in which the Hsp83 function evolved becauseof its possible role as a mechanism for evolvability.This suggestion, however, shares a problem withsimilar ideas for biological mechanisms with a po-tential influence on evolvability, like recombina-tion or mutation rates (Maynard Smith, ’78). Theinformal consensus among population geneticistsis that evolvability is difficult to select for even ifin principle possible (Dawkins, ’89; Kauffman, ’93;Wagner and Altenberg, ’96). The problem is thatthe advantage only comes into effect during thepotentially rare episodes of adaptive evolution. Infact, in phases of adaptive equilibrium many ofthese traits are selected against. The question,then, is whether the benefits in terms of evolv-ability outweigh the disadvantages these traitshave in equilibrium, and the answer to this de-pends on how often the population experiencesdirectional selection. To answer this question re-quires theoretical work to assess what pattern ofenvironmental changes is necessary to give evolv-ability enhancers a (geometric) fitness advantage.These studies need to be supplemented by care-ful field observations like those gathered onDarwin’s finches on the Galapagos (Grant, ’86) tocompare the predicted with the actual patternsof environmental changes.

Is Hsp90 a regulator of evolvability? Hsp90clearly holds great promise as a model system forunderstanding the genetic basis for evolvability,and indeed shows some indications of (broad-sense) adaptation for this purpose. However, it istoo early to conclude that the Hsp90 system is aclear-cut example of a mechanism directly favor-ing evolvability. Fortunately, the theoretical de-velopments and experimental evidence needed tofirmly test this idea are within our reach.

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