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First published online March 2, 2006
Journal of Experimental Biology 209, 1064-1073 (2006)
Published by The Company of Biologists 2006
doi: 10.1242/jeb.02129
The relative contributions of developmental plasticity and adult acclimation to physiological variation in the tsetse fly, Glossina pallidipes (Diptera, Glossinidae)
1 Spatial, Physiological and Conservation Ecology Group, Department of
Botany and Zoology, University of Stellenbosch, Private Bag X1, Matieland,
7602, Stellenbosch, South Africa
2 Centre for Invasion Biology, University of Stellenbosch, Private Bag X1,
Matieland, 7602, Stellenbosch, South Africa
* Author for correspondence (e-mail: jst{at}sun.ac.za)
Accepted 26 January 2006
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Key words: intra-specific variation, metabolic rate, phenotypic plasticity, water balance, beneficial acclimation hypothesis, climatic stress resistance, tsetse fly
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;
Hoffmann et al., 2005b), but
could also significantly affect the nature of evolutionary responses of
populations (Price et al.,
2003) and their likely survival, especially in the face of rapidly
changing modern climates (Helmuth et al.,
2005; Somero,
2005). Physiologists have long assumed that acclimation (or
acclimatization, both are forms of plasticity) to a particular environment
enhances performance in that environment (e.g.
Hochachka and Somero, 2002;
Prosser, 1986) such that
acclimation is beneficial (see Leroi et
al., 1994). However, in recent years the generality of this
beneficial acclimation hypothesis has been questioned because most formal
tests of the predictions thereof have been negative
(Gibert et al., 2001;
Gilchrist and Huey, 2001;
Leroi et al., 1994;
Sibly et al., 1997;
Woods, 1999;
Woods and Harrison, 2001;
Zamudio et al., 1995) (but see
Nunney and Cheung, 1997;
Thomson et al., 2001).
Subsequently, Wilson and Franklin (Wilson
and Franklin, 2002) argued that many of these tests actually
addressed the adaptive significance of developmental plasticity rather than of
acclimation. That is, the tests examined the consequences for the adult
phenotype of altered rearing environments, rather than the consequences of
environmental variation within a given life-stage, and more specifically, the
adults. Therefore, they should not be considered tests of the beneficial
acclimation hypothesis because acclimation is typically defined as a
reversible, facultative response to changes in the adult environment
(Spicer and Gaston, 1999;
Wilson and Franklin, 2002).
Given the wide range of conditions under which adaptive phenotypic plasticity
is thought to arise (Berrigan and Scheiner,
2004; Scheiner,
1993), these alternative perspectives on beneficial acclimation
have proven controversial.
One outcome of the debate is the realization that few studies have sought
to examine the effects of acclimation in the rearing stage (developmental
plasticity) relative to those in the adult or within a given life-stage (adult
acclimation, sometimes also known as phenotypic flexibility) (see
Piersma and Drent, 2003), and
the nature of the interactions between them. The most detailed work to date
has been that of Fischer and coworkers
(Fischer et al., 2003;
Zeilstra and Fischer, 2005).
In the first study (Fischer et al.,
2003), rearing temperature had a substantial effect on the size of
eggs laid by Bicyclus anynana butterflies. However, these effects
were largely reversible after adults were held at different temperatures.
Thus, the magnitude of the developmental plasticity and adult acclimation
effects was similar. The second study
(Zeilstra and Fischer, 2005)
used Lycaena tityrus and again found that the effects of
developmental plasticity on recovery time from cold shock were largely
reversible. They also found that as long as newly eclosed adults were not
exposed to cold shock, adult temperatures (i.e. an adult exposure and a
rearing exposure to different temperatures) also affected recovery time,
though these times tended to be shorter than those for butterflies exposed to
cold shock immediately after eclosion (i.e. a rearing exposure only). Few
other studies of this nature have been undertaken (though for discussion of
early work, see Spicer and Gaston,
1999; and see Tracy and
Walsberg, 2001), and therefore their generality is not
certain.
In this study we therefore examine the effects of altering treatment
temperatures during pupal development (developmental plasticity), during the
adult stage (adult acclimation) and during both stages (combined plasticity)
on four traits, viz. critical thermal minimum and maximum, water loss
rate and metabolic rate, in the tsetse fly Glossina pallidipes
Austen. This species was chosen for several reasons. First, variation in the
traits of interest across the Insecta is typically partitioned at family and
order levels (Chown et al.,
2002). Therefore, investigations of non-lepidopteran species are
likely to provide a rapid way of determining how general previous findings are
likely to be. Second, temperature and water availability are important
correlates of the distribution of G. pallidipes and play major roles
in influencing its population dynamics directly
(Hargrove, 2004;
Rogers and Robinson, 2004) and
indirectly via metabolic rate
(Bursell et al., 1974;
Bursell and Taylor, 1980;
Terblanche et al., 2004).
Investigating the mechanistic bases of the responses of flies to their
external environment is therefore a significant link in the causal chain of
reasoning from environment to population dynamics. Finally, as a vector of
animal disease, G. pallidipes has important effects on animal health,
and thus indirect effects upon socio-economic development in Africa
(Maudlin et al., 2004).
Understanding the likely current and future determinants of its abundance and
distribution is therefore of considerable significance for ongoing development
of the continent (Maudlin et al.,
2004).
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).
It is these pupae that were subjected to an acclimation treatment, or simply
maintained close to the original conditions in which they were held at the
Seibersdorf laboratory.
On arrival, pupae were immediately placed in Petri dishes (N=50
per dish) on paper towels, which were stored inside two plastic containers
with non-airtight lids and transferred to a climate chamber held at 25°C
(mean ± s.d.: 24.8±1.0). They were retained at this temperature
except in the case of the developmental plasticity treatments, in which pupae
were moved to incubators set at either 21°C or 29°C (see below), but
otherwise identical to those described here. Relative humidity (RH) inside
containers was regulated by means of saturated salt (NaCl) solutions located
within each container to give 76% RH
(Winston and Bates, 1960). At
the first sign of eclosion, pupae were transferred to 10–12 mesh cages
(10 cm diameter, N<50 per cage) and either retained at the
original temperature or moved later to a different temperature for an adult
acclimation treatment (details below). Cages were stored inside closed,
non-airtight plastic containers with relative humidity regulated as above.
The adults were fed using a membrane-tray system (see
Gooding et al., 1997) every
alternate day, similar to the methods described elsewhere
(Terblanche et al., 2004), and
subsequently container locations were randomized within climate chambers. Care
was taken to ensure that all treatment groups were handled for the same
duration during transfer from the climate chamber to the feeding area, and
spent a similar amount of time outside of the climate chambers whilst feeding
(
25 min per group). Temperatures during shipment and acclimation were
recorded using Thermocron iButtons (Dallas Semiconductors, Dallas, Texas, USA;
sampling rate=15 min; temperature during shipment: 24.2±2.9°C, mean
± s.d.).
Treatments
Three experimental treatments were undertaken: a developmental plasticity
treatment, involving manipulation of the temperatures that the pupae
experienced; an adult acclimation treatment in which pupae developed at a
common temperature, but adults were exposed to different temperatures; and a
combined treatment where both pupae and adults were exposed to a given
temperature (Fig. 1). In the
case of developmental plasticity the following protocol was followed: Pupae
received a 6-day acclimation in climate chambers set to 21°C (mean
± s.d.: 20.5±1.0°C), and 29°C (28.0±0.2°C)
starting approximately 12–13 days prior to the expected date of
emergence (i.e. just over halfway through the pupal stage). Pupae were also
kept at 25°C, though handled in the same way as the other groups, to
provide insight into the changes relative to baseline conditions (for
rationale, see Sinclair and Chown,
2005). After 6 days of acclimation, all groups were returned to
25°C for emergence. This treatment does not constitute a full rearing
exposure (as in Fischer et al.,
2003), but nonetheless has a marked effect on the pupae (as
confirmed in pilot trials). Moreover, it was undertaken because the time
course of critical thermal minima Tcrit,min has been
examined in adult G. pallidipes previously. This revealed that a
maximum response is typically obtained within 5 days of exposure, and does not
reflect a graded (temperature-dependent) response, but rather demonstrates a
distinct temperature threshold (Terblanche
et al., in press). Eclosion commenced in the warmest acclimation
group c. 7–8 days before the coolest acclimation group. Thus, depending
on the experimental temperature in each developmental plasticity group, a
total of 12–19 days passed between the end of the acclimation period and
the onset of the experimental assessments. For G. pallidipes, 10 days
at 25°C is sufficient to completely reverse adult acclimation responses in
Tcrit,min after 9 days at 19–21°C (i.e. adult
acclimation responses for this trait were reversible)
(Terblanche et al., in press).
After 30–40% of the flies in an experimental group had begun to eclose,
flies were taken through three blood meals, or `hunger cycles' (6 days),
and used for experimental assessment on approximately the eighth day of the
adult stage in a fasted, post-developmental (non-teneral) state. This group
was considered the `developmental plasticity' treatment
(Fig. 1).
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On the day following the experimental assessment, the remaining flies were
fed as usual and were then transferred back to the same temperature received
during the pupal stage (e.g. flies which were exposed to 21°C during the
pupal stage were returned to 21°C) and left to acclimate for a further 6
days, whereupon the same physiological measures were assessed. These flies
were labelled the `combined plasticity' treatment
(Fig. 1). Preliminary
experiments using Tcrit,min in adult G.
pallidipes showed that acclimation responses can be fully induced after 6
days and Tcrit,min does not change further with longer
duration of acclimation (up to 12 days)
(Terblanche et al., in
press).
The final treatment was the adult acclimation treatment (Fig. 1). Pupae left at 25°C (24.8±1.0°C) until after eclosion were divided into groups and placed in climate chambers set to 21, 25 and 29°C. The flies were switched to these cabinets on day 6 after three hunger cycles (as above), and left at these temperatures for 6 days before traits were assessed.
Experimental assessments
All experimental assessments were performed on adult flies. In each
experiment, care was taken to randomly select flies from as many cages as
possible. In cases where the number of flies required was higher than the
number of available cages, fly selection was balanced among cages. In
addition, all acclimation groups were handled with similar duration and
vigour. Although no cagextreatment effects could be detected in
preliminary experiments (Terblanche et
al., in press), the present treatments, in conjunction with
frequently randomized cage locations, probably prevented any significant
cagextreatment interactions.
Critical thermal limits
An insulated system with eleven double-jacketed isolation chambers was
connected to a programmable water bath (LTD 20 with PZ1 programmer, Grant
Instruments, Cambridge, UK), which regulated water flow around the chambers. A
single fly was placed in each of the ten chambers. A 40 SWG type-T
thermocouple was inserted into a control chamber to measure chamber
temperatures. The flies were allowed to equilibrate for 10 min at either 12 or
35°C before the commencement of the respective minimum and maximum
critical thermal limit assessments. Because of their small body mass, the body
temperature of flies was considered equivalent to the chamber temperature.
Moreover, there is no significant difference between body temperature and
ambient temperature across the range 25–45°C under high humidity
conditions in G. morsitans (Edney
and Barrass, 1962). After equilibration, the chamber's temperature
was decreased or increased at a rate of 0.25°C min–1.
Critical thermal minima (Tcrit,min) were defined as the
loss of coordinated muscle function at decreasing temperatures and critical
thermal maxima (Tcrit,max) as the onset of muscle spasms
at increasing temperatures (Klok and
Chown, 1998). These endpoints are readily identifiable for any
species once an observer is practiced
(Lutterschmidt and Hutchison,
1997). Typically the variance about the endpoints is low, and here
a single observer (C. J. Klok) undertook all of this work. Moreover, this
experimental procedure has been verified using thermolimit respirometry
(Klok et al., 2004), is widely
used to assess thermal limits (Chown and
Nicolson, 2004), and the observer typically was not informed which
acclimation treatment was being assessed. Groups of ten flies were assessed
together and the temperature at which these limits were observed was recorded
for each individual fly. Preliminary experiments using adult flies found no
effect of gender, age or feeding status on critical thermal limits (C. J. Klok
and J. S. Terblanche, unpublished data), and we assumed that this was also the
case in the present study.
Desiccation rate
To remove possible confounding effects of sex or pregnancy, only male flies
were used in desiccation experiments. Flies (N=16), individually
contained in 5 ml cuvettes, were subjected to desiccation in flowing air
(<2.5% RH) for 10 h at 25.0±1.0°C in a climate chamber
(Labotech, Pretoria, South Africa). Air flow, produced by an aquarium pump,
was directed through a scrubbing column containing silica gel and Drierite
(Xenia, OH, USA) as desiccants to remove residual water, and then into a mass
flow-controller to control the air flow rate. The mass flow-controller outlet
was connected to a Sable Systems (Las Vegas, NV, USA) MF8 airflow manifold.
Each outflow channel of the manifold was further split in two so that two
cuvettes, each containing a fly, were attached per manifold channel. The air
flow rate through each cuvette, tested using a second mass flow-controller,
was regulated to 100 ml min–1. Experiments took place during
the night (21:00 h–07:00 h) because this represents a period of minimal
activity in tsetse (Brady,
1988; Kyorku and Brady,
1993). Mass was recorded before and after an experiment using an
electronic microbalance (0.1 mg, Avery Berkel FA 304T, EU, Fairmont, MN, USA)
and the difference was assumed to be a result of water loss (acknowledging
that some mass loss is due to substrate catabolism but that this is negligible
relative to the quantity of water loss over the time scales investigated here
(Bursell, 1957)). Flies were
dried to constant mass (50–60°C for 72 h) and re-weighed
to give dry body mass.
Metabolic rate
Metabolic rate was recorded using flow-through respirometry. A calibrated
LI-6262 (LiCor; Lincoln, NN, USA) infra-red gas analyzer (IRGA) was connected
to a Sable Systems eight channel multiplexer inside a temperature-controlled
cabinet, as described previously
(Terblanche et al., 2004). The
first seven channels regulated the flow-through respirometry for individual
flies and channel eight was used as an empty reference channel for
CO2 and H2O baseline measurements. These recordings were
performed for fasted, post-development males at 25°C in each acclimation
group. Mass was recorded before and after respirometry recordings as described
in the desiccation experiments. Airflow was regulated to 100 ml
min–1 using a mass flow-controller and the outside air was
scrubbed through sodalime, silica-gel and Drierite columns to remove water and
CO2. A Sable Systems AD-1 activity detector was connected to the
first cuvette only to compare active and resting gas exchange traces in all
flies. Previous studies have shown that in this species activity can be
reliably detected from the
CO2 trace without an
activity detection system (Terblanche et
al., 2004), which was confirmed here. Sable Systems Datacan V
software was used to extract and analyze standard (resting) metabolic rate
(SMR) data from 5–8 individuals per treatment group.
Statistical analyses
For all traits, the effect of the treatment (21 or 29°C) relative to
the standard rearing conditions (25°C) was assessed. One way analyses of
variance (ANOVA) followed by post-hoc tests for homogeneity were used
to compare the three temperature groups in the case of
Tcrit,min and Tcrit,max. Because body
mass influences both metabolic rate and water loss rate
(Addo-Bediako et al., 2001;
Addo-Bediako et al., 2002)
analyses of covariance were used for these traits. These analyses provided
primary insight into the effects of developmental plasticity, adult
acclimation, and the combined treatment. However, to assess the relative
contributions of plasticity type and treatment temperature to trait variation,
two pure Model II (random effects) nested ANOVAs were undertaken
(Sokal and Rohlf, 1995, p.
274). In the first analysis, treatment temperature was nested within
plasticity type using data for developmental plasticity and adult acclimation
only. In the second analysis, all three plasticity types were assessed nested
within treatment temperature. These two nested ANOVAs present complimentary
perspectives on the relative importance of plasticity type and treatment
temperature for variation in each of the traits. Previous work demonstrated
that the effects of developmental plasticity can be altered following adult
acclimation for several traits (Fischer et
al., 2003; Zeilstra and
Fischer, 2005). Therefore we expected equal contributions of
plasticity types to total variance. Sample sizes for physiological assessments
ranged from 9–20 (Tcrit,max), 12–20
(Tcrit,min), 8–16 (water balance) and 5–8
(metabolic rate) individuals per treatment per temperature.
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Treatment temperature at the pupal stage had little effect on water loss rate, and this was also true for adult acclimation, except for a strong decline following adult acclimation at a treatment temperature of 29°C (Fig. 4). The combined plasticity treatment at 29°C also resulted in a substantial increase in water loss rate. In consequence, plasticity type explained a larger proportion of the variance when when all three plasticity types were included in the nested ANOVA (Table 2). However, surprisingly little of the variance in desiccation rate could be explained by plasticity type when only developmental plasticity and adult acclimation were considered (Table 1).
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When only developmental plasticity and adult acclimation were considered, the only significant effect was a response of adult metabolic rate to the 21°C treatment (Fig. 5). In consequence, treatment temperature explained most of the variance (Table 1) among treatment temperature and plasticity type. This increase in metabolic rate following the 21°C treatment was also found when both adults and pupae were exposed to acclimation, resulting in a more even partitioning of variance between plasticity type and treatment temperature (Table 2).
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; Slabber and Chown,
2005) or acclimatization
(Hoffmann et al., 2005b)
effects on thermal limits. Although a more pronounced acclimation effect on
lower than on upper limits is not universal [e.g. the converse has been found
the caterpillars of the moth Embryonopsis halticella
(Klok and Chown, 1998)], it
does seem to be typical of most ectotherms
(Kingsolver and Huey, 1998).
In insects, greater variation in lower than in upper thermal tolerances is
typical not only of acclimation responses, but also of intra- and
interspecific geographic variation in thermal limits, and in the responses of
these limits to selection (Addo-Bediako et
al., 2000; Chen et al.,
1990; Chown, 2001;
Gaston and Chown, 1999;
Gilchrist et al., 1997;
Hercus et al., 2000;
Hoffmann et al., 1997;
Kimura, 2004;
Stanley et al., 1980;
Terblanche et al., 2005b).
Increases in metabolic rate of the G. pallidipes adults with adult
exposure to low temperature seems to be typical of this group of tsetse flies
because it has also been found in Glossina morsitans. Exposure of
adult G. moristans to 29°C had little effect on metabolic rate
relative to adults held at 24°C, but exposure to 19°C resulted in a
significant increase in metabolic rate across a wide range of test
temperatures (Terblanche et al.,
2005a). Increases of metabolic rate in insects exposed to low
temperatures are not uncommon (reviewed in
Addo-Bediako et al., 2002;
Chown and Gaston, 1999).
However, the causes, consequences and likely significance of such whole
organismal metabolic upregulation remain controversial and poorly investigated
(Chown et al., 2003;
Clarke, 2003;
Hodkinson, 2003). Why
metabolic rate should increase following exposure to a relatively low
temperature in tsetse adults is not clear, but might contribute to the absence
of this species (and G. morsitans) from low temperature areas.
Elevated metabolic rates will result in increased use of lipid reserves, lower
tolerance of starvation, and increased pressure for foraging, all of which are
likely to enhance the chances of mortality
(Rajagopal and Bursell, 1966),
likely limiting the ability of the flies to survive in low temperature
environments (though a lack of pupal development is also important for
restricting flies to warmer areas; for reviews, see
Hargrove, 2004;
Rogers and Robinson,
2004).
Perhaps more unusual than the responses of the other traits to acclimation
was the change in desiccation rate in response to the temperature treatments.
Previous studies have shown strong responses of insect desiccation rate to
changes in relative humidity (e.g. Gibbs
et al., 2003; Hoffmann,
1990). However, responses of water balance parameters to different
temperature regimes are more variable. In some cases treatment temperature has
elicited either no response in water balance-related traits
(Terblanche et al., 2005b), or
a response that is expressed in some traits but not others, such as cuticular
hydrocarbon profile changes but no alteration of water loss rates in
cactophilic Drosophila (Gibbs et
al., 1998; see also
Cloudsley-Thompson, 1969). In
others, pronounced responses in desiccation rate have been found. This is true
of D. melanogaster from the east coast of Australia, where in several
populations in response to summer acclimation temperatures, individuals showed
greater desiccation resistance than flies exposed to constant or winter
temperatures (Hoffmann et al.,
2005b). Similar declines in desiccation resistance with increasing
treatment temperature have also been found for D. takahashii and
D. nepalensis (Parkash et al.,
2005). Here, we found that when exposed to 29°C, either as
adults or as pupae, adult G. pallidipes had significantly reduced
rates of water loss compared to flies held at 21°C or 25°C (and
similar to results described in Terblanche
et al., in press). Neither the ultimate, nor proximate causes of
this change have been fully investigated. It seems likely that in the former
case a reduction in desiccation rate at high temperatures should increase
survival, given that high temperatures can be associated with low water
availability in the habitats occupied by these flies. Indeed, inter-population
comparisons of G. pallidipes in Kenya have shown that populations
from drier areas tend to have the lower water loss rates than those from
moister areas (Terblanche et al., in
press). How the acclimation or inter-population differences are
effected mechanistically has not been well studied, but differences in the
amount and composition of cuticular lipids might be significant, as has been
found in other species (Gibbs et al.,
1991; Rourke,
2000). Preliminary studies in this species have indicated that
variation in cuticular hydrocarbon profiles and desiccation rates can occur
among populations; however, desiccation rates in these flies are not
correlated with cuticular lipid mass (i.e. quantity) (R. Jurenka, G. Marquez,
J. Odera, J. Terblanche, C. Klok, S. Chown and E. Krafsur, unpublished
data).
Whilst the extent of the acclimation responses varied among traits, several
generalizations can be made about the effects of plasticity type on the traits
measured in the adults. Responses to treatment temperature showed an
asymmetric effect across all responsive plasticity types, with either the low
treatment temperature or the high treatment temperature having a significant
effect, but responses to both were found only for
Tcrit,min. In consequence, treatment temperature typically
explained less than half of the variance in the measured traits, except in the
case of Tcrit,min, where it accounted for more than 80% of
the variance. However, the symmetric Tcrit,min response
was found only for the combined plasticity treatment, and in this case
appeared to comprise a response to the 21°C treatment in the adults, and a
response to the 29°C treatment in the pupae, but not vice versa.
This finding deserves further exploration. Nonetheless, strong opposing
responses to high and low temperature treatments for lower thermal limits have
been found in several other studies, though typically these examined
acclimation effects on a single stage only (e.g.
Ayrinhac et al., 2004;
Hoffmann et al., 2005b;
Klok and Chown, 2003;
Slabber and Chown, 2005;
Terblanche et al., 2005b;
Zeilstra and Fischer, 2005).
Likewise, asymmetric or threshold effects of treatment temperature have
previously been recorded for Tcrit,max
(Hoffmann et al., 2005b;
Klok and Chown, 1998;
Slabber and Chown, 2005),
metabolic rate (Hoffmann,
1985; Terblanche et al.,
2005a) and desiccation rate
(Rourke, 2000).
By contrast, plasticity type usually accounted for substantial proportions
of the variation in the response of the traits to acclimation. Developmental
plasticity following the 29°C temperature treatments was significant and
irreversible for Tcrit,min and desiccation rate, but the
pupal treatment was either reversed or had little effect following the
21°C treatment for these traits and for both temperature treatments in the
other traits. The absence of reversibility in Tcrit,min
following exposure to 29°C is unlike the situation in Lycaena
tityrus, in which lower thermal limits that change within the pupal stage
are typically reversible in the adults
(Zeilstra and Fischer, 2005).
No other work has examined the effects of developmental plasticity on
desiccation rate in insects, but clearly it is significant. Developmental
plasticity is known from a wide variety of other traits, e.g. morphological,
behavioural, locomotion performance
(Atkinson, 1996;
Crill et al., 1996;
Nijhout, 2003;
Sheeba et al., 2002), and
generally is assumed to be fixed in the adult stage (e.g.
Gibert et al., 2000; discussed
in Wilson and Franklin, 2002).
Although early work assumed that such plasticity would be beneficial, most
recent studies have shown that this is rarely the case (for a discussion, see
Huey et al., 1999;
Wilson and Franklin, 2002).
The present design did not enable us to fully explore this hypothesis, though
clearly the responses shown by Tcrit,min and desiccation
rate could initially be considered beneficial, given that vapour pressure
deficit would increase at higher temperatures (for a given quantity of water
in the air) (Addo-Bediako et al.,
2001), and that a trade-off between the extent of lower thermal
limits and starvation resistance has been found
(Hoffmann et al., 2005a).
Adult acclimation effects were most common following low temperature
exposures, although for desiccation rate it was only the high temperature
exposure that had an effect. Although the time course of the persistence of
changes in physiological tolerances has not yet been investigated, other work
on the same species has shown that plasticity in Tcrit,min
is reversible within 10 days (Terblanche
et al., in press). The generality of these findings for other
traits of physiological tolerance in this and other insect species warrants
further attention. These findings are in keeping with what has been found in
many other studies (reviewed in Spicer and
Gaston, 1999; see also Wilson
and Franklin, 2002), and confirm the notion that adult acclimation
is typically reversible, while developmental plasticity is not
(Piersma and Drent, 2003).
What was perhaps most significant is that the combined plasticity treatments
did not seem to have noticeable additive effects. Rather, the extent of the
response following the combined treatments was either quite similar to the
pupal or adult response following exposure, or in the case of the response of
critical thermal limits to the low temperatures, seemed to have a negative
effect on tolerance. What the basis for this negative interaction might be is
not clear. However, it is known that basal and induced cold tolerance
responses are linked in some species
(Chown and Nicolson,
2004).
In conclusion, this study has shown that the stage at which acclimation
takes place has significant, though often different, effects on several adult
traits that are likely to modify environmental effects on populations of
G. pallidipes. These show that it is not only important to
distinguish between developmental plasticity and adult acclimation
(Wilson and Franklin, 2002;
Piersma and Drent, 2003) and
consider possible interactions among them, but also to consider the direction
of the responses and their significance from a life-history perspective
(Fischer et al., 2003).
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References |
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Addo-Bediako, A., Chown, S. L. and Gaston, K. J. (2000). Thermal tolerance, climatic variability and latitude. Proc. R. Soc. Lond. B 267,739 -745.[Medline]
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