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First published online October 5, 2006
Journal of Experimental Biology 209, 4129-4139 (2006)
Published by The Company of Biologists 2006
doi: 10.1242/jeb.02492
Delayed development and lifespan extension as features of metabolic lifestyle alteration in C. elegans under dietary restriction
1 NASA Ames Research Center, M/S 239-11, Moffett Field, CA 94035-1000,
USA
2 Department of Biological Sciences, University of Pittsburgh, Pittsburgh,
PA 15260, USA
3 Wyle Laboratories, M/S 239-11, Moffett Field, CA 94035-1000,
USA
4 Department of Developmental Biology, Stanford University, Stanford, CA
94305, USA
* Author for correspondence (e-mail: nate{at}alumni.cmu.edu)
Accepted 15 August 2006
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Summary |
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Key words: health, life style, growth and development, aging, axenic animals
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Introduction |
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TOPSummaryIntroductionMaterials and methodsResultsDiscussionReferences |
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).
Relatively little is known of the natural history of C. elegans, as
distinct from that of nematodes in general, but it is generally accepted that
C. elegans is an R-strategist
(Hodgkin and Barnes, 1991). It
has been noted that the nearly exclusive use of one standard culture system
for growing C. elegans (using Escherichia coli as a food
source) has brought immense success, but has also potentially biased our
understanding of C. elegans biology
(Hodgkin and Doniach, 1997).
For example, when grown on a bacterial diet, C. elegans displays
essentially invariant development with a predictable cell lineage
(Sulston et al., 1983), cell
number and nervous system (White et al.,
1986); we do not know if these properties remain invariant under
other growth conditions.
Following the pioneering work of Dougherty
(Dougherty, 1959), a chemically
defined axenic medium, C. elegans maintenance medium (CeMM), has been
established for liquid culturing of C. elegans
(Lu and Goetsch, 1993;
Szewczyk et al., 2003). The
use of liquid culture techniques allows automation, an important factor for
spaceflight and high throughput experimentation and analysis. Additionally,
the chemically defined nature of CeMM is advantageous to the use of C.
elegans as a biological environmental monitor
(Custodia et al., 2001), in
pharmaceutical discovery (Kaletta and
Hengartner, 2006), and/or in toxicity testing
(Dengg and van Meel, 2004)
where secondary effects of E. coli metabolism can be a confounding
variable (Dougherty, 1959).
Past liquid culturing of C. elegans employing undefined axenic diets
have found increased lifespan, decreased body mass, reduced fecundity and
increased metabolic rate per unit mass, relative to worms cultivated on
bacteria (Houthoofd et al.,
2002). The similarity of these results to C. elegans
grown on reduced amounts of E. coli
(Klass, 1977), suggests that
axenic growth may be `unhealthy' for C. elegans. Alternatively,
axenic cultivation of C. elegans may simply result in an altered life
history in response to altered diet.
Theoretically, a species entering a new environment has a choice in how
best to attempt to thrive (MacArthur and
Wilson, 1967). On the one hand, the intrinsic rate of population
growth could be maximized, at the expense of environmental resource depletion,
by adopting a life history of rapid development and large number of progeny as
is seen with cultivation of C. elegans on a bacterial diet. On the
other hand, the organism could maximize the efficiency of exploitation of the
carrying capacity of the environment resulting in delayed development and a
prolonged reproductive period.
In order to test directly if C. elegans undergo a life history
alteration in response to diet, we have compared life history on two diets. We
show that worms grown in chemically defined medium (CeMM) do indeed undergo a
life history alteration. Animals in CeMM take longer to develop and exhibit a
prolonged reproductive period. Additionally, animals have decreased fecundity,
increased lifespan, reduced metabolic stores, and an increased lifespan. As
development, reproductive period, and lifespan are all prolonged in CeMM, we
find that developmental stages and reproductive period remain fixed
percentages of the 90th percentile of maximum lifespan regardless of diet.
This conservation of life stages as a percentage of lifespan suggests that the
alterations seen in CeMM are adaptive to the diet present and not simply
random pathological alterations (i.e. CeMM-grown animals have a decreased
`rate of living') (Finkel and Holbrook,
2000; Pearl,
1928). Our findings imply that C. elegans can adopt at
least two adult life histories that are each normal and healthy for a specific
diet. As `rate of living', broodsize and metabolic stores are decreased in
CeMM-grown animals, we suspect that metabolic resources available, per unit
time, for various biochemical reactions are decreased as the result of animals
adopting a life history of more efficient exploitation of the carrying
capacity of the environment. Prior studies have shown that the forkhead-class
transcription factor DAF-16 (McElwee et
al., 2003) can control all the adult phenotypes we observe are
altered in CeMM. Microarray analysis indicates increased levels of
daf-16 transcript in CeMM grown animals, suggesting that DAF-16,
whose activity is regulated by insulin signaling
(Ogg and Ruvkun, 1998),
modulates the metabolic life history alteration we observe in response to
altered diet. Our observations demonstrate that animals grown in CeMM are
`healthy but different' from animals grown on nematode growth medium (NGM) and
introduce a powerful system for automation of experimentation on healthy
C. elegans and for systematic analysis of the profound impact of diet
on animal physiology.
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Materials and methods |
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). To control for potential maternal effects of temperature,
N2 and CC1 were allowed to grow for three generations at 15°, 20° or
25°C prior to the onset of these studies. Growth of animals in CeMM at
15°C results in approximately a 10% frequency of arrest at the L4 stage or
development into sterile adults and about 2% male development. These animals
were not excluded from the longevity or broodsize assessments. To assess the growth of individual animals we allowed five wild-type adults to lay eggs for 2 h on NGM plates or for 24 h in 200 µl of CeMM (Fisher) in a well of a 24-well tissue culture plate. Each assessment was completed in triplicate and averaged.
To assess brood size, 100 individual animals were allowed to develop separately on NGM plates or in 200 µl of CeMM in a well of a 24-well tissue culture plate at 15°, 20° or 25°C. Animals were transferred to new plates or wells as needed to distinguish the parent from the progeny (approximately every day or two for NGM and every 3–5 days for CeMM). The total number of L1 stage progeny of each animal was scored and the total number of progeny for each 100 animals was averaged. These animals were also scored once a day for death. Death was determined by a lack of pharyngeal pumping and a lack of movement in response to physical stimulation.
Acute transfer of animals from NGM to CeMM was accomplished by picking early adults off an NGM plate, into a small Petri dish containing 3 ml of BU buffer (70 mmol l–1 potassium phosphate, 70 mmol l–1 NaCl, pH 7.0) with added streptomycin (200 µg ml–1) and kanamycin (20 µg ml–1). After a few minutes of swimming in the BU, individual animals were pipetted into a well of a 24-well tissue culture dish containing 200 µl of CeMM with added streptomycin (200 µg ml–1) and kanamycin (20 µg ml–1) and broodsizes determined as above. We also confirmed that CeMM growth curves, as described above, are unaltered by the addition of streptomycin (200 µg ml–1), kanamycin (20 µg ml–1), or nystatin (500 i.u. ml–1), either individually or together in any combination at 20°C (not shown).
Total RNA for microarray analysis was isolated using TRI Reagent (Molecular
Research Center, Cincinnati, OH, USA) following the supplied protocol. mRNA
isolation was accomplished using the MACS mRNA isolation kit and protocol
(Miltenyi Biotec, Cologne). Isolated RNA was labeled
(DeRisi et al., 1997) as
previously described for hybridization to near full genome C. elegans
DNA microarrays (Jiang et al.,
2001). Microarray processing was preformed as previously described
(Wang and Kim, 2003) and the
data are available from the Stanford Microarray Database
(Gollub et al., 2003).
Reliable data (r>0.6 across the spot, mean value of normalized
intensity above background) was obtained for 10204 cDNAs on at least one
array, 6484 on two, and 1202 on all three. This represents at least one
reliable data point for a little over half the genome and three reliable data
points for a little over 5% of the genome. As displayed here, data from three
reliable data points were averaged with the mountains as previously assigned
(Kim et al., 2001).
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), animals grown in defined CeMM at 20°C display a
developmental delay (Szewczyk et al.,
2003). To assess the nature of this developmental delay we created
a developmental profile for CeMM-grown animals.
Because the rate of C. elegans development is temperature
dependent (Byerly et al.,
1976), we examined the growth rate at three common temperatures of
culture. Animals grown on either an NGM or CeMM diet displayed temperature
dependency of growth and reached early adulthood in times consistent with
those previously published for NGM or complex axenic media
(Fig. 1)
(Byerly et al., 1976;
DeCuyper and Vanfleteren,
1982). Although growth in liquid CeMM may be different in many
respects from growth on CeMM solidified with agar, it has previously been
reported that the physical form of CeMM does not influence the growth rate at
20°C (Szewczyk et al.,
2003).
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). The lengths of CeMM liquid-grown animals at moults
(Fig. 2) differ from the
calculated lengths of C. elegans on NGM plates
(Byerly et al., 1976). It may
simply be that fewer resources are available for cuticle formation in CeMM.
However, unlike any other trait we have measured, length at time of moult is
relatively temperature insensitive for both NGM and CeMM grown animals. The
simplest explanation for this is that length dictates moulting, with some
unknown factor setting an appropriate length in response to diet. This would
explain why the distribution patterns of expression of moulting genes
(Frand et al., 2005) and
length at moults (Fig. 2) look
strikingly similar.
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A lengthening of the reproductive period in defined medium
The general slowing of development in response to culturing in CeMM is
consistent with the developmental delay predicted from a life history
alteration toward maximization of the exploitation of the carrying capacity of
the environment (MacArthur and Wilson,
1967). We therefore examined the reproductive period of CeMM grown
animals and found it was likewise lengthened
(Table 2), consistent with a
life history alteration. Broodsize is an inverse function of temperature but
is not a strong diagnostic of temperature of cultivation
(Fig. 3;
Table 2). The same curve
describes the relationship between broodsize and temperature for both diets
(not shown), indicating conservation of at least some of the processes
underlying reproduction. However, the lengthening of the reproductive period
did not allow CeMM-grown animals to produce as many eggs as NGM-grown
counterparts. This may suggest that, for C. elegans, there is a
`reproductive cost' to shifting toward a life history that maximizes
exploitation of the carrying capacity of the environment.
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Broodsize and lifespan are independently responsive to the environment
Growth of C. elegans in undefined medium has previously been shown
to result in decreased fecundity, and these animals have been shown to have an
increased lifespan (DeCuyper and
Vanfleteren, 1982). Animals of various species with increased
lifespan as the result of dietary restriction or reduction of function
mutations in an insulin signaling pathway have led to the hypothesis that
broodsize may directly influence lifespan as the result of a `cost of
reproduction' (Partridge et al.,
2005). To test directly the hypothesis that C. elegans
broodsize influences lifespan, we compared broodsize and lifespan of animals
grown on NGM and CeMM. Consistent with prior reports, we found that C.
elegans exhibits both reduced fecundity and increased lifespan when on an
axenic diet (Fig. 3). Because
C. elegans broodsize and lifespan are temperature-dependent traits
(Byerly et al., 1976;
Klass, 1977) we chose to apply
a standard systems biology approach of altering a second variable
(Sachs et al., 2005),
temperature, to understand the relationship between fecundity and lifespan.
Broodsize and lifespan covary for both temperature and diet but there is a
direct correlation for temperature and an inverse correlation for diet
(Fig. 3). Thus, there is not a
clear relationship between broodsize and lifespan. It may be that the
covariance results from simple coregulation or that a feedback system
maintains a balance between the two.
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), animals grown in defined CeMM have a lengthened lifespan
compared to those grown on NGM (Fig.
4A). As is also true for animals grown in complex axenic media
(Vanfleteren et al., 1998),
CeMM survival curves are best fit using a three-parameter logistic equation. A
question was posed (Vanfleteren et al.,
1998) as to whether or not such curves were only appropriate for
axenic culture. We find our NGM survival curves are also best fit using this
method, although the actual values of the parameters are quite different. This
point can easily be seen when survival is plotted against the median lifespan
(Fig. 4B).
The observation that many animals die sooner as temperature increases has,
in part, led to the hypothesis that there is a `rate of living'
(Finkel and Holbrook, 2000;
Pearl, 1928). Consistent with
this hypothesis, we find that temperature dependence of population death
curves holds regardless of diet (Fig.
4A).
In contrast to the temperature dependence of population death curves, the
temperature dependence of population survival curves is quite different for
animals grown on NGM versus in CeMM
(Fig. 4C). Animals always die
sooner as temperature increases (Fig.
4A) but the fraction of animals dying at a given age (i.e. the
steepness of the curve) increases with temperature for NGM- but not CeMM-grown
animals (Fig. 4C). Since the
cause of the temperature dependence of population survival curves is unknown
for NGM-grown animals, we can only speculate as to why temperature dependence
is lost in CeMM-grown animals. The simplest explanation is that E.
coli toxicity (Walker et al.,
2005) is temperature dependent. This possibility could be tested
by utilizing UV-killed E. coli, which are already known to increase
lifespan at 20°C (Gems and Riddle,
2000). An alternative explanation is that failure of critical
biochemical processes, which are inherently temperature dependent, underlies
death in C. elegans. Theoretically, increased efficiency in
biochemical processes as the result of increased efficiency in exploitation of
the carrying capacity of the environment in CeMM could mask apparent
temperature sensitive failure of these processes (i.e. CeMM growth shifts the
temperature tolerance of survival). This latter explanation could explain why
CeMM-grown animals die sooner at 25°C than predicted from the 15°C and
20°C data.
Taken together, the data in Fig.
4 add weight to the suggestion
(Kirkwood et al., 2005) that
C. elegans population survival curves describe a stochastic process
where individual deaths occur randomly across the adult lifespan. The data in
Fig. 4B suggest that this
process can be altered by diet, so as to affect the likelihood that time of
death has a large deviation from the median.
Fig. 4C further suggests that a
bacterial diet prolongs survival, making deaths of individuals appear less
randomly distributed over the population lifespan, whereas an axenic diet
prolongs lifespan. An ironic interpretation of these data is that the cost of
reduced susceptibility to stochastic death is a shortened lifespan.
Reproduction comes with a higher risk of death in defined medium
When grown in defined CeMM, a small percentage of animals appear to live
substantially longer than others (Fig.
4A), as reported for animals grown in other, chemically undefined,
axenic medium (DeCuyper and Vanfleteren,
1982). To determine why some animals appear to live longer, we
examined the cause(s) of death. We found that animals grown in CeMM are twice
as likely to die from prolapsed or ruptured vulva and internal hatching than
are NGM grown animals (Table
3). When these deaths are omitted from death curves such as in
Fig. 4A, the curves still
depict a stochastic process but the extent to which a small percentage of
animals appear to live much longer than others is greatly reduced (not shown).
This suggests that apparent tails in death curves of axenically grown C.
elegans are observed, in part, because reproduction comes at large cost
(i.e. risk of death) to a population in axenic medium.
Whereas reproduction may come at a huge `cost' to an individual animal, there is not an obvious `cost' that can account for the decreased broodsizes of CeMM-grown animals as even those animals that do not die as the result of reproducing do not produce broodsizes comparable to those of NGM grown animals (Fig. 3). The general slowing of development, and lengthening of the reproductive period suggest that mobilization of the resources needed to proceed through each developmental stage and to produce a zygote takes longer. The hypothesis that CeMM-grown animals have fewer resources available, per unit time, for developmental processes and zygote production predicts that stored resources are decreased.
Decrease metabolic storage in defined medium
Like other axenically grown animals
(Houthoofd et al., 2002),
those grown in CeMM are thinner than counterparts on NGM. To test the idea
that CeMM-grown animals place fewer metabolic resources in storage, we
examined lipid and protein stores.
Using Nile Red to visualize lipid stores
(Ashrafi et al., 2003), we find
CeMM-grown animals have decreased lipid stores in the intestine
(Fig. 5A). Ninety percent of
animals fed on CeMM had lipid stores at or below the level observed in animals
starved for 24 h after having been fed on NGM. The remaining ten percent had
higher lipid stores but not as high as those of animals fed on NGM.
|
). We
employed unc-54::lacZ transgenic animals to examine a
muscle-expressed transgenic ß-galactosidase fusion protein that remains
stable in fed animals on NGM, and which is catabolised upon starvation
(Zdinak et al., 1997). Nearly
all animals fed on NGM displayed normal X-gal staining in the body wall
muscles and complete loss of staining following 48 h of starvation
(Fig. 5B). By contrast, only
20% of animals fed on CeMM displayed normal staining in the body wall muscles,
with the remaining 80% displaying no staining in the body wall muscles.
Microarray analysis (see below), suggests that decreased synthesis of the
reporter protein is sufficient to account for the decreased staining. These
results are consistent with CeMM-grown animals having decreased protein stores
in muscle.
The lipid and protein staining patterns do not appear to represent the
effects of continued starvation, as staining does not decrease with age
throughout the reproductive period. The reduced stores could simply represent
decreased commitment to storage, or a more complex situation. In the case of
the proteins, microarray analysis (see below) suggests decreased expression of
muscle proteins, which is consistent with decreased commitment to storage. The
reduced lipid stores however, might themselves alter endocrine signaling
(Dowell et al., 2005).
Manipulation of C. elegans diet by changing the composition of
chemically defined media should provide more insight into the mechanism(s) of
resource allocation to various biological processes (e.g. reproduction,
storage, movement, etc.) than single diet based studies.
Development and reproductive period are a relatively constant fraction of lifespan
The slowed development, extended reproductive period, conserved
relationship between broodsize and temperature, extended lifespan, and the
decreased metabolic stores are consistent with adjustment of a "rate of
living" that is determined by metabolism
(Finkel and Holbrook, 2000;
Pearl, 1928). If such an
adjustment has occurred, then the reproductive period should be a scaled
percentage of lifespan regardless of diet. We examined reproductive period as
a percentage of the 90th percentile of maximum lifespan to avoid any
statistical error introduced by fluctuation in the day of death of the last
survivor. Surprisingly, both developmental stages and reproductive period are
constant percentages, within experimental error, of the 90th percentile of
maximum lifespan regardless of diet (Table
1,
2). Thus, the amount of time
C. elegans spends in any developmental stage or the reproductive
period can be used to predict accurately the maximum population (though not
necessarily individual) lifespan, when temperature and diet are held constant.
Intriguingly, the temperature-dependent increase in time spent in the L1 stage
observed for NGM-grown animals remains even when development is viewed in the
context of lifespan (Table 1).
This may be consistent with L1 diapause, like dauer diapause, being regulated
by both temperature and diet.
Short term plasticity of life history traits?
While we have shown that C. elegans can adopt two alternative
adult life histories as a long-term response to two different diets, an
interesting and important question remains: Are life history traits,
individually and/or collectively, also plastic as a short-term response to
abrupt changes in diet and/or temperature? C. elegans lifespan and
rates of behavior appear to be set early in life
(Dillin et al., 2002),
suggesting that once selected, life history is committed. However, prior
studies using complex axenic media suggest life history has some level of
plasticity. Acute diet shift results in lifespan alteration
(Houthoofd et al., 2005b) and
acute temperature shift results in broodsize alteration
(Houthoofd et al., 2005a); in
both cases the alteration is toward the predicted value for animals raised
exclusively on the new diet or temperature and away from the value for animals
raised on the original diet or temperature. In keeping with these past
results, we find acute shift from NGM to CeMM at 20°C, prior to egg
laying, results in broodsizes that are much smaller than on NGM. These
broodsizes are also smaller than those from animals grown exclusively in CeMM,
but this difference is not statistically significant (41±15,
N=10; compare with Table
2). An appropriate increase in the reproductive period is also
observed (7.5 days; compare with Table
2). Systematic alteration of diet and temperature throughout the
lifespan should clarify the nature and extent of such lifestyle
plasticity.
Transcript levels of daf-16 are increased in defined medium
The question of how the phenotypes observed in response to diet are tied to
the molecular mechanism of metabolic alteration led us to probe gene
expression. Roughly 10% of the genome, 1881 cDNAs, had a reliable and
significant (at least a fourfold) expression difference between NGM and CeMM
grown animals in at least one of three trials. Given the large-scale changes
in gene expression in response to diet, any discussion of the results is
necessarily limited by our incomplete understanding of the function of each
gene in the C. elegans genome and our current limited ability to
appreciate how simultaneous changes in 2000 genes translate into organism
scale changes. Therefore, the full microarray dataset is available as
supplementary material or from the Stanford Microarray Database or upon
request from the authors.
The overlapping set of genes that is reliably, significantly, and
reproducibly altered on all three trials is displayed in
Table 4. Consistent with the
reduced lipid and protein stores, 8 of the 26 genes identified as
downregulated have a putative role in metabolism based upon sequence identity.
Experimental evidence confirms this for three: daf-18
(Ogg and Ruvkun, 1998) and
F25D1.1 and K05C4.5 (Wang and Kim,
2003). Similarly, 8 of the 22 genes identified as upregulated in
the overlapping dataset are important for recovery from the metabolically
altered dauer state and/or are downstream targets of the transcription factor
DAF-16, which is known to regulate metabolic alterations
(McElwee et al., 2003;
Murphy et al., 2003;
Wang and Kim, 2003). The
predicted effects of the genes identified by the overlapping dataset are
consistent with our life history and metabolic storage data.
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The overlapping dataset identifies daf-16 and three known
downstream targets as being upregulated. Previous studies of daf-16
have suggested a role in regulation of lifespan and metabolism
(Ogg and Ruvkun, 1998) and
transgenic overexpression yields thinner animals with delayed development,
increased reproductive period and decreased broodsize
(Henderson and Johnson, 2001).
Therefore upregulation of daf-16 is consistent with the morphological
and physiological changes observed in CeMM-grown animals.
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;
Murphy et al., 2003),
suggesting that these genes may be involved in the lifespan extension seen in
CeMM-grown animals.
DAF-16 (FOXO) as a modulator of three C. elegans life histories
As discussed above, DAF-16 is known to control all the adult phenotypes we
have reported as being altered in CeMM. The microarray analysis demonstrates
that daf-16 is upregulated and that known downstream targets of
daf-16 are both up and downregulated (note that daf-16 is
known to both up- and downregulate various downstream targets)
(Lee et al., 2003a;
Lee et al., 2003b;
McElwee et al., 2003;
Murphy et al., 2003). In other
words, upregulation of daf-16 may be sufficient to account for our
observations and alteration of daf-16 and its downstream targets
correlate with the alteration of diet. Thus, it is reasonable to conjecture
that daf-16 and its downstream targets are involved in the responses
to growth on CeMM.
Here, we have compared C. elegans development, metabolism, gene
expression and physiology in two different systems of culture and found two
life histories for C. elegans. On NGM, C. elegans adopts a
life history of rapid development and a large number of progeny. On CeMM,
C. elegans adopts a life history of efficient exploitation of the
carrying capacity of the environment. The modulation of these two extremes of
life history strategies appears to involve DAF-16. This may not be surprising
given the appreciated role of insulin-like signaling in response to levels of
food. What is potentially surprising is that DAF-16 may modulate three
alternative life histories: the two distinct developmental courses represented
by normal and dauer larva development
(Wolkow et al., 2000), and
third, the choice between the life histories displayed during growth on NGM or
CeMM. An attractive model is that under conditions of limited resources, such
as in CeMM, DAF-16 levels, not just activity, are elevated across the
developmental stages of C. elegans, and are poised to push animals to
an L1 or dauer arrest should conditions further deteriorate. In the future, it
may be of interest to examine the life histories of C. elegans
expressing different, quantifiable, levels of DAF-16.
Implications of life history alteration on studying the genetic control of lifespan
Years of studying the genetic basis of lifespan control in C.
elegans have identified an insulin-like signaling pathway acting,
via the action of daf-16 upon metabolism, as a principal
mechanism controlling C. elegans lifespan
(Hansen et al., 2005). The
details of the link between levels and activities of metabolic enzymes and
lifespan remain to be studied in detail. However, our finding that C.
elegans can adopt two distinct adult life histories suggests that many of
the genes identified as regulating C. elegans lifespan actually act
via global life history alteration rather than solely upon lifespan.
It should prove a matter of much interest to determine to what extent the
various life history parameters (developmental timing, reproductive period,
brood size, metabolic stores and lifespan) are coordinately controlled. Our
observations suggest that genes newly identified as controlling lifespan
should be examined for effects on all life history parameters and in animals
exhibiting both life histories, so as to distinguish if genes that extend
lifespan are invariably associated with global life history alteration.
C. elegans `health' is defined by diet
During the development of axenic media for nematode cultivation, the field
was driven by the urge to identify missing nutrients. Perhaps because of this,
the observation that measured parameters of animals grown in axenic media
resemble those of animals grown on reduced amounts of E. coli is
often interpreted as indicating that axenic cultivation is unhealthy for
C. elegans. Indeed, a cursory look at Tables
1,
2 and
3 gives the impression that
animals grown in CeMM are less healthy than their NGM-grown counterparts.
Development takes longer, broodsize is decreased, and deaths associated with
reproduction are increased in CeMM-grown animals relative to NGM-grown ones.
However, if development is represented as a function of maximum lifespan
rather than days, it is striking that both the length of developmental stages
(Table 1) and the reproductive
period (Table 2) remain
constant percentages of maximum lifespan regardless of NGM or CeMM diet. These
observations suggest that both CeMM- and NGM-grown animals are normal and
`healthy'. In other words, C. elegans is capable of exhibiting two
very different life histories each of which is adaptive for the diet present.
Thus, `health' is conditional upon environment such that what is healthy in
one environment is not in another. This idea has been suggested to be true of
all animals (Geist, 1979).
Perhaps what is most remarkable of our observations is that C.
elegans can change between these states of `health' or `fitness'. As
previously mentioned, it should prove a matter of much interest to clarify the
nature and extent of lifestyle plasticity.
In addition to providing a mechanism for exploring lifestyle plasticity,
the combination of C. elegans genetics and dietary manipulations
offers a powerful platform on which to study the profound impact of diet on
animal physiology, as originally suggested by Dougherty
(Dougherty and Calhoun, 1948).
When making dietary manipulations, chemically defined medium or E.
coli variants such as strains deleting specific genes of interest (for
example coenzyme Q) (Larsen and Clarke,
2002), appear to be attractive approaches. In order to decide
whether an E. coli variant or CeMM is a more appropriate tool for a
particular manipulation, it is important to learn how much of what is known of
C. elegans biology on E. coli is applicable to C.
elegans biology in CeMM. Our data provide a starting point to guide such
decisions.
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Acknowledgments |
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Footnotes |
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References |
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Ashrafi, K., Chang, F. Y., Watts, J. L., Fraser, A. G., Kamath, R. S., Ahringer, J. and Ruvkun, G. (2003). Genome-wide RNAi analysis of Caenorhabditis elegans fat regulatory genes. Nature 421,268 -272.[CrossRef][Medline]
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Custodia, N., Won, S. J., Novillo, A., Wieland, M., Li, C. and Callard, I. P. (2001). Caenorhabditis elegans as an environmental monitor using DNA microarray analysis. Ann. N. Y. Acad. Sci. 948,32 -42.[Medline]
DeCuyper, C. and Vanfleteren, J. R. (1982). Nutritional alteration of life span in the nematode C. elegans.Age 5,42 -45.[CrossRef]
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