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Heat-shock and genetic dominance

Fri Jul 29 08:56:29 EST 1994

   This certainly seems to be a hot topic! I am getting many reprint requests
for a paper I wrote on this topic. It is not possible to respond to them all.
So papers this electronic publication can satisfy some of the demand.

Sincerely,  Don Forsdyke


J. Theor. Biol. 167, 1-5 (1994)

           The heat-shock response and the molecular basis of genetic dominance

                                     D. R. FORSDYKE
                      Department of Biochemistry, Queen's University,
                             Kingston, Ontario, Canada K7L3N6


Wild type alleles are usually dominant over deleterious mutant alleles. For a
particular pair of
such alleles possible populations include a wild-type homozygote population, a
population, and a mutant homozygote population. Fisher's theory that dominance
would evolve
by selection acting on the heterozygote subpopulation has lost ground in favour
of the "dose-
response" theory under which dominance is an incidental consequence of selection
acting on the
wild type homozygote population. This postulates a "margin of safety" in the
quantity of wild-
type gene product so that heterozygotes with only one copy of a wild-type allele
still have
sufficient product for normal function. The selective force postulated to lead
to the evolution of
this margin of safety is some unspecified "extreme environmental disturbance".
I have proposed
elsewhere that the heat-shock response evolved very early as part of an
intracellular system for
self/not-self discrimination. I here propose that the rapid decrease in quantity
of most normal
proteins occurring in the heat-shock response would have provided a sufficient
selective force
for the margin of safety to have evolved.

                                    1. Two Theories of Dominance

Wild type alleles are usually dominant over deleterious mutant alleles. Possible
mechanisms of
dominance have been the subject of much debate, notably the classic dual between
Fisher and
Wright. Fisher (1931) proposed that heterozygotes begin with codominant
expression of alleles,
which are then subject to modification by the products of other genes (modifier
eventually, following evolutionary selection acting on the heterozygote
subpopulation, only the
wild-type phenotype is observed in heterozygotes. Growing evidence against the
generality of
Fisher's theory (Charlesworth, 1979; Orr, 1991), makes it appropriate that an
alternative theory,
the "dose-response" or "physiological" theory (Haldane, 1930; Muller, 1932;
Wright, 1977),
should receive closer scrutiny.  A detailed enzyme kinetic analysis of the
latter has been
provided by Kacser & Burns (1980). A simpler enzyme kinetic model presented here
a better basis for understanding the possible role of heat shock proteins in the
evolution of

                                     2. The Dose-Response Theory

  The dose-response theory states that the quantity of the product of a dominant
wild type gene
in a homozygote is so much in excess of the needs of the organism that halving
this quantity,
as might occur in a heterozygote containing a wild type allele and a mutant
allele (encoding a
non-functional product), will not change the phenotype. Figure 1 shows a plot of
quantifiable phenotypic feature against the dose of the product of a gene
concerned with that
phenotype. At low doses of gene product (indicated by point X on the curve) the
                 |         * * * * * * * *             Figure 1
                 |       *   Y        Y'
                 |     *
   Phenotype     |    * X
   (Rate of      |   *
    enzyme       |  * X'
    product      | *
    formation)   |________________________

                  Dose of gene product (quantity of enzyme)
                                                     [Legend below]
depends directly on gene product quantity. Halving the dose (point X') halves
the phenotypic
parameter. At high doses of gene product a plateau is approached when some other
factor becomes limiting (e.g.
the availability of substrate A; see later). Under these conditions increasing
the quantity of gene
product has a minimal effect on the phenotype. In the case of the wild type
homozygote it is
postulated that the normal amount of gene product corresponds to a point well
along the plateau
of the curve (indicated by point Y). This creates a "factor of safety" (Haldane,
1930), or
"margin of stability and security" (Muller, 1932), so that halving the amount of
the product (to
point Y') has no effect on phenotype.

              3. Difference between Rate-Limiting and Non-Rate-Limiting Steps

  In many cases the phenotype will be the result of a series of enzyme catalyzed
reactions such
as shown in Figure 2. Substrate A is converted to end product D through a series
Figure 2
             /                  \
         _  /       E2    E3    |
           |     B ---> C --->  D
           E1   /
      ----> A
intermediates (B, C), catalyzed by enzymes E1, E2, and E3. The first step of the
pathway, the
rate-limiting step, is subject to feed-back inhibition by D. Using radioactively
labelled A and
short incubation periods, it has been shown experimentally that the system may
be modelled as
if in vivo the intermediate steps had no effect on the reaction rate (Forsdyke,
  Figure 3 shows hypothetical in vivo substrate dose-response curves for the
three steps in the
                  | E1              E2                 E3
                  |       * * * *           * * * *            * * * *
Rate of Enzyme    |     *       ^         *                  *
product formation |    *                 *                  *
                  |   *                 *^                 *^
                  |  *                 *                  *
                  | *                 *                  *
                  |______________    ______________     ________________
                         [A]              [B]                [C]
                                  Substrate concentration

pathway. The vertical arrows indicate the normal substrate concentration
existing in vivo. In the
case of the rate-limiting enzyme E1 the in vivo concentration of A must, by
correspond with the plateau of the dose-response curve, so that enzyme
concentration, and not
substrate concentration, is rate-limiting. This would correspond to point X on
the plot of reaction
rate versus enzyme concentration (Fig. 1). In the case of the non-rate-limiting
enzymes E2 and
E3, the normal substrate concentration corresponds to the ascending limbs of the
substrate dose-response curves (Fig. 3). There is ample enzyme to accommodate
fluctuations in
availability of the substrates B and C (the products of E1 and E2,
respectively). This quantity of
enzyme would correspond to point Y on a plot of reaction rate versus enzyme
(Fig. 1). Thus, after a molecule of A has squeezed through the "bottle-neck" E1
to become B,
subsequent chemical modifications by E2 and E3 do not influence the rate of
accumulation of the
end-product D.
  It can be seen that the situation with the non-rate-limiting enzymes (E2, E3)
corresponds to the
"margin-of-safety" scenario. In the case of the rate-limiting enzyme E1, halving
of gene-product
concentration (X --> X' in Fig. 1) would have consequences for the phenotype.
However, in
general, rate-limiting enzymes are subject to complex controls by products of
metabolism. The most common of these is end-product inhibition (Fig. 2). A
decrease in D
would decrease the inhibition. This would increase the activity per molecule of
E1, so that the
reaction rate would become the same as in the homozygote. Thus, in this case,
inhibition provides another "margin of safety" allowing a heterozygote to
maintain the wild-type

                                 4. "Extreme environmental disturbances"

  A major problem with what we might now call the margin-of-safety theory, is in
what selective forces would have created and sustained the margins of safety.
Some have argued
that this can be explained entirely in metabolic terms (Kacser & Burns, 1980).
In the case of a
rate-limiting enzyme (E1) the margin could indeed be a simple consequence of the
evolution of
metabolic controls. It is in the case of non-rate-limiting enzymes that a
further explanation must
be sought. What sustains the enzyme activity in a wild-type homozygote at point
Y rather than
at point Y' (Fig. 1)?
  Haldane interpreted dominance in metabolic terms, but adopted Fisher's view
that weak
selective evolutionary forces acting on heterozygotes would be sufficient to
maintain the margin
of safety. Thus Haldane (1930) wrote: "If we imagine a race whose genes were
only just doing
the work required of them, then any inactivation of one of a pair of genes would
lead to a loss
of total activity. Thus if A1A1 can just oxidize all of a certain substrate as
fast as it is formed,
its inactivation will produce a zygote A1a which can only oxidize about half. If
now A1 mutates
to A2, which can oxidize at twice or thrice the rate of A1, if necessary, no
effect will be
produced, i.e. A1A2 and A2A2 zygotes will be indistinguishable from A1A1. But
A2a will be
normal. Hence A2a zygotes will have a better chance of survival than A1a, and A2
will be
  Muller (1932), however, thought that the evolutionary selection would act at
the homozygote
level. He postulated "that the mutations favouring dominance........have been
selected and are
maintained, not so much for their specific protection against heterozygosis at
the locus in
question, but as to provide a margin of stability and security, to insure the
organism against
weakening or excessive variability of the character by other and more common
influences -
environic and probably also genetic".  Along similar lines Wright (1977) stated
that "because
of extreme environmental disturbances, a considerable excess [of gene product]
is advantageous.
This is likely to be so great that the correlated response of the rare
heterozygote is also brought
fairly close to the asymptote, thus giving a high degree of dominance." The
possible nature of
the "extreme environmental disturbances" was not specified.

                                       5. The Heat Shock Response

  The heat shock response follows a sudden change in various physical or
chemical features of
the environment, and is particularly notable following an increase in
temperature (Nover, 1989).
The response is detected as a rapid increase in the intracellular concentrations
of a set of
evolutionarily conserved "heat-shock" proteins, which is accompanied by a
decrease in the
concentrations of most normal proteins. The notion that the response is
predominantly concerned
with protection against thermal and other types of "stress" has recently lost
ground (Fisher et
al., 1992; Bader et al., 1992).
  I propose elsewhere (Forsdyke, 1985, 1991, 1992, 1993a), that the response has
evolved as
part of a mechanism for distinguishing the proteins of intracellular pathogens
("not-self") from
normal intracellular proteins ("self"). The proteins of the crowded cytosol
exert a collective
pressure tending to make individual protein species aggregate when their
concentrations exceed
their individual solubility limits. These concentrations have been fine-tuned
over evolutionary
time so as not to exceed these limits. Not-self proteins more readily "trip" an
surveillance system because their concentrations have not been so fine-tuned.
  The aggregations, however, being primarily entropy-driven (Lauffer, 1975), are
increased by an increase in temperature. The organism exploits this (e.g. fever)
to promote the
aggregation of the proteins of a foreign pathogen (Nguyen et al., 1989). In this
process self
proteins might also be aggregated. To avoid this, the concentrations of normal
proteins decrease.
However, in turn, this decreases the collective pressure exerted by the
cytosolic proteins to make
the proteins of the pathogen aggregate. To compensate for this, a special set of
proteins, the
heat-shock proteins, are produced (Forsdyke, 1993a).
  The main point to be made about the heat-shock response in the present context
is that it
probably reflects a fundamental process which appeared early in evolution when
sets of
replicators encased in a membrane (prototypic cells) had to be protected against
invasion by
foreign replicators (prototypic viruses; Forsdyke, 1991). All subsequent
developments would potentially be influenced by this pre-existing system. The
sudden general
fall in the concentration of normal self proteins as part of the heat shock
response would
severely compromise cell function if there were not a margin of safety regarding
function. Thus
the heat shock response would constitute a powerful evolutionary force acting on
homozygotes. This would lead to the general evolution of proteins of a specific
activity sufficient
to sustain (or facilitate the recovery of), cell function, at a time when
protein concentration had
fallen. An incidental outcome of this would be that a heterozygote would
normally have
sufficient gene product so that it would be phenotypically indistinguishable
from the wild-type.

  However, having only one copy of the allele in question, a heterozygote would
have sacrificed
part of its margin of safety and this might have some impact on viability.
Indeed, heterozygotes
for lethal mutations do show some reduction in viability compared with the
homozygous wild-
types and may be more temperature-sensitive (Plunkett, 1932; Simmons & Crow,
1977). A viral
infection might trigger a fever and an associated heat-shock response; the
quantity of a non-rate-
limiting protein might then fall from level Y' to X in Figure 1. Thus a protein
unable to respond
to normal metabolic controls (e.g. show increased activity in response to a loss
of end-product
inhibition), would now become rate-limiting. The consequence of this would
depend on timing
and the nature of the end-product involved. A viral infection at a key
developmental stage might
have disastrous consequences.

                            6. X Chromosome Dosage Compensation

  The hypothesis offers a new way of looking at the problem of X chromosome
compensation. This was first described as the process by which the function of
the single X
chromosome in male fruit flies is made equivalent to the function of both X
chromosomes in
females (Muller, 1948). Without dosage compensation, the situation would be
equivalent to the points Y (in females) and Y' (in males) as shown in Figure 1.
postulated that these "exceedingly minute" phenotypic differences (differences
between the point
Y and point Y' phenotypes) would constitute a sufficient selection pressure for
compensation to have evolved. The heat-shock response, however, in shifting
(male) gene product concentrations from point Y' to point X (Fig. 1), would have
created a
much greater selection force for the evolution of a margin of safety in males.
Although this
might have been a factor in the evolution of dosage compensation, I argue in the
paper (Forsdyke, 1993b) that the major factor is probably the need to fine-tune
concentrations, rather than protein functions, to be equal in male and female

  (1992). Inhibition of heat shock gene expression does not block the
development of
  thermotolerance. J. Cell. Physiol. 151, 56-62.
CHARLESWORTH, B.(1979). Evidence against Fisher's theory of dominance. Nature
278, 848-
FISHER, B., KRAFT, P., HAHN, G. M. & ANDERSON, R. L. (1992). Thermotolerance in
  the absence of induced heat shock proteins in a murine lymphoma. Cancer Res.
52, 2854-61.
FISHER, R. A. (1931). The evolution of dominance. Biol. Revs. 6, 345-368.
FORSDYKE, D. R. (1971). Application of the isotope-dilution principle to the
analysis of
  factors affecting the incorporation of 3H-uridine and 3H-cytidine into
cultured lymphocytes.
  Biochem. J. 125, 721-732.
FORSDYKE, D. R. (1985). Heat shock proteins defend against intracellular
pathogens: a non-
  immunological basis for self/not-self discrimination. J. Theoret. Biol. 115,
FORSDYKE, D. R. (1991). Early evolution of MHC polymorphism. J. Theoret. Biol.
150, 451-
  456 (1991).
FORSDYKE, D. R. (1992). Two signal model of self/not-self discrimination: an
update. J.
   Theoret. Biol. 154, 109-118.
FORSDYKE, D. R. (1994a). Entropy-driven protein self aggregation as the basis
  interspecies discrimination in the crowded cytosol. J. Biol. Systems (in
FORSDYKE, D. R. (1994b). Relationship of X chromosome dosage compensation to
  intracellular self/not-self discrimination: a resolution of Muller's paradox.
J. Theoret. Biol.
  167, 7-12.
HALDANE, J. B. S. (1930). A note on Fisher's theory of the origin of dominance
and on a
  correlation between dominance and linkage. Amer.Naturalist 64, 87-90.
KACSER, H. & BURNS, J. A. (1980). The molecular basis of dominance. Genetics 97,
LAUFFER, M. A. (1975). Entropy-Driven Processes in Biology. New York:
MULLER, H. J. (1932). Further studies on the nature and causes of gene
mutations. In Proc.
  6th Internat. Cong. Genetics, 2, 213-255. (Jones, D. F. ed), Menasha,
Wisconsin: Banta Pub.
MULLER, H. J. (1948). Evidence on the precision of genetic adaptation. Harvey
Lect. 43, 165-
NOVER, L. (1989). 125 years of experimental heat shock research. Genome 31,
NGUYEN, V. T., MORANGE, M. & BENSAUDE, O. (1989). Protein denaturation during
heat shock and related stress. E. coli ~ galactosidase and Photinus puralis
inactivation in mouse cells. J.Biol.Chem. 264, 10487-10492.
ORR, H. A. (1991). A test of Fisher's theory of dominance. Proc.   Natl. Acad.
Sci. USA   88,
PLUNKETT, C. R. (1932). Temperature as a tool of research in phenogenetics. In
Proc. 6th
   Internat. Cong. Genetics, 2, 158-160. (Jones, D. F. ed), Menasha, Wisconsin:
Banta Pub. Co.
SIMMONS, M. J. & CROW, J. F. (1977). Mutations affecting fitness in Drosophila
   populations. Annu. Rev. Genet. 11, 49-78.
WRIGHT, S. (1977). The evolution of dominance. In: Evolution and the Genetics of
   Populations. 3, 498-526. Chicago: The University of Chicago Press.


  FIG. 1  Hypothetical in vivo dose-response curve showing some quantitative
measure of
phenotype (such as the rate of formation of enzyme product) as a function of the
dose of a gene
product which contributes to that phenotype (such as the quantity of an enzyme).
The vertical
arrow indicates the normal concentration of a non-rate-limiting gene product in
a diploid wild-
type homozygous cell. Halving this dose (Y --> Y') has a minimal effect on
phenotype. X and
X' indicate corresponding points for a rate-limiting gene product. Added in
italics is a chart of
the flow of information from gene to phenotype, as considered in this paper. It
is assumed that
in most circumstances gene product concentration is directly proportional to
gene dosage.

  FIG. 2  A hypothetical metabolic pathway catalyzed by enzymes E1, E2 and E3. A
is the
substrate of E1, which catalyzes the rate-limiting step in the pathway. A is
also metabolized by
another pathway. B is the product of E1 and the substrate of E2, which is
normally not rate-
limiting. C is the product of E2 and the substrate of E3, which is also normally
not rate-limiting.
D, the product of E3, is responsible for the phenotype and for pathway
regulation by feed-back
inhibiting E1.

  FIG. 3  Hypothetical in vivo dose-response curves for enzymes E1, E2, and E3.
Rates of
formation of products (B, C, and D) are expressed as functions of the
concentrations of
substrates A, B, and C, respectively. The vertical arrows refer to the normal in
concentrations of these substrates as they interact with the enzymes.

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