ExperimentalGerontology,Vol. 31, No. 6, 623~43, 1996
Copyright© 1996 ElsevierScienceInc.
Printed in the USA.All rights reserved
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ELSEVIER
PII S0531-5565(96)00096-4
Mini-Review
F A C T O R S CONTRIBUTING TO THE PLASTICITY OF THE
E X T E N D E D LONGEVITY P H E N O T Y P E S OF DROSOPHILA
ROBERT ARKING 1, ALLAN G. FORCE,1'3 STEVEN P. DUDAS I, STEVEN BUCK 1 and
GEORGE T. BAKER, III 2,4
1Department of Biological Sciences, Wayne State University, Detroit, Michigan 48202 and
2Gerontology Research Center, National Institute of Aging, Baltimore, Maryland 21224
Abstract - - A number of laboratories have constructed independently derived long-lived
strains of Drosophila, each of which have similar but not identical patterns of variability in
their adult longevity. Given the observed plasticity of longevity within each of these strains,
it would be useful to review the operational and environmental factors that give rise to this
phenotypic plasticity and ascertain whether they are common or strain specific. Our review
of the more extensively analyzed strains suggests that the allelic composition of the initial
genomes and the selection/transgene strategy employed yield extended longevity strains with
superficially similar phenotypes but which are probably each the result of different proximal
genetic mechanisms. This then offers a plausible explanation for the differential effects of
various environmental factors on each strain's particular pattern of phenotypic plasticity. It
also illustrates that the species has the potential to employ any one of a number of different
proximal mechanisms, each of which give rise to a similar longevity phenotype. Copyright
© 1996 Elsevier Science Inc.
Key Words: longevity, life span, Drosophila, genetic control of aging, genetic plasticity, phenotypic
plasticity, environment effects
INTRODUCTION
IN 1891, A u g u s t W e i s m a n n asked what factors accounted for the significant differences in life
span observed b e t w e e n different species and a m o n g different m e m b e r s of the same species
( W e i s m a n n , 1891). Although R a y m o n d Pearl (1928) and others began to develop answers to his
question, the task was soon a b a n d o n e d in favor of more experimentally tractable problems in
genetics, evolution, embryology, biochemistry, and the like. Indeed, it was not until some
insight had been attained into molecular and developmental genetic m e c h a n i s m s that the prob3Current address: Department of Biology, University of Oregon, Eugene, OR 97403
aDeceased
Correspondence to: Robert Arking
(Received 6 November 1995; Accepted 15 July 1996)
623
624
R. ARKING et at.
lem of aging and longevity could be put in a proper perspective. Even then, the choice of an
experimentally appropriate model system raised some problems. A number of investigators
chose to examine the reasons underlying the shortened longevity of various mutant strains
(Baker et al., 1985), believing that their analysis would provide insights into the mechanisms
regulating aging and senescence in normal lived organisms. This approach was not productive,
because a shortened longevity may arise from any one of a number of different developmental
pathologies, none of which had much to do with normal aging (Mayer and Baker, 1985). In the
last decade or so, a number of different groups decided to use artificial selection to create
extended longevity strains, and use these as a model system with which to examine the mechanisms underlying aging and senescence. This approach has been more successful. One implication of this approach was the transformation of longevity into just another phenotype. As such,
it is as variable as any other quantitative trait. It is the purpose of this paper to (1), catalog the
sources of this phenotypic plasticity as observed within our own selected long-lived L strain; and
(2), compare them to the other selected Drosophila strains and determine whether these individual sources of plasticity are strain specific or are common to the several different long-lived
strains.
Genetic approaches are often used to uncover a causal pathway leading from the gene to the
phenotype, and vice versa. Drosophila, as the premier genetic organism, has been used in aging
research since 1913 (Lints and Soliman, 1988). An impressive amount of data have been
collected that bear directly on the problems of aging and longevity in this one organism. The
review by Baker et al. (1985) lists almost 400 literature references, and a current comprehensive
review would probably need to list several hundred more. Yet despite this effort we still do not
fully understand the mechanisms of aging in Drosophila. Part of the reason for this slow
progress might have to do with the variability of past results reported in different strains (Baker
et al., 1985). This variability might arise from the plasticity of the aging process itself (defined
as the extent to which the expression of an individual's genotype can be modified by environmental factors (Scheiner, 1993), or from the possibility that the different strains studied by
different investigators might actually age in different ways, or both. In any event, the variability
implies that it might be misleading to extrapolate across strains without some assurance that the
strains are, in fact, comparable; that they are not affected in different ways by the same
environmental variables, and/or that they are not using different mechanisms to regulate their
extended longevity phenotype, as was the case with the Oregon-R and Canton-S strains studied
by Ganetzky and Flannagan (1977). Two possible strategies by which a comparative genetic
approach can minimize the effect of such variability are to (1), investigate some presumptive
factors affecting longevity across a number of replicate strains; and/or (2), investigate the factors
affecting longevity on a defined set of related strains with significantly different longevities. We
shall discuss examples of both strategies.
Usefulness of Drosophila as a model system for the study of aging
Johnson et al. (1993) have questioned the utility of Drosophila as a model system that can
afford insights into the mechanisms of longevity extension operative in other organisms including humans. These objections were based on the alleged failure to identify candidate genes, the
assumed involvement of large numbers of genes with heterotic effects, the large difference in
life histories between mammals and insects, and the alleged inability to ascertain causality of
extended longevity in Drosophila. The data presented in this article and others suggests that
these objections are not well founded. The genes comprising proximal longevity mechanisms
PLASTICITY OF LONGEVITY
625
such as the antioxidant defense system have been identified as candidate genes (Dudas and
Arking, 1995). The minimum number of genes necessary to significantly increase longevity is
two (Orr and Sohal, 1994). There are no heterotic effects noted in our strains, the genes involved
being normal recessives (Buck et al., 1993a). Causality is being addressed with respect to the
antioxidant defense system and shows every sign of being successful (Arking et al., unpublished
data). There is undoubtedly a large difference in life history strategies between mammals and
dipteran insects; yet the disposable soma theory of aging (Kirkwood, 1990), suggests that
longevity is the outcome of a series of trade-offs between reproduction and repair/defense
processes. Given the fundamental similarity of basic biological processes, then it is not surprising that enhanced antioxidant defenses have been implicated in the extended longevity
mechanisms probably operative in fungi, nematodes, flies, and mammals. We suggest that
appropriate investigations into the extended longevity of Drosophila may provide insights
useful for analysis of similar mechanisms in mammals, including humans.
Sources of variation f o r any phenotype
There are three sources of variation for any phenotype. There are two genetic variables, the
additive and the nonadditive factors, which interact with the environmental factors so as to yield
the observed final phenotype (Falconer, 1981). Scheiner (1993) has reviewed the definitions,
measurements and models of phenotypic plasticity in general; the reader is referred to that article
for an overall assessment of the topic. Curtsinger (1990) has estimated, on the basis of population genetic studies, that about 20% of the phenotypic variance in Drosophila longevity is due
to additive variance, about 20% to nonadditive variance, and about 60% to environmental
factors. We can operationally identify environmental plasticity of a phenotype by examining the
data for the existence of genotype by environment interactions that result in the significant
modulation of the phenotype. A complete analysis of the mechanisms regulating aging and
longevity in our strains requires that we identify and characterize all components of these three
variables. Our experiments have been designed with this goal in mind and our results are
generally congruent with those of Curtsinger (1990). Thus, our discussion of environmental
plasticity leads us to also discuss the variability of the genetic system on which it acts. Analysis
of the extended longevity phenotype (ELP) in our strains of Drosophila has led us to conclude
that the ELP is a genetically determined, environmentally modulated, event dependent, developmental process. We have previously determined that the critical genetic factors are located on
the third chromosome (c3), but their expression is positively and negatively regulated by
nonadditive factors located on the first and second chromosomes (cl and c2) (Arking et al.,
1993; Buck et al., 1993a). In addition, the expression of these strain specific genetic factors is
modulated by the environmental components such as larval density (Buck et al., 1993b),
ambient temperature (Arking et al., 1988), and nutrition (see below), among others . The data
reviewed below will provide the factual support for this empirical definition of the aging process
in these strains.
Finally, the allelic composition of the initial or baseline strain must restrict the potential
response of the population to a given selection pressure. Our own baseline R strain was constructed from recently caught wild flies in Michigan. Sampling this baseline strain nine times
over a 10-year period with two different selection paradigms has yielded a number of long-lived
strains, each of which appear to depend primarily on enhanced antioxidant resistance as their
proximal mechanism. Other laboratories find a similar conformity of response among their
sublines but involving some other proximal mechanism. The species response is broader than
626
R. ARKING e t al.
that of any given population. Hence, the allelic composition of the initial population must
predict and constrain their response to any particular selection or transgenic intervention.
Different methods of constructing long-lived strains
Artificial Selection The use of artificial selection as a means of constructing long-lived strains
can be viewed as a proof of the correctness of the evolutionary theory of aging (Rose, 1991).
Although some attempts have not been successful (i.e., Lints and Hoste, 1974; Baret and Lints,
1993), those failures cannot be logically used to disprove the general possibility of using such
a procedure to generate extended longevity strains, especially in the face of the data (Arking and
Buck, 1995).
In most cases, different investigators have indirectly selected for extended longevity by
directly selecting for delayed female fecundity (Luckinbill et al., 1984; Rose, 1984; Arking,
1987; Partridge and Fowler, 1992). However, it has also been shown possible to directly select
for dessication resistance (Hoffmann and Parsons, 1989) or starvation resistance (Graves et al.,
1992; Rose et al., 1992), and thus, indirectly select for extended longevity. It seems likely that
these different selection scenarios would be likely to give rise to similar but not identical
phenotypes, and this variability in selection paradigms might also account for some of the
genetic plasticity of longevity in Drosophila. We will present data below to support this
statement.
Even when different investigators believe they are using the same selection procedure, they
may be mistaken. For example, the different selection regimes may not differ simply in age at
reproduction but may also manifest other phenotypic factors that were coselected as well. A
clear example is provided in comparing the long-lived and short-lived strains from Roper et al.
(1993), as well as those from our own laboratory (Buck and Arking, unpublished data). Both
labs found that their strains were found to have been selected inadvertently for fast development
time as well as for extended longevity. It is possible that other such traits unrelated to longevity
may also have been inadvertently selected in these and other strains. This situation gives rise to
two possibilities. First, if the changes in some inadvertently selected physiological parameter in
both late- and early-reproduced populations occur and are not controlled, then the comparisons
between such lines may give rise to spurious positive or negative relationships between any
character and longevity. It seems likely that some characters may be associated with different
aspects of the selection procedure but may not be associated with longevity per se. Second, the
inadvertent inclusion of other phenotypic characters within the generic extended longevity
phenotype may give rise to differing patterns of gene interactions between the several ostensibly
identical strains. In the absence of explicit information regarding the existence of the inadvertent
factors, the differing gene expression patterns may be wrongly interpreted as evidence of
different genetic mechanisms. Very clearly, either of these situations may give rise to a spurious
interpretation of genetic plasticity of longevity in Drosophila.
On the other hand, Draye et al. (1994) have demonstrated that natural populations of Drosophila are genetically different for at least some life history traits when measured in the
laboratory as soon as possible after capture. This finding implies that the species as a whole is
polymorphic in these traits and, thus, that laboratory populations founded from different natural
populations may respond differently to the same experimental situation because they are genetically different from one another. Such a finding gives rise to the expectation that there
should be substantial genetic plasticity of longevity in laboratory strains derived from different
founder populations.
627
PLASTICITY OF LONGEVITY
The long-lived strains constructed by different laboratories appear to have much in common
with each other. For example, the strains constructed via a direct selection for delayed female
fecundity all appear to bring about the extension of longevity via a delayed onset of senescence
(Luckinbill e t al., 1984; Arking and Wells, 1990; Rose, 1984; Partridge and Fowler, 1992). Such
a process affects the age of onset of senescence but does not affect the rate of aging. The
Gompertz curves shown in Fig. 1 illustrate this in the case of our long-lived L and normal-lived
R strains, where the L strains have the same slope but a much lower intercept than do the R
strains. From a demographic point of view, this means that the extended longevity arises from
a lowering of the mortality rate at all ages, as was shown by Curtsinger (1995) for sister lines
to our strains.
Is it possible to use artificial selection to construct a strain that lives long, not because it
delays the age of onset of senescence but because it slows down the rate of aging? It is clear that
gene expression during the adult stage is tightly regulated and exhibits what appear to be both
early life- and late life-specific gene activity patterns (Fleming e t al., 1993; Tower, 1993; Dudas
and Arking, 1995; Helfand e t al., 1995). It should be possible in principle to use molecular
genetic techniques to isolate genes (and mutants) with altered late life specific gene activity
patterns such that they would live longer due to a slowing of the rate of senescence, in a manner
similar to that used by Egilmez et al. (1989) and D ' M e l l o et al. (1994) in the isolation of the
LAG-1 genes of yeast. Tower (1993) is using a similar approach in that he is using insertional
mutagenesis to isolate mutants with late life gene expression patterns. A related approach was
taken by Zwaan (1995), who directly selected for long-lived individuals. These strains and
mutants have not yet been fully analyzed but when they are, it is likely that the mutants and/or
2,
o
f
j
-2,
0
20
40
60
80
OLa
100
AGE
FIG. 1. The Gompertz plots of a normal-livedcontrol strain (RA) compared to that of a genetically
selected long-lived strain (LA). The former is the normal-lived baseline strain from which the
long-livedL strain was derived. Both strains were raised at high larval density. Note that the slopes
of the two strains are almost parallelto one anotherbut that their interceptsare significantlydifferent.
This observationsuggests that the significantfactors in the extended longevityof the L animals are
a lower mortalityrate and a delayedonset of senescencebut not a decrease in their rate of aging. This
conclusionis consistentwith those obtainedfrom an analysis of biomarkerdata (Arking and Wells,
1990) and from mortalitydata on sister lines (Curt.singeret al., 1995). Based on maximumlikelihood
analysis of data presented in Buck et al. (1993a,b).
628
R. ARKING et al.
strains isolated by these paradigms would show an extended longevity phenotype significantly
different from that observed in the present long-lived L strain. Conversely, proving that two or
more fundamentally different mechanisms, each derived by similar selection strategies, but
which each give rise to a similar generic phenotype would also point out another source of
genetic plasticity of longevity in Drosophila.
Transgenic Experiments In Drosophila, transgenic experiments have been done with the
specific aim of testing the antioxidant theory of aging (Harman, 1956). Reveillaud et al. (1991)
made transgenic animals carrying an extra copy of the bovine Cu-Zn superoxide dismutase
(SOD) gene under the control of an actin 5c promoter. The resulting animals expressed both
mammalian and Drosophila SOD. Some, but not all, of the transgenic strains exhibited an
increased resistance to exogenous paraquat along with a modest increase in the mean lifespan.
There was no effect in the maximum lifespan observed in the experimental and control lines,
suggesting that the effect of the treatment was to decrease premature mortality. This was
confirmed by a later experiment showing that the bovine SOD gene had the ability to rescue a
SOD null mutant from early lethality (Reveillaud et al., 1994). An essentially similar result was
reported by Orr and Sohal (1992, 1993) as a result of their construction of animals transgenic
for either a Drosophila SOD gene or a Drosophila catalase (CAT) gene. However, constructing
an animal transgenic for both the Drosophila SOD and CAT genes, each under the control of
an actin promoter but also subject to the effects of position variegation, gave rise to a strain that
was not only significantly resistant to exogenous paraquat but also exhibited a significant
increase in both mean and maximum longevity (Orr and Sohal, 1994). In addition, the Gompertz
curve for these doubly transgenic strains had the same intercept but a lower slope compared to
the controls. This indicates that the extended longevity of the transgenic animals might come
about via a slower rate of aging and not via a delay in the age of onset of senescence as is the
case with our selected strains (Fig. 1). This suggests that even though these two strains both
appear to rely on the antioxidant defense system for their extended longevity, the actual mechanisms by which this enhanced antioxidant gene activity exerts its physiological effects may well
be quite different in these two strains. This difference in the phenotype is likely to manifest itself
as a genetic plasticity of longevity in Drosophila.
Webster (1986) first proposed that elongation factor 1~ (EF-I) was causally involved in
bringing about the onset of senescence in Drosophila. Their hypothesis was initially confirmed
by a transgenic experiment (Shepherd et al., 1989) in which an extra copy of the EF-I gene
inserted in a wild type fly yielded a strain with a significant increase in mean and maximum
lifespan relative to their controls. Subsequently, however, other more detailed analyses revealed
that the effects were not as straight forward as was initially thought. Shikama et al. (1994)
reported that their transgenic EF-1 flies do not synthesize more EF-1 mRNA or protein even
though there still exists a reproducible difference in lifespan between the experimental and
control animals. This finding suggests that the EF-I gene products themselves may not play a
causal role in the longevity extension of the genetically manipulated strain. A series of EF-I
transgene experiments was done by Stearns and Kaiser (1993), during which they observed
negligible and/or variable effects on life span as a function of strain, sex, position, etc. It is now
known that the EF-1 transgene was not expressed in these experiments and, thus, the observed
effects must represent nonspecific insertional effects (Curtsinger et al., 1995). Finally, an
analysis of the expression of the normal EF- 1 gene products in a genetically selected long-lived
strain led Dudas and Arking (1994) to conclude that the expression of the EF-1 genes is not
associated with the expression of the extended longevity phenotype in that strain. One important
PLASTICITY OF LONGEVITY
629
point of these several analyses is that the interpretation of a transgene experiment may not be
as simple as our preconception of the experimental situation might initially suggest. The organism represents a buffered physiological and metabolic system (Clark and Keith, 1988).
Furthermore, many genetic systems are under the control of cis and/or trans regulatory loci, such
as has been demonstrated for antioxidant defense systems in lower organisms (Munkres, 1990,
1992; Ames et al., 1993). Therefore, the random insertion of structural genes may disrupt
regulatory homeostatic mechanisms and lead to compensatory responses that confound interpretation. For these reasons, transgenes may well have unexpected effects in different strains.
Gene regulatory circuits are interconnected and can respond in unexpected ways to a physical
or regulatory perturbation. The diversity of proximal mechanistic responses to such perturbations is a source of genetic plasticity for longevity in Drosophila.
Genetic analysis o f different long-lived strains
Transgenic Strains The transgenic experiments discussed above lead to the conclusion that
altering the expression of as few as two particular genes, SOD and CAT, can lead to the
expression of an extended longevity phenotype superficially similar to that produced by artificial selection. The transgenic experiment of Orr and Sohal (1994) also provides strong support
for the involvement of free radicals in governing the rate of aging. Not only is their data
consistent with our molecular data, but it is also consistent with our genetic estimates suggesting
that, while extended longevity is a polygenic trait, it probably does not involve a large number
of genes (Buck et al., 1993a). On the face of it, this experiment with its minimalist number of
genes stands opposed to the concept that aging is a highly polygenic phenotype dependent on
the integrated functioning of a large number of different genes, such as has been proposed by
Rose and his colleagues (see below). Our own selected strains also seem to depend on an
enhanced antioxidant defense system response, but as discussed below, they do this in a somewhat different mode. Some of the observed plasticity in the longevity of Drosophila may stem
from the fact that there appears to be more than one way in which an organism can marshall its
antioxidant defense mechanisms to yield an extended longevity phenotype. Even within a
common proximal mechanism, there appears to be sufficient genetic variability that also serves
as a source of additional environmental plasticity for longevity in Drosophila.
Selected Strains Luckinbill et al. (1987) initially reported that at least one gene was involved
in the expression of extended longevity in sister stocks to our strains. This minimum estimate
was later discarded in favor of a polygenic mechanism primarily involving the third chromosome (Luckinbill et al., 1988). Although chromosome localizations were not performed, Rose
and his colleagues (Hutchinson and Rose, 1991; Hutchinson et al., 1991) used two different
types of quantitative genetic analyses to show that the transmission and expression of the
extended longevity phenotype in their selected strains could be adequately explained as being
due to the effects of additive genes averaged over some unspecified number of loci. Based on
their analysis of the mobility and expression patterns of 321 proteins of the long-lived O strains
of Rose (1984), Fleming et al. (1993) concluded that about six proteins had statistically different
expression patterns in the long-lived and control strains, or about 2% of the total. Based on the
assumption that this is a representative number that one may extrapolate to the whole genome,
they then estimated that perhaps 200 to 400 loci that can postpone aging might exist in their
strains. If the Orr and Sohal (1994) experiment sets a lower limit to the number of genes
involved in extended longevity, then the experiment of Fleming et al. (1993) can be viewed as
630
R. ARKING et al.
setting an upper limit. The resolution of this debate is important, driving as it does both our
concepts and our experimental strategies of identifying the genes involved.
Data obtained from the analysis of our selected strains supports the concept of a hierarchy of
regulatory genes involved in the expression of a long-lived phenotype but suggests that (a), the
number of structural genes involved may be substantially smaller than the estimate of Fleming
et al. (1993) and (b), that selection acted so as to alter the nature of certain key regulatory genes
in the experimental strain relative to its baseline control. Table 1 summarizes the results of some
of the 27 isochromosomal lines constructed from the 1st, 2nd, and 3rd chromosomes (c 1, c2, c3)
of a normal-lived R control strain and from a long-lived L strain selected for late-life fecundity
and under high-density developmental conditions (see Wells et al., 1987; Buck et al., 1993a,
1993b; Arking et al., 1993, for details). The analysis of these and other data led to the following
conclusions. First, the genes essential to the expression of the extended longevity phenotype
(ELP) are located only on the L strain c3 and map somewhere to the left of ebony. Second, these
genes are recessive. Third, there exists a complex pattern of epistasis such the longevity enhancing recessive genes on c3 are repressed by (unknown) genes on c2, which can, themselves,
be repressed by (unknown) genes on cl. Thus, the c3 genes can be expressed either in the mutual
presence of cl and c2, or in the absence of c2 (Arking et al., 1993; Buck et al., 1993). Fourth,
larval density can also modulate the c3 gene expression, the critical period for its appliction
being from 60 to 120 h after egg laying (Buck et al., 1993b), and is believed to act directly on
the c3 (Arking et al., 1993). The regulatory circuits derived from these studies are shown in Fig.
2. The negative effect of c2 on c3 is supported by the report of Graf and Ayala (1986), in which
they showed that the levels of SOD on c3 are under the control of (unknown) loci on c2. In a
similar manner, Bewley and Laurie-Ahlberg (1984) have shown that the expression of CAT on
c3 is also regulated by (unknown) loci on c2. The report of Graf and Ayala (1986) further
suggests that one such difference between the R and L strain c3 might possibly involve a
cis-acting element, such as their S O D Cat mutation, which was shown to significantly reduce
TABLE I. LONGEVITY AND CHROMOSOMECOMPOSITION
Mean LongeviD"
( +-SEM)
Chromosome Composition
Strain
000
222
001
002
220
1st
2nd
3 rd
R/R
L/L
R/R
R/R
L/L
R/R
L/L
R/R
R/R
L/L
R/R
L/L
R/L
LFL
R/R
Male
52.4
66.9
44.7
67.7
60.3
_+ 0.7
-+ 1.3
+ 2.8
-+ 1.3
_+ 1.7
Female
49.2
64.3
47.8
64.3
54.6
_+0.8
_ 1.
+ 2.7
_+ 1.5
_+ 1.6
A n illustration of the connection between c h r o m o s o m e c o m p o s i t i o n a n d longevity. A total
o f 27 i s o c h r o m o s o m a l lines, involving all possible c o m b i n a t i o n s o f the I st, 2nd, and 3rd
c h r o m o s o m e s o f a short-lived R control strain and a long-lived L strain, were constructed
a n d their longevity measured. The table s h o w s data o f four representative strains. The
strain designation is b a s e d on a three digit n u m b e r where each place represents c l , c2, or
c3 respectively; and the value at e a c h place (0, 1, or 2) represents the n u m b e r o f L type
c h r o m o s o m e s present. Note that the 222 and 002 strains have identical L type life spans,
a n d that the latter is significantly different f r o m the 000, 001, and 220 values. B a s e d on
data presented in B u c k et al. (1993a).
PLASTICITY OF LONGEVITY
631
U)
!
C1 L
I
e2 L
%
FIG. 2. A summary of the chromosomaland environmental interactions involved in the expression of
the extended longevity phenotype. Loci on cl L and c2e interact with c3L, both positively (cl) and
negatively (c2), respectively, such that cl represses c2, which in turn, represses c3. The recessive
genes on c3L are absolutely required for the delayed onset of senescence and the expression of the
ELP, which may be fully expressed in the mutual presence and the mutual absence of cl L and c2L.
The positive and negative roles of larval density are indicated by HD and LD. Based on data
presented in Buck et al., (1993a,b) and in Arking et al. (1993). See text for discussion.
SOD protein levels. Finally, while the molecular genetic regulation of anioxidant systems is one
important and confirmed approach to extended longevity, other hierarchies of regulatory genes
controlling systems that may act in conjunction with antioxidant systems are likely to exist.
Indeed, caloric restriction is a good example of an experimental paradigm known to enhance
longevity and overall physiological vigor, and which undoubtedly has its own regulatory hierarchy. The effectiveness of different proximal mechanisms suggests additional sources of genetic and environmental plasticity affecting the expression of the extended longevity phenotype.
Molecular analysis o f different long-lived strains
In nonselected standard laboratory strains, transgenic techniques have been used to increase
selectively the dosage of certain antioxidant defense system (ADS) genes and thereby discern
its effects on longevity. As discussed above, increasing the gene dosage of the SOD gene alone
has only minimal effects on longevity (Seto et al, 1990; Reveillaud et al., 1991; Orr and Sohal,
1992). However, Orr and Sohal (1994) found that simultaneously increasing the dosage of both
the SOD and CAT ADS genes brings about an increase in longevity comparable to that observed
in our genetically selected strains. Their data certainly make the free radical theory a probable
proximal theory of aging in Drosophila.
That proximal explanation of extended longevity is applicable to some but not all selected
long-lived strains. We have assayed the changes in the m R N A and/or enzyme activity levels of
a number of loci during the development and early adult life of our normal-lived R and
long-lived L strains (Dudas and Arking, 1995). The m R N A data, shown in Table 2, demonstrates that, at day 5 in the L strain, there appears to be a coordinately regulated significant
increase in the m R N A levels of CuZnSOD, MnSOD, CAT, and xanthine dehydrogenase (XDH).
These increases in m R N A levels are accompanied by significant increases in the enzyme activity
of CuZnSOD, CAT, and glutathione-S-transferase (GST) (Table 2). In addition, our ongoing
work shows that the SOD-specific protein increases proportionately as well (Burde, Haft, and
Arking, unpublished data). Thus, it seems reasonable to conclude that these alterations in gene
expression are probably the result of a transcriptional level change. It is well known that all four
of these gene products are involved in antioxidant defense, and that null mutants at each locus
632
R.
ARKING et ~tl.
TABLE 2. RELATIVE LEVELS OF THE EXPRESSION OF ANTIOXIDANT GENES 1N
NORMAL-LIVED AND LONG-LIVED STRAINS OF Drosophila AT DAY 5 OF
ADULT LIFE*
Relative Level ~[' lz)-pression in:
Gene Product
ADS mRNAs:
CuZnSOD
MnSOD
CAT
GST
XDH
A D S enzymes:
SOD
CAT
GST
Non-ADS mRNAs
ADH
haywire
R strain
L strain
Significant
(p < 0.05)
1.00
1.00
1.00
1.00
1.00
1.88
1.52
1.27
1.17
2.45
yes
yes
yes
no
yes
1.00
1.00
1.00
1.95
1.13
2.49
yes
yes
yes
1.00
1.00
0.90
0.98
no
no
*Data taken f r o m D u d a s and A r k i n g (1995), D u d a s (1993), and J u n g and
A r k i n g (unpublished).
render the organism very sensitive to paraquat dependent oxygen stress (see Phillips and Hilliker, 1990; Weinhold et al., 1990, for review and references). The mRNA's levels for other
gene loci assayed in these same strains, such as the EF-1 (Dudas and Arking, 1994), the alcohol
dehydrogenase (ADH) or haywire loci (Table 2), show different patterns of expression through
the same developmental stages, thus indicating that the phenomenon observed is specific for the
antioxidant defense genes. That these changes in ADS gene expression are important to the
expression of the ELP is suggested by the following four facts. First, a twofold higher resistance
to exogenous paraquat, a known free-radical generator, is observed in the young adults of our
long-lived L strains relative to their normal lived controls (Arking et al., 1991). Second, the
long-lived L strains have a enhanced temporal expression of paraquat resistance relative to the
controls, maintaining significant levels of resistance for the first four to five weeks of adult life
(Soliman and Arking, 1996). Third, long-lived L strains have a delayed onset and a lower level
of oxidative protein damage relative to their normal lived controls (Soliman and Arking, unpublished). In addition, the long-lived L strain has a significantly lower rate of lipid peroxidation
than does the R strain, and the final level of lipid peroxidation attained is also significantly lower
in the L strain relative to the R strain (Soliman and Arking, unpublished data). Finally, it has
been found that feeding the long-lived L strain animals with aminotriazole, a known specific
inhibitor of CAT, makes the treated L animals respond to paraquat as if they were the low
resistant R types (Dudas and Arking, 1995).
Electrophoretic studies have shown that there is no such allozyme difference for SOD or CAT
in our selected long-lived strains relative to the normal controls (Arking et al., 1993; Dudas,
1993). Such evidence suggests that the ADS structural genes in the L A and Ra strains are
probably identical to one another (Dudas and Arking, 1995). It then follows that the selection
process may have involved the fixation in the L A strain of regulatory gene sequences different
PLASTICITY OF L O N G E V I T Y
633
from those commonly found in the R A strain. Operationally, we then would expect to find
a number of polymorphic regulatory sites that differ between the two strains and which,
when isolated, would be candidate longevity assurance regulatory genes, Such projects are
now underway. In addition, the fact that the XDH gene is coordinately activated along with the
other ADS genes suggests that the regulatory genes known to modulate XDH activity in terms
of its effects on eye color and purine metabolism may also be operative in regulating its activity
in terms of the ELP. If confirmed, then this suggestion would significantly extend and complicate the antioxidant gene regulatory system and expand the sources of additional genetic
plasticity.
On the other hand, Rose and his colleagues (Tyler et al., 1993) found that their selected
long-lived strains contained a known high-activity allele of the SOD gene. However, although
enzyme activity measurements have been reported for this allele in other genetic backgrounds,
there has been no report of their activity in the O strains background. As described elsewhere
in this paper and in their writings, their analyses have led them to consider other metabolic and
biochemical factors as constituting proximal explanations for the expression of extended longevity in Drosophila. It is entirely possible that artificial selection operating on two different
progenitor stocks may have operated so as to bring about a generically similar extended longevity phenotypes each dependent on different proximal mechanisms. Some of the observed
plasticity of longevity may have its origins in this situation.
Environmental factors affecting the expression of the extended longevity phenotype
Some Environmental Factors Have A Minor Effect on Expression of the ELP It has long been
known that the life span of poikiothermic animals can be significantly affected by changes in the
adults' ambient temperature (see David, 1988, for review and references). Such environmental
manipulations were once extensively used as a means for the experimental manipulation of
longevity. We examined the effects of ambient temperature on adult life span to determine if
manipulation of this factor would make the R strain control animals more similar to the
long-lived L strain experimental animals (Arking et al., 1988). The results were instructive.
Animals raised at 18°C have a mean and maximum life span that is about twice as great as it
is for genetically identical animals raised at 28°C. Thus, the adult life span for both strains is
changed by ambient temperature in the qualitative manner that one would expect on the basis
of past work, both strains yielding an inverse relationship between the two variables with
roughly equivalent slopes (see Fig. 1 of Arking et al., 1988). However, following a recent
detailed reanalysis of these data, it is now quite apparent that the two strains also differ
quantitatively in their response to ambient temperature. Computing the change in life span per
degree C between different measured temperature regimes, as listed in Table 3, shows that these
two genotypes differ in their response to ambient temperature in several important ways, as
follows:
First, the L strain animals show a larger response to temperature over the entire 18-28°C
tested range than do the R strain controls, the actual value depending on other genetic (i.e., sex)
and environmental (i.e., developmental temperature) factors, but ranging from a 13 to a 39%
increase.
Second, within the 18-28°C range, the L strain animals consistetly show their largest response in the 22-25 ° segment, while the R strain controls show their largest response in the
18-22 ° segment.
634
R. ARKING et al.
TABLE3. GENOTYPE BY ENVIRONMENT EFFECTS OF AMBIENT TEMPERATURE ON ADULT LONGEVITY*
Change in the M e a s u r e d Lifespan/1 °C
Over the M e a s u r e d Temperature Differential
Female
Strain
LE
RE
LI
Rl
LTso
LT9o
LTso
LT9o
LTso
LT9o
LTso
LT9o
Male
18-28 °
1~22 °
22 25 °
22-28 °
18-28 °
18-22 °
22-25 °
5.7
8.7
4.6
6.7
7.0
7.8
4.3
5.6
4.2
9.5
10.0
2.7
3.0
5.5
9.2
6.4
8.1
7.0
7.8
4.3
5.8
3.2
8.5
7.8
13.7
6.5
6.7
8.2
3.3
4.2
6.2
8.7
3.0
4.0
10.5
8.3
8.5
6.3
8.0
16.3
8.3
10. 7
8.3
3.7
11.7
11.5
8.0
7.0
8.2
8.5
10. 7
6.5
6.8
8.3
3.0
3.3
22-28 °
7.0
9.7
3.7
5.8
6.2
7.3
3.3
3.7
*Based on a reanalysis of data originally presented in Fig. 1 of Arking et al. (1988). E strains were kept at the indicated
temperature from oogenesis to death; I strains were kept at 25°C until eclosion when they were transfered to the
indicated temperature. The italicized numbers identify the temperature range in which that genotype exhibited its
m a x i m u m response. See text for discussion.
Third, both R and L strain animals raised at the same temperature throughout their entire life
(i.e., the E series of data in Arking et al., 1988) usually showed a larger response to ambient
temperature than did their sibs raised at 25°C during the developmental periods and then
switched to the appropriate ambient temperature for the rest of their adult life span (i.e., the I
series of data in Arking et al., 1988). Thus, stage specific temperature treatments may have
delayed and cumulative effects on the adult.
The two strains obviously react differently to environmental temperatures. This different
genotype by environment interaction leads to a significant increase in the L strain lifespan at
lower temperatures relative to the R strain. Although both the L and the R strain display an
inverse relationship between their life spans and the ambient temperature, the actual response of
the two genotypes is quite different in detail.
Temperature is thought to exert its effects through a generalized alteration of the metabolic
rate. There is an obvious inverse relationship between ambient temperature and mean daily
metabolic rate (MDMR) such that animals raised at 18°C have a significantly lower M D M R
than do animals of the same strain raised at 28°C (Arking et al., 1988). However, there is no
statistically significant difference in the M D M R between the R and L strains raised at the same
temperature even though there are such differences in longevity. The enhanced antioxidant
defense of the L strain may explain part of this apparent paradox.
The differential temperature dependency of longevity may come about, not because the
temperature dependent rate of free radical production is different, but because the intrinsic
activity of the ADS is different in these two strains. If temperature modulates the life span of
Drosophila by affecting the metabolic rate, then the presumed increased production of free
radicals will have a more deleterious effect in the R strain relative to the L strain because it is
known that the latter strain has a more active ADS (Table 2; Dudas and Arking, 1995). This is
consistent with the report of Helfand et al. (1995) suggesting that temperature can affect the
timing but not the pattern of differential gene expression in the adult.
PLASTICITY OF LONGEVITY
635
Some Environmental Factors Have Very Strong Effects on Expression of the ELP Larval
density is known to affect life span in wild strains of D. melanogaster (Miller and Thomas,
1958; Lints and Lints, 1971; Economos and Lints, 1984; Zwaan et al., 1991), as well as in
selected long-lived strains (Clare and Luckinbill, 1985; Buck et al., 1993b; Graves and Mueller,
1993). The data suggest that there is a generalized response of Drosophila longevity to larval
density that alters adult longevity. Our selected strains also exhibited an obvious density dependent longevity (Clare and Luckinbill, 1985). The tested isogenic lines showed a mean
increase of 16.9 days in their life span (Buck et al., 1993b), a response that was much greater
than the five to seven day increase observed in wild strains. This suggests that selection may
have acted so as to increase the magnitude of the density response in these animals. The genetic
variability uncovered by larval crowding must be related to the genetic variability for longevity,
presumably via some common genetically controlled factor(s) such as oxidative stress or developmental time. The L strain animals do develop faster that the R strain animals under both
HD and LD conditions (Buck and Arking, in preparation), although we must point out that this
may have been an unintended consequence of the design of our original selection experiment
(see Arking, 1987, for discussion). Thus, the two strains react differently to this environmental
variable as well. This constitutes another source of plasticity, at least to the extent that developmental time has an effect on longevity. Tigerstedt (1969) has shown that selection for fast
development increases population fitness and liberates additive genetic variance, a finding not
fully supported by Mueller et al. (1991). Dominguez and Albernoz (1987) were able to draw no
general conclusions about the effect of density on the five strains they tested, suggesting the
possible existence of extensive species plasticity for this environmental variable. Our use of
density dependent selection to create the L strain also leads to the prediction that the density
dependent aspect of the phenotype has the potential to become genetically assimilated (Waddington, 1940), thereby decreasing the environmental plasticity of the phenotype by incorporating the environmental effect into the genetic component. We have used density shift experiments (Buck et al., 1993b) to define the existence of a critical period in larval life that begins
no later than 60 h after egg laying and that ends no later than 120 h after egg laying, and during
which the developing larvae must be exposed to high density conditions if the extended longevity phenotype is to be expressed. The nature of the inductive event implied by the existence
of the critical period is still unknown but it might well involve the antioxidant defense system.
During this same time period, we have found that the HD food has an increased redox potential,
relative to the LD food, as indicated by nitroblue tetrazolium measurements on food samples
(Force and Arking, unpublished data). We have also observed that caloric restriction in the
larvae is responsible for the induction of a significantly higher level of adult paraquat resistance
(see Table 4). These observations suggest a role for oxidative stress during the larval stages as
well as in the adult stages as discussed above and in Table 2. This presents a very interesting
developmental problem regarding environmental induction of specific gene activities--an obvious and potentially useful source of plasticity for the extended longevity phenotype. Although
there exists a strong density dependent genotype by environment interaction for lifespan in the
B and O stocks of Rose (1984), this interaction has not been shown to involve as obvious a
density threshold as was shown by Clare and Luckinbill (1985) for the L strains. The differences
between these and other long-lived strains suggest that the presence of this density effect is not
essential to the expression of the extended longevity phenotype. We have confirmed this
supposition by manipulating one of our density dependent long-lived strains so as to produce a
density independent (i.e., constitutive) long-lived strain that has an identical extended longevity
phenotype (Arking, unpublished data). Larval density may be viewed as a condition that is
636
R. ARKING et al.
TABLE 4.
Group
No.
1
2
3
4
5
6
7
8
9
EFFECT OF LARVAL YEAST RESTRICTION ON BODY WEIGHT AND ON PARAQUAT RESISTANCE
n
mg Yeast
per 10 Eggs
203
208
208
208
196
184
206
209
207
0.01
0.025
0.05
0.10
0.25
0.50
0.75
1.0
2.0
mg Body Weight
of 5-Day Adult
on 10 mM
1.40
1.41
1.42
1.41
1.44
1.44
1.39
1.42
1.44
1.41
+_0.01
1.43
+_0.02
Median Survival Time
Paraquat
43.0
42.8
42.7
42.4
37.4
38.6
40.6
34.8
40.9
42.7
+_0.03
38.5
+_2.5
Longevity
48.3
51.0
47.4
50.6
42.0
46.5
44.5
43.0
44.7
Median
49.2
+_1.6
44.1
+_1.7
All animals wee raised under LD conditions (10 eggs/vial) on standard sucrose agar media supplemented
with top yeast as indicated. Our standard nutrition conditions are approximately equivalent to group 7 or
8. There is a discontinuous effect of diet such that feeding less than 0.1 mg/yeasWl0eggs significantly
extends the paraquat response and the median longevity. This threshold is suggested by the dotted line.
The responses of groups 14 (indicated by the italicized means and standard deviations) are significantly
different from those of groups 5-9 (also indicated by italicized numbers) by the Kolmorov-Smirnovtest,
except for the effect of diet restriction on adult body weight, which is not significant.
associated with, but not always essential for, the expression of the extended longevity phenotype. Certainly, some o f the plasticity associated with longevity in different strains of
D r o s o p h i l a may be attributed to the alterations in gene expression patterns associated with
this environmental factor, some of which may have effects on longevity as well as on other
density dependent traits. Finally, Bakker (1961) has shown that larval density had no effect
on adult body weight in his stocks when compensation was made for the effects of competition
for food. This implies that larval density must be considered in the context of dietary restriction
effects.
Dietary restriction is known to be a highly effective method of bringing about extended
longevity in mammals and other forms (Austad, 1989; Finch, 1991). Economos and Lints (1984)
have investigated the effect of larval dietary restriction on the adult longevity of the wild type
(i.e., normal lived) S N O W strain. They found that the strain's maximum longevity of ca. 63 days
was obtained at a submaximal level of ca. 40 mg yeast per 120 eggs. We have observed a similar
effect in our long-lived strains. We have found (unpublished data) that varying the amount of
yeast available to the larvae affects their performance on our standard bioassay of longevity. L
strain animals were raised under LD conditions and allowed to feed on standard sucrose/yeast/
agar media containing concentrations of " t o p y e a s t " ranging from 2.0 mg to 0.01 mg per vial.
The resulting adults were then divided into two subsets for each group. One subset was tested
at five days of age for their paraquat resistance, a known bioassay of longevity in our strains
(Arking et at., 1991); and the other subset was assayed for their adult longevity. We observed
a consistent and statistically significant trend such that larvae fed on yeast concentration of 0.10
mg/vial or less resulted in a higher paraquat resistance and an increased adult life span (Table
4). The data suggest the existence of a dietary threshold below which the antioxidant defense
system genes may possibly be activated. It would be instructive to determine whether the
environmental triggers of larval dietary restriction and larval density exerted their effects
PLASTICITYOFLONGEVITY
637
trhough similar or different mechanisms. There was no observable effect of the yeast concentration on body weight, suggesting that this dietary effect may be rather specific in its effects.
One complication of doing feeding experiments on larvae is that of separating out the direct
effects of nutrition on growth of the adult imaginal discs and internal organs from those affecting
adult longevity via some other means. For example, the metabolic fate of ethanol derived
carbons has been determined in D. melanogaster (Frerikesen et al., 1991, 1994; Heinstra and
Geer, 1991; Geer et al., 1993). These data suggest that the larval ADH activity exerts an almost
complete control over the flux of carbons from ethanol into lipid, its flux control coefficient
being approximately 1.0 (Freriksen et al., 1991; but see Freriksen et al., 1994). This finding
implies that there should be a direct correlation between ADH activity and lipid content. It also
implies that larval dietary restriction and/or ADH gene activity changes may inadvertently affect
the adult lipid content and body weight. Because these latter factors have been hypothesized to
consititute part of the causal nexus underlying extended longevity (Service et al., 1985), then it
follows that larval dietary restriction might affect the longevity of the adult by altering the
animal's biochemistry and, thus, its potential ability to resist stresses such as starvation (Graves
et al., 1992). On the other hand, however, our L strain animals are associated with a significantly
reduced larval ADH mRNA and enzyme activity relative to the R strain (Dudas, 1993; Dudas
and Arking, 1995). Accordingly, one might predict that the resulting adults should have a
relative decrease in their lipid content which, as summarized below, is exactly what we have
observed. Thus, comparatively small difference in the activity of stage- and locus-specific gene
products can bring about substantial changes in the resulting adult; changes that are thought by
some to be causally involved in the determination of adult longevity. It is reasonable to believe
that these different genotypes will respond quite differently to identical environmental factors
such as larval dietary restriction, yeast, and alcohol content of the food, and so forth. Thus, an
understanding of the processes involved in larval stage metabolism should enhance our understanding of the mechanisms modulating the plasticity of the adult longevity phenotype.
Chippindale et al. (1993) have investigated the role of dietary restriction during the adult
stages, and report that dietary restriction enhanced the mean and maximum life spans of both
control and long-lived strains by between 7.1 to 13.3 days. This increase is similar to that seen
in our L and R strains as a result of larval density, although the two environmental factors
presumably impinge on different regulatory gene hierarchies. The effect of dietary restriction in
the adult is a very interesting result in view of the fact that there is very little mitotic activity
in the adult; thus, one should be able to use this tool to investigate metabolic effects on longevity
without the complications of cell replacement. If dietary restriction in the fly works in a manner
similar to that observed in vertebrates, then one can predict that adult dietary restriction should
alter the patterns of adult gene expression such as has been reported by Dudas and Arking (1995)
and by Helfand et al. (1995). However, it should be noted that Le Bourg and Medioni (1991)
have shown that, in their hands, adult dietary restriction had no effect on the longevity of their
wild-type laboratory strain. This result suggests that different genotypes may respond differently
to this environmental variable as well.
Finally, it must be noted that Leroi et al. (1994) have reported that strong genotype by
environment interaction can obscure what was thought to be a fundamental trade off of certain
life-history characters (early fecundity and extended longevity), a finding that suggests that the
continued presence of negative genetic correlations in a selected population may depend on the
particularities of the genotype by environment interactions. If this turns out to be generally true,
then whether a particular environmental factor is judged to have a major or minor effect upon
the expression of the ELP may depend on when the question is asked.
638
R. ARKING et al.
Comparison o f extended longevity mechanisms in different long-lived strains
Several studies by different laboratories on different strains have implicated various biochemical and stress resistant features as being positively associated with increased longevity
(Service et al., 1985, 1987; Graves et al., 1988, 1992; Luckinbill et al., 1988, 1989; Service et
al., 1988; Hoffmann and Parsons, 1989). Service et al. (1985) and Service (1987) found that
increased lipid content as well as increased starvation and dissiccation resistance were associated with increased longevity in their strains. It has been shown that sister lines to one of our
long-lived L strains exhibited increased flight duration relative to a related but short-lived (E)
strain (Graves et al., 1988). In a separate study using the independently derived long-lived O
strain of Rose (1984), Graves et al. (1992) noted that increased flight duration and higher
glycogen content were positively correlated with increased longevity. The authors concluded
that there were at least two distinct physiological mechanisms that extend longevity in Drosophila: one associated with starvation resistance and lipid content, and one associated with
dessication resistance, increased flight endurance, and higher glycogen content. This conclusion
suggests that the extended longevity phenotype can arise from any one of several mechanisms,
each of which act so as to reduce mortality associated with specific environmental stresses.
We have examined various biochemical (protein, lipid, and glycogen content) and stress
resistance (ability to survive starvation, dessication, and exogenous paraquat) parameters of 10
sister lines of five of our different Drosophila strains, four pairs of which were deliberately
selected so as to express either a short-lived (M, 2E) or a long-lived (L, 2L) phenotype, while
the fifth pair (R) was deliberately maintained in a nonselected state and served as the baseline
strain to which all others were compared (Force et al., 1995). Our analysis of the data obtained
from this comparative survey led us to the following conclusions regarding the biological
processes underlying the expression of this differential longevity in our strains, as follows:
First, the protein content is not significantly correlated with the strains selected for either early
or late age of reproduction in our strains.
Second, a reduced lipid content and a lower body weight are statistically associated with the
expression of extended longevity in our strains, in contrast to the reports of other laboratories
using other strains (Service, 1987).
Third, starvation resistance is not correlated with the strains selected for either early or late
age of reproduction in our strains, in contrast to the reports of other laboratories using other
strains (Graves et al., 1992).
Fourth, glycogen content and dessication resistance are positively correlated with longevity
in our strains. However, this association does not appear to be diagnostic for either trait, because
identical values for either trait may be found in strains that have been subjected to diametrically
opposed selection regimes.
Fifth, an enhanced resistance to exogenous paraquat is clearly and significantly associated
with extended longevity in our strains, a finding that confirms and extends our previous findings
regarding the coordinate upregulation of antioxidant gene activities in our long-lived strains as
discussed above (Dudas and Arking, 1995). Paraquat resistance is a reliable diagnostic character, clearly separating extended longevity strains from all others. All of the available data
strongly suggests that our L strains delay the onset of senescence due, at least in part, to their
having a more effective antioxidant defense system.
It is entirely possible that artificial selection operating on these two different progenitor stocks
may have operated so as to bring about two superficially similar extended longevity phenotypes
each dependent on different proximal mechanisms. Some of the observed plasticity of longevity
PLASTICITY OF LONGEVITY
639
may have its origins in this situation, whereby the variety of operative genetic mechanisms, each
with its own set of genotype-environment interactions, greatly increases the longevity in Drosophila and allows the population as a whole to respond appropriately to multiple environmental
signals. However, it is not yet possible to determine whether the presumably independent
proximal mechanisms involved in the expression of extended longevity in these different strains
may actually have some deep genetic identity. Such a possibility is not totally implausible, given
the data of Clark and Keith (1988) showing the interrelated expression of genes coding for
various metabolically important enzymes.
CONCLUSIONS
We have demonstrated that different long-lived strains of Drosophila show a complex but
understandable pattern of variability in their adult longevity as a result of manipulation of
particular genetic and/or environmental factors. Within each strain, each of these factors must
be properly defined, and their interactions understood, if we are to attain a proper understanding
of the mechanisms that regulate the expression of the extended longevity phenotype. The genetic
factors responsible for the extended longevity phenotype(s) are under investigation in several
laboratories and presumably will be soon understood. The environmentally based phenotypic
plasticity may itself reflect the different genotypes involved (Thompson, 1991). Scheiner and
Lyman (1991) have shown that under these conditions, the genetic basis of plasticity appears to
arise from epistatic gene interactions. The question arises as to whether the phenotype of
extended longevity observed in different long-lived strains within the species may actually arise
via different mechanisms. Do similar phenotypes arise via similiar or dissimiliar mechanisms?
A comparative analysis of existing data suggests that the latter is the case. However, there seems
to be an assumption in the literature (e.g., Parsons, 1995) that generalized stress resistance may
be the main mechanism by which adult longevity is extended throughout the entire species.
There is a problem here in the induction of a general conclusion from fairly specific research.
This proposal forces one to lump together mechanisms as disparate from one another as resistance to oxidative stress and resistance to starvation. There is no known generalized stress
resistance mechanism in Drosophila. Expression of the extended longevity phenotype by unrelated strains developed in different laboratories appear to involve different genetic, molecular,
and physiological mechanisms. Thus, it seems prudent not to extrapolate data between unrelated
strains without the support of empirical data. The existence of different mechanisms in independent strains derived from independent progenitor stocks would do much to explain the
obvious disagreements between the data obtained from several laboratories each using strains
that appear to be expressing the same generic extended longevity phenotype. The assumption
that all extended longevity strains of Drosophila are dependent upon the same proximal mechanisms is not supported by the available data, but it appears to have been uncritically accepted
(e.g., Dixon, 1993). This is unfortunate to the extent that it inhibits future experimental strategies. Rose (1991, and elsewhere) has emphasized the importance of replicated selection lines
in aging research. We wish to emphasize the importance of doing comparisons between selection systems. All long-lived stocks of Drosophila are not the same.
Our future work will involve the experimental verification of the genetic-environmental
circuitry discussed here, using molecular and mutational techniques to define, characterize, and
isolate the genes involved in the expression of the extended longevity phenotype in this strain.
It will be of great interest to determine if the regulatory and structural genes operative in our
strain are also operative in other long-lived strains, and, thus, provide a deep genetic identity.
640
R. ARKINGet al.
It is p o s s i b l e t h a t e n e r g y a l l o c a t i o n m e c h a n i s m s
m a y p l a y s u c h a role. S u c h a p h e n o m e n o n
w o u l d f o r c e u s to r e c o n s i d e r i f w h a t w e n o w v i e w as d i f f e r e n t p r o x i m a l m e c h a n i s m s
variations on a common
are but
t h e m e . I f s u c h a d e e p g e n e t i c i d e n t i t y d o e s not, in f a c t , e x i s t , t h e n w e
m a y c o n s i d e r t h e s e g e n e s to b e u n i q u e s t r a i n - s p e c i f i c i d e n t i f i e r s o f q u a l i t a t i v e l y d i f f e r e n t e x tended longevity phenotypes.
Acknowledgments--This paper was initially presented in a Gerontological Society of America sponsored Symposium
on Genetic Plasticity of Longevity in 1991. It has been substantially revised and updated to December 31, 1995. We
acknowledge the helpful comments of Michael R. Rose and an anonymous reviewer on an earlier version of the
manuscript. We also acknowledge the assistance of Dr. Elaine Hochman (WSU Research Support Lab) with the
statistical analysis. Portions of the work described were supported by a Shock Foundation Fellowship to S.P.D., by a
Howard Hughes Underagraduate Research Fellowship to A.G.F., by a WSU President's Excellence Award to R. A., and
by NIH grant AG 08834 to R.A.
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