AMER. ZOOL., 39:857-864 (1999)
Organismal, Ecological, and Evolutionary Aspects of Heat-Shock Proteins
and the Stress Response: Established Conclusions and Unresolved Issues'
MARTIN E. FEDER 2
Department of Organismal Biology & Anatomy, The Committee on Evolutionary Biology,
and The College, The University of Chicago, 1027 East 57th Street, Chicago Illinois 60637
SYNOPSIS. HOW heat-shock proteins function in diverse organisms from diverse
environments, and how this diversification has evolved, is an emerging focus of
research on molecular chaperones. As molecular chaperones, heat-shock proteins
play diverse cellular roles, typically in minimizing dysfunction that may occur
when other proteins are in non-native conformations. The standard aspects of these
roles in vitro, in isolated cells, and in typical model organisms in the laboratory
are now well-established, as are the ubiquity of heat-shock proteins in organisms,
the range of stresses that induce heat-shock proteins, the major families of heatshock proteins, their expression in nature, and their variation along natural gradients of stress. These aspects may no longer require extensive examination. By
contrast, the frequency of natural expression of heat-shock proteins, their exact
physiological roles in stress tolerance at levels of biological organization above the
cell, the exact molecular mechanisms by which heat-shock protein expression and
function has become tuned to the prevailing level of environmental stress, and the
fitness consequences of heat-shock protein expression in nature are among the
numerous unresolved issues in this area.
gions in other proteins and so lead to agAll organisms are critically dependent on gregations of proteins that at worst are cyproteins for maintenance, growth, reproduc- totoxic and at best reduce the pool of function, and the synthesis of other essential tional protein in the cell. This problem is as
biomolecules. In turn, the biologically rel- ancient and widespread as life itself—and
evant function of proteins is critically de- so is one major biological solution: molecpendent on the formation/dissolution of ular chaperones. Molecular chaperones are
"weak" chemical bonds (Hochachka and a class of proteins that function to minimize
Somero, 1984; Somero, 1995). This re- the problems that arise when other proteins
quirement irrevocably sensitizes organisms are in non-native conformations (Nover,
to environmental factors (heat, concentra- 1991; Gething and Sambrook, 1992; Mortion, toxicants) that affect weak bonds; if imoto et al, 1994; Feder et al, 1995; Hartl,
these factors are present in excess or in in- 1996; Gething, 1997; Feder and Hofmann,
sufficient quantity, the result is departure 1998, 1999). Molecular chaperones can recfrom the native structure of proteins, with ognize (typically at exposed hydrophobic
ultimately devastating consequences for residues) and bind to non-native proteins
protein (and all other biological) functions and release them in highly regulated fash(Hochachka and Somero, 1984; Somero, ion, allowing the bound proteins to attain/
1995). One particular problem is that non- re-attain their native conformation and/or
native proteins expose regions that are sat- be targeted for degradation and removal
isfied in the native structure; these exposed from the cell. In so doing, molecular chapunsatisfied regions can bind other such re- erones minimize the probability of other
proteins forming unproductive or cytotoxic
1
From the Symposium Organismal, Ecological and aggregations.
Evolutionary Significance of Heat Shock Proteins and
Molecular chaperones were originally
the Heat Shock Response presented at the Annual
characterized,
named, and investigated as
Meeting of the Society for Comparative and Integrative Biology, 6-10 January 1999, at Denver, Colorado. "heat-shock proteins" (Hsps) because heat
2
can induce those first discovered. To date,
E-mail: [email protected]
INTRODUCTION
857
858
MARTIN E. FEDER
the major focus of research on molecular
chaperones and Hsps has been on the central tendencies or generalities of their structure, function, regulation, encoding genes,
and role in health and disease as elucidated
by laboratory study of the major model organisms {E. coli, yeast, Drosophila, mammalian cells in culture, etc.) or purified molecular chaperones in vitro. Interest in these
topics has spawned a massive community
of investigators and a correspondingly large
body of discovery (now including more
than 15,000 references) (Nover, 1991; Gething and Sambrook, 1992; Morimoto et
al, 1994; Feder et al., 1995; Hartl, 1996;
Gething, 1997; Feder and Hofmann, 1998;
Feder and Hofmann, 1999). An alternative
focus, on how and why molecular chaperones and Hsps differ among diverse tissues,
organs, individual organisms, and higher
taxa undergoing ecologically and evolutionarily relevant stresses, has long received
some attention but is now achieving critical
mass due to the joint efforts of molecular
chaperone investigators and researchers
from outside this community. The intent of
the accompanying group of symposium
presentations is to highlight this achievement and build upon it. Here I provide a
general background for these papers, offer
opinions on several pitfalls and conclusions
that may be so well-established as to require little additional substantiation, and
suggest several possibilities for future research.
Particularly for researchers who are new
to molecular chaperones and heat-shock
proteins, comprehending existing findings
can be critical for making future discoveries
and avoiding redundant or naive research.
Unfortunately, the existing findings (see
above references) are so massive that they
are difficult to master. This symposium's organizers have compiled a database (Feder
and Hofmann, 1998) comprising perhaps
the 10% of literature references (as of mid1998) that are most relevant to the organismal, ecological, and evolutionary diversity of the heat-shock response, and urge
that those contemplating research in this
area take time to scan the titles. Among the
salient conclusions emerging from this literature are (Feder and Hofmann, 1999):
• Molecular chaperones are a universal aspect of living things. All organisms studied to date have genes that encode and
express chaperones.
• Molecular chaperones play diverse roles
in the unstressed cell and are induced by
or cope with every form of environmental stress that has been studied.
• Molecular chaperones and their encoding
genes are extraordinarily highly conserved. Most can be assigned to families
on the basis of molecular weight, sequence homology, and function: hspllO,
hsplOO, hsp90, hsp70, hsp60, hsp40,
hsplO, small hsp, and many others. In eukaryotes, some families have many
members, often playing diverse roles. Often, these proteins accomplish their functions as chaperone machines involving
many individual polypeptides.
• Molecular chaperones, while important,
are one of many molecular mechanisms
of stress tolerance.
These findings have important implications for the choice of research problems
and methodologies involving organismal,
ecological, and evolutionary diversity of
the heat-shock response (Feder and Hofmann, 1999). Finding that an as-yet-unexamined species expresses a member of a
well-known Hsp family in response to heat
shock (or, for that matter, another stress) is
no longer particularly novel. The diversification of Hsps within each family necessitates use of techniques that can differentiate
among family members, and the multiplicity of Hsps and non-Hsp mechanisms of
stress tolerance increasingly requires genetic or experimental manipulation to establish
the function of any single Hsp and its consequences. Such techniques and manipulations are often unavailable or problematic
for non-standard organisms. Despite these
pitfalls, however, the relative paucity of
study of non-standard organisms represents
an awesome opportunity for significant discovery.
NATURAL HSP-INDUCING STRESS
Because so much of the research on Hsps
has been undertaken in the laboratory, the
question naturally arises as to whether and
HEAT-SHOCK PROTEINS: INTRODUCTION
how frequently organisms in nature undergo Hsp-inducing stress. Whether organisms
undergo such stress in the wild is no longer
equivocal, having been demonstrated for
thermal stress in corals, various intertidal
invertebrates, brine shrimp, and fishes in
aquatic ecosystems; and in certain plants,
Drosophila larvae and pupae, ants, other insects undergoing laboratory diapause at natural temperatures, salamanders, and birds
and mammals undergoing physiological
levels of heterothermy in terrestrial ecosystems (Feder and Hofmann, 1999). In the
present symposium, contributions by Hofmann (1999) and Carey et al. (1999) exemplify such research, and Clos and Krobitsch (1999) expand its scope to parasitic
organisms serially infesting ectothermic
and endothermic hosts. Virtually every
stress studied can induce Hsps (Feder and
Hofmann, 1999), and thus a relationship between Hsps and endocrine stress responses
in fish, as Iwama et al. (1999) report, is not
surprising. Clegg et al. (1999) implicate the
massive accumulation of an a-crystallin
Hsp family member in enabling encysted
brine-shrimp (Artemia) embryos to survive
years of natural anoxia. Indeed, numerous
investigators and commercial concerns
have begun to exploit this feature to assess
biological and anthropogenic stress (Hightower, 1998). How frequently wild organisms undergo Hsp-inducing stress, by contrast, is largely unknown for several reasons: First, due to their large size, human
investigators can fundamentally misperceive the environmental stress impinging on
non-human organisms, which on average
are much smaller than humans. Second, human investigators interested in stress quite
naturally choose to study subjects that likely undergo stress, thereby potentially overestimating the frequency of stress. For example, a common theme of all the investigations of natural/semi-natural Hsp expression listed above is that their subject taxa
have been chosen on the basis of presumed
if not demonstrable exposure to natural
temperature stress. Thus, we do not know
if natural exposure to stress and consequent
gene expression is routine, frequent, or rare;
and whether the studied species are typical
or deviant in their exposure to stress and
859
gene expression. Such knowledge is needed
because it is fundamental in evaluating the
effects of stress and their significance.
Moreover, it requires a speciomic approach
(i.e., a comprehensive survey of species or
species sampling regime that is unbiased
with respect to the variable under study,
likelihood of stress).
VARIATION ALONG MICROGEOGRAPHIC,
GEOGRAPHIC, AND CLIMATIC STRESS
GRADIENTS
If organisms in nature undergo variation
in stress along gradients of latitude, altitude,
season, rainfall, competitive intensity, etc.,
then a potential outcome is that such organisms may demonstrate corresponding differences in their stress-induced expression
of molecular chaperones. Expression could
vary in at least five non-exclusive ways
(Fig. 1), in the: (a) Magnitude of expression, with organisms from a high-stress environment expressing more Hsps than organisms from low-stress environments; (b)
Threshold for expression, with organisms
from a low-stress environment expressing
Hsps at lower levels of stress than organisms from high stress environments; (c)
Breadth of expression, with organisms from
variable-stress environments expressing
Hsps over a broader range of stresses than
organisms from environments with constant
levels of stress; (d) Kinetics of expression,
with organisms from a high-stress environment expressing Hsps more rapidly and recovering more rapidly than organisms from
a low-stress environment; (e) Efficacy of
function, with the Hsps of organisms from
high-stress environments being more effective chaperones than equimolar amounts of
Hsps of organisms from low-stress environments, all else equal. Interspecific comparisons of amount and/or threshold of expression are most numerous, yielding expected
outcomes for archaebacteria, eubacteria, algae, yeast, coelenterates, mollusks, insects,
crustaceans, fish, lizards, and plants. In the
present symposium, for example, Hofmann
(1999) demonstrates such variation in intertidal marine organisms, Hightower et al.
(1999) in desert fishes, Krebs and Bettencourt (1999) in Drosophila species, and
Heckathorn et al. (1999) within several ter-
860
MARTIN E. FEDER
A. Magnitude
D. Kinetics
1
Inducing stress (Intensity, duration)
B. Threshold
\
A/V
''"ii,
Time (min)
E. Efficacy
Inducing stress (intensity, duration)
C. Breadth
High
Low
stress
stress
'species'
'species'
Inducing stress (Intensity, duration)
FIG. 1. Expected variation in magnitude of heat-shock protein (Hsp) expression or function beiween organisms
from differing environments. Organisms could be from different species, higher taxa, populations of a species,
individuals from a population, or different developmental stages of an individual. In A-C, Hsp expression
increases and then decreases with increasing environmental stress, but the shape and position of these functions
differ. In A, organisms from environments with large amounts of stress (solid line) express more Hsp than
organisms from environments with lesser amounts of stress (broken line). In B, organisms from warm environments (solid line) begin to express Hsps at lower temperatures than organisms from cool environments (broken
line). In C, organisms from environments that vary greatly in the level of ambient stress (solid line) express
Hsps over a broader range of stress levels than organisms from environments with relatively constant levels of
stress (broken line), where the mean level of stress is identical. In D, organisms from environments with large
amounts of stress (solid line) express Hsps more rapidly and recover more rapidly than organisms from environments with lesser amounts of stress (broken line). In E, equimolar amounts of Hsps of organisms from
environments with large amounts of stress (solid line) are more effective in refolding denatured proteins than
Hsp of organisms from environments with lesser amounts of stress (broken line). These patterns are not mutually
exclusive; a species pair could exhibit some or all of these patterns simultaneously. Many actual studies yield
results corresponding to A and B; the other patterns re less well-investigated. In each case, only two organisms
are shown for clarity; rigorous studies often require more comparisons to avoid various problems (Garland and
Adolf, 1994). Also, none of the curves need be symmetric or even regular as shown.
restrial plant species. Additionally, Hsp expression can vary along "internal" gradients of stress during parasitism, symbiosis,
development, aging, disease, and hibernation (Carey et ai, 1999; Clos and Krobitsch, 1999; Tatar, 1999). Countergradient
variation is less well-examined for breadth
of expression, and has not yet been examined for efficacy of function. Also, significant gaps in our knowledge are conspicuous, although several of the accompanying
papers begin to fill them. Seldom have interspecific studies controlled for the confounding influence of phylogeny. Although
stress and Hsp expression are potentially
major factors in determining species distribution and abundance, systematic studies of
Hsp expression and underlying stress across
a species' ranges are lacking, as are rigorous comparisons of the stress response in
widespread and narrowly distributed spe-
cies. The prospect of global climate change
further increases the importance of such
studies.
Given that patterns of Hsp expression
vary among species and along environmental gradients, a related issue is how evolution has engineered these changes; i.e.,
what genes have been modified to result in
differing Hsp expression, and in what
ways? Laboratory studies of Hsp expression and its regulation have elucidated a
host of candidate mechanisms that could be
modified: the coding regions of the hsp
genes themselves, non-coding regions (promoter; 3'- and 5'-untranslated regions,
which affect message stability), hsp gene
copy number, trans-acting factors (e.g.,
heat-shock factors, heat-shock binding protein), and the stability of proteins whose denaturation triggers the stress response,
among others. Ecological and evolutionary
HEAT-SHOCK PROTEINS: INTRODUCTION
variation in these mechanisms is just beginning to receive scrutiny. In addition to the
work that Hofmann (1999), Heckathorn et
al. (1999), and Hightower et al (1999) report in their accompanying papers, the research of Thomas Bosch and colleagues on
Hydra is a superb example. In congeneric
species inhabiting relatively variable and
constant environments, the constant-environment species exhibits greatly reduced
Hsp70 expression, which Brennecke et al.
(1998) attribute to reduced message stability encoded in the untranslated region of the
hsp70 gene. Recently, ecological variation
in activation of the heat-shock factor(s)
(HSF, which coordinately regulates the expression of heat-shock genes) is receiving
considerable attention (Sarge et al., 1995;
Airaksinen et al, 1998).
FROM HEAT-SHOCK PROTEINS TO FITNESS
While heat-shock proteins are often invoked as contributing to evolutionary fitness by enhancing stress tolerance and/or as
putative adaptations, rigorous examinations
of these claims are surprisingly few. A first
step in establishing adaptation is demonstrating a direct cause-and-effect relationship between an individual Hsp and enhanced stress tolerance. As Feder and Hofmann (1999, p. 258) state: "Literally thousands of studies report correlations between
Hsp expression, diverse biological functions in the face of stress, and stress tolerance—but these typically conclude that
their findings are at best consistent with a
role of one or more Hsps in stress tolerance." Increasingly, unambiguous tests of
Hsps' fitness consequences require tools for
genetic and experimental manipulation of
single Hsps in whole organisms or parts
thereof, but these tools are not yet available
for many organisms of ecological and evolutionary interest. Nonetheless, a growing
number of studies of typical model organisms in the laboratory is revealing tissue-,
organ-, and organism-level stress tolerance
phenotypes that are clearly attributable to
Hsps (Table 1 in Feder and Hofmann,
1999). These phenotypes include stress tolerance, as originally assumed by the discoverers of the stress response, but many
others as well. For example, increased ex-
861
pression of Hsp70 is sufficient to extend
lifespan in Drosophila, as Tatar demonstrates in his accompanying manuscript.
Surprisingly (Feder, 1996), manipulations
of a single Hsp (or the gene encoding it)
are often sufficient for a major effect upon
stress tolerance.
Although the foregoing work clearly establishes that Hsps have diverse protective
or restorative effects in tissues, organs, and
whole organisms, how these effects arise is
remarkably poorly understood. Presumably,
stress-intolerant organisms have some especially vulnerable protein(s) or other cellular component(s) that fail under stress,
and Hsps confer inducible stress tolerance
by somehow repairing this damage or preventing it from occurring through their
function as molecular chaperones. For example, recent work has implicated the digestive organs (Krebs and Feder, \991b;
Feder and Krebs, 1998; Krebs and Feder,
1998) and development of Drosophila and
the photosynthetic apparatus of plants (see
accompanying work of Heckathorn et al,
1999) as especially vulnerable targets with
correspondingly distinctive patterns of Hsp
expression {e.g., Marcuccilli et al, 1996),
but the exact identity of their most vulnerable molecules (presumably proteins) remains to be discovered. Many mammalian
diseases are thought to be due to defects in
chaperoning of essential proteins (Thomas
et al, 1995), but comparable explanations
of Hsp-mediated protection of key structures under stress remains a goal for the future.
Increased expression of Hsps, however,
is not uniformly beneficial. Increasing numbers of studies report deleterious effects of
Hsps (Krebs and Feder, 1997a; Feder and
Hofmann, 1999; Krebs and Bettencourt,
1999), attributable to at least two potential
mechanisms. First, expression of Hsps,
which can be massive, may consume so
much biosynthetic substrate and occupy so
much of the protein expression apparatus
that not enough remains for other important
biosynthesis (Heckathorn et al, 1996a, b).
Second, presumably by binding other proteins too tenaciously, Hsps can be toxic
(Feder et al, 1992; Krebs and Feder,
1997a). Although the generality and details
862
MARTIN E. FEDER
of both mechanisms are yet to be established, the deleterious consequences themselves are clear and apparently trade off
against the benefits of Hsps in an evolutionary sense, either constraining directional selection for increased Hsp expression or
necessitating especially effective autoregulation of Hsp expression.
A second step in rigorously establishing
the adaptive significance of an Hsp is showing how it affects fitness in the wild. In this
regard, the demonstration that the subject
organism undergoes stress in nature (see
above) becomes critical, as does demonstrating that altered stress tolerance affects
fitness in natural populations. The tolerance-fitness relationship, however, can be
complex indeed. Unlike laboratory organisms, wild organisms may seldom encounter single stresses and combinations of
stresses may have unexpected impacts. The
impact (and the ability of Hsps to ameliorate it) may be in terms other than survival
vs. death of the affected organisms. For example, in natural populations of Drosophila
melanogaster, natural heat shock of embryos, larvae, and pupae may induce severe
morphological abnormalities in the adults
that eclose from these stages (Roberts and
Feder, 1999). The affected flies are alive,
but are likely to have greatly reduced fitness. In a sibling species, Drosophila simulans, natural heat shock and Hsp expression may well mediate release from a bacterially-mediated reproductive incompatibility (Feder et al, 1999). To date, even
demonstrations of simple relationships between Hsps, thermotolerance, and fitness
are still extremely rare (Hashmi et al.,
1998).
A final step in establishing the adaptive
nature of an Hsp is in showing that it actually satisfies the other criteria for origin
and maintenance by natural selection: interindividual variation and heritability. The
conundrum here is that Hsps and their encoding genes are extremely ancient and
highly conserved, which could override
small-scale variation. A surprisingly large
number of studies have now established
both interindividual variation and heritability of Hsps and their encoding genes (Favatier et al., 1997; Feder and Hofmann,
1999). In many of these cases, moreover,
interindividual variation is correlated with
stress tolerance. For example, Heckathorn
et al. (1999) report genotypic variation in a
small Hsp that is an important determinant
of photosynthetic thermotolerance.
Given these findings, Hsps should be
amenable to experimental evolution. Indeed, laboratory evolution at high temperatures results in correlated changes in
Hsp70 expression and thermotolerance in
Drosophila melanogaster (Bettencourt et
al., 1999), and selection for resistance to
thermal paralysis alters the frequency of
polymorphisms in at least two heat-shock
genes in this species (McColl et al., 1996;
McKechnie et al, 1998).
LARGE-SCALE EVOLUTIONARY VARIATIONS
IN HSPS
The extraordinary conservation of hsp
gene sequences and Hsp function make
these genes and proteins superb subjects for
historical studies of evolutionary change
(Feder and Hofmann, 1999). Hsps are recognizable in all kingdoms of living things,
and in species resembling those thought to
have arisen early in the history of life on
earth. Apparently, the need for molecular
chaperoning of proteins is as old as proteins
themselves. Comparative studies have now
suggested how the extant hsp genes have
arisen and diversified from ancestral genes,
have taken on novel roles as chaperones,
and have assumed non-chaperone functions
{e.g., a modified Hsp is a major component
of the lens of the eye (de Jong et al, 1993)).
In turn, hsp genes can yield distinctive insights into the phylogenetic relationships
and evolutionary origins of the major
groups of extant organisms and their organelles (Gupta, 1995).
Have heat-shock proteins themselves affected the course of evolution? One theme
in evolutionary biology is that phenotypic
plasticity (of which Hsps can be a significant component) buffers organisms against
continuous evolution in response to routine
environmental stress, leaving organisms especially vulnerable to large-scale changes
in environmental stress (Schlicting and Pigliucci, 1998). Alternatively, Hsps and stress
together may actually potentiate evolution
HEAT-SHOCK PROTEINS: INTRODUCTION
of novel traits (Rutherford and Lindquist,
1998). Rutherford and Lindquist posit that,
in the absence of stress, Hsp90 enables developmentally defective proteins to function normally and so preserves their encoding genes, which selection would otherwise
modify. Upon stress, the ensuing damage
titrates Hsp90 away from these proteins,
leaving them free to initiate abnormal development. Whereas the consequent abnormal development is usually harmful if not
lethal, occasionally it may represent beneficial phenotypic novelty that can then be
assimilated genetically. Future study will no
doubt elucidate the importance of such phenomena for evolution in nature.
CONCLUSION
In too many cases, "molecular," "functional," and "evolutionary" approaches
have either proceeded along separate paths
or have been adversarial (Wilson, 1994;
Watt, 1999). The recent explosion of interest in organismal, ecological, and evolutionary aspects of the heat-shock response
(as typified by the accompanying papers) is
not unique in blending these approaches,
but is an especially powerful demonstration
of the benefits of doing so. Moreover, this
particular focus on the heat-shock response
is still sufficiently novel that many issues
of great significance are virtually unexamined, although other issues may have now
received so much scrutiny that additional
work is not justifiable. A challenge for all
investigators in this area will be to keep
abreast of the massive amount of information being generated individually in the molecular, functional, and evolutionary realms
and to conduct research that meets the rising standards of each discipline. Nonetheless, as the accompanying papers demonstrate, meeting this challenge offers extraordinary insights into a molecular response to
stress and how it has evolved.
ACKNOWLEDGMENTS
The Symposium Organismal, Ecological
and Evolutionary Significance of Heat
Shock Proteins and the Heat Shock Response was supported by National Science
Foundation grant IBN99-04107 and the Society for Integrative and Comparative Bi-
863
ology; all participants are extremely grateful for this support. The author's research
was supported by NSF grant IBN97-23298.
I thank B. Bettencourt, G. Hofmann, D.
Lerman, and S. Roberts for helpful comment. Opinions expressed in this manuscript are my own and not necessarily
shared by the other symposium participants.
REFERENCES
Airaksinen, S., C. M. I. Rabergh, L. Sistonen, and M.
Nikinmaa. 1998. Effects of heat shock and hypoxia on protein synthesis in rainbow trout (Oncorhynchus mykiss) cells. J. Exp. Biol. 201:25432551.
Bettencourt, B. R., M. E. Feder, and S. Cavicchi. 1999.
Experimental evolution of Hsp70 expression and
thermotolerance in Drosophila melanogaster.
Evolution 53:484-492.
Brennecke, T., K. Gellner, and T. C. G. Bosch. 1998.
The lack of a stress response in Hydra oligactis
is due to reduced hsp7O mRNA stability. Eur. J.
Biochem. 255:703-709.
Carey, H. V., N. S. Sills, and D. A. Gorham. 1999.
Stress proteins in mammalian hibernation. Amer.
Zool. 39:825-835.
Clegg, J. S., J. K. Willsie, and S. A. Jackson. 1999.
Adaptive significance of a small heat shock/acrystallin protein (p26) in encysted embryos of
the brine shrimp, Anemia franciscana. Amer.
Zool. 39:836-847.
Clos, J., and S. Krobitsch. 1999. Heat shock as a regular feature of the life cycle of Leishmania parasites. Amer. Zool. 39:848-856.
de Jong, W. W., J. A. Leunissen, and C. E. Voorter.
1993. Evolution of the alpha-crystallin/small heatshock protein family. Mol. Biol. Evol. 10:103—
126.
Favatier, E, L. Bornman, L. E. Hightower, E. Gunther,
and B. S. Polla. 1997. Variation in hsp gene expression and Hsp polymorphism: do they contribute to differential disease susceptibility and stress
tolerance? Cell Stress & Chaperones 2:141-155.
Feder, M. E. 1996. Ecological and evolutionary physiology of stress proteins and the stress response:
The Drosophila melanogaster model. In I. A.
Johnston and A. F. Bennett (eds.), Animals and
temperature: Phenotypic and evolutionary adaptation, pp. 79—102. Cambridge University Press,
Cambridge.
Feder, J. H., J. M. Rossi, J. Solomon, N. Solomon, and
S. Lindquist. 1992. The consequences of expressing Hsp70 in Drosophila cells at normal temperatures. Genes & Development 6:1402-1413.
Feder, M. E. and G. E. Hofmann. 1998. Evolutionary
and ecological physiology of heat-shock proteins
and the heat-shock response: A comprehensive
bibliography. Supplement to Ann. Rev. Physiol.
61: http://www.annurev.org/sup/material.htm.
Feder, M. E. and G. E. Hofmann. 1999. Heat-shock
proteins, molecular chaperones, and the stress re-
864
MARTIN E. FEDER
sponse: Evolutionary and ecological physiology.
Ann. Rev. Physiol. 61:243-282.
Feder, M. E., T. L. Karr, W. Yang, J. M. Hoekstra, and
A. C. James. 1999. Interaction of Drosophila and
its endosymbiont Wolbachia: Natural heat shock
and the overcoming of sexual incompatibility.
Amer. Zool. 39:363-373.
Feder, M. E. and R. A. Krebs. 1998. Natural and genetic engineering of thermotolerance in Drosophila melanogaster. Amer. Zool. 38:503—517.
Feder, M. E., D. A. Parsell, and S. L. Lindquist. 1995.
The stress response and stress proteins. In J. J.
Lemasters and C. Oliver (eds.), Cell biology of
trauma, pp. 177-191. CRC Press, Boca Raton,
FL.
Garland, T. and S. C. Adolf. 1994. Why not to do twospecies comparative studies: Limitations on inferring adaptation. Physiol. Zool. 67:797-828.
Gething, M. J. (ed.) 1997. Guidebook to molecular
chaperones and protein-folding catalysts. Oxford
University Press, Oxford.
Gething, M. J. and J. Sambrook. 1992. Protein folding
in the cell. Nature 355:33-45.
Gupta, R. S. 1995. Phylogenetic analysis of the 90 kD
heat shock family of protein sequences and an examination of the relationship among animals,
plants, and fungi species. Mol. Biol. Evol. 12:
1063-1073.
Haiti, F. U. 1996. Molecular chaperones in cellular
protein folding. Nature 381:571-580.
Hashmi, S., G. Hashmi, I. Glazer, and R. Gaugler.
1998. Thermal response of Heterorhabditis bacteriophora transformed with the Caenorhabditis
elegans hsp70 encoding gene. J. Exp. Zool. 281:
164-170.
Heckathorn, S. A., C. A. Downs, and J. S. Coleman.
1999. Small heat-shock proteins protect electron
transport in chloroplasts and mitochondria during
stress. Amer. Zool. 39:865-876.
Heckathorn, S. A., G. J. Poeller, J. S. Coleman, and R.
L. Hallberg. 1996a. Nitrogen availability alters
patterns of accumulation of heat stress-induced
proteins in plants. Oecologia 105:413-418.
Heckathorn, S. A., G. J. Poeller, J. S. Coleman, and R.
L. Hallberg. 1996b. Nitrogen availability and vegetative development influence the response of ribulose 1,5-bisphosphate carboxylase/oxygenase,
phosphoenolpyruvate carboxylase, and heat-shock
protein content to heat stress in Zea mays L. Int.
J. Plant Sci. 157:588-595.
Hightower, L. E. 1998. The promise of molecular biomarkers for environmental monitoring. Biol.
Chem. 379:1213-1215.
Hightower, L. E., C. E. Norris, P. J. dilorio, and E.
Fielding. 1999. Heat shock responses of closely
related species of tropical and desert fish. Amer.
Zool. 39:877-888.
Hochachka, P. W. and G. N. Somero. 1984. Biochemical adaptation. Princeton University Press,
Princeton.
Hofmann, G. E. 1999. Ecologically relevant variation
in induction and function of heat shock proteins
in marine organisms. Amer. Zool. 39:889-900.
Iwama, G. K., M. M. Vijayan, R. Forsyth, and P. Ackerman. 1999. Heat shock proteins and physiological stress in fish. Amer. Zool. 39:901-909.
Krebs, R. A. and B. R. Bettencourt. 1999. Evolution
of thermotolerance and variation in the heat shock
protein, Hsp70. Amer. Zool. 39:910-919.
Krebs, R. A. and M. E. Feder. 1997a. Deleterious consequences of Hsp70 overexpression in Drosophila
melanogaster larvae. Cell Stress & Chaperones 2:
60-71.
Krebs, R. A. and M. E. Feder. 1997i>. Tissue-specific
variation in Hsp70 expression and thermal damage in Drosophila melanogaster larvae. J. Exp.
Biol. 200:2007-2015.
Krebs, R. A. and M. E. Feder. 1998. Hsp70 and larval
thermotolerance in Drosophila melanogaster:
How much is enough and when is more too much?
J. Insect Physiol. 44:1091-1101.
Marcuccilli, C. J., S. K. Mathur, R. I. Morimoto, and
R. J. Miller. 1996. Regulatory differences in the
stress response of hippocampal neurons and glial
cells after heat shock. J. Neurosci. 16:478-485.
McColl, G., A. A. Hoffmann, and S. W. McKechnie.
1996. Response of two heat shock genes to selection for knockdown heat resistance in Drosophila
melanogaster. Genetics 143:1615—1627.
McKechnie, S. W., M. M. Halford, G. McColl, and A.
A. Hoffmann. 1998. Both allelic variation and expression of nuclear and cytoplasmic transcripts of
hsr-omega are closely associated with thermal
phenotype in Drosophila. Proc. Natl. Acad. Sci.
U.S.A. 95:2423-2428.
Morimoto, R. I., A. Tissieres, and C. Georgopoulos.
(eds.) 1994. Heat shock proteins: Structure, function and regulation. Cold Spring Harbor Lab.
Press, Cold Spring Harbor, NY.
Nover, L. (ed.) 1991. Heat shock response. CRC Press,
Boca Raton, FL.
Roberts, S. P. and M. E. Feder. 1999. Natural hyperthermia and expression of the heat-shock protein
Hsp70 affect developmental abnormalities in Drosophila melanogaster. Oecologia (In press)
Rutherford, S. L. and S. Lindquist. 1998. Hsp90 as a
capacitor for morphological evolution. Nature
396:336-342.
Sarge, K. D., A. E. Bray, and M. L. Goodson. 1995.
Altered stress response in testis. Nature 374:126.
Schlicting, C. D. and M. Pigliucci. 1998. Phenotypic
evolution: A reaction norm perspective. Sinauer
Associates, Inc., Sunderland, MA.
Somero, G. N. 1995. Proteins and temperature. Ann.
Rev. Physiol. 57:43-68.
Tatar, M. 1999. Evolution of senescence: Longevity
and the expression of heat shock proteins. Amer.
Zool.: 39:920-927.
Thomas, P. J., B. H. Qu, and P. L. Pedersen. 1995.
Defective protein folding as a basis of human disease. Trends Biochem. Sci. 20:456-459.
Watt, W. B. 1999. Avoiding paradigm-based limits to
knowledge of evolution. Evol. Biol. (In press)
Wilson, E. O. 1994. Naturalist. Island Press/Shearwater Books, Washington, D.C.