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First published online October 7, 2008
Journal of Experimental Biology 211, 3272-3280 (2008)
Published by The Company of Biologists 2008
doi: 10.1242/jeb.022467
Control of discontinuous gas exchange in Samia cynthia: effects of atmospheric oxygen, carbon dioxide and moisture
1 Centre for Invasion Biology, Department of Botany and Zoology, Faculty of
Science, Stellenbosch University, Private Bag X1, Matieland 7602, South
Africa
2 Department of Animal Physiology, Humboldt-Universitat zu Berlin,
Philippstrasse 13, 10115 Berlin, Germany
* Author for correspondence at present address: Department of Conservation Ecology and Entomology, Faculty of AgriSciences, Stellenbosch University, Private Bag X1, Matieland 7602, Stellenbosch, South Africa (e-mail: jst{at}sun.ac.za)
Accepted 13 August 2008
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Summary |
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Key words: trachea, respiration, metabolic rate, cyclic gas exchange, ventilation rate
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INTRODUCTION |
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) (see also Klok et
al., 2002) and several hypotheses (mostly adaptive, but with one
non-adaptive explanation) (Chown and
Holter, 2000) have been proposed to explain the origin and
maintenance of DGE (Lighton,
1996; Chown et al.,
2006). The major adaptive hypotheses are: the hygric hypothesis
– DGE acts as a water savings mechanism; the chthonic hypothesis –
DGE facilitates gas exchange under hypoxic and/or hypercapnic conditions; the
chthonic–hygric hypothesis, a combination of the two former hypotheses
– but specifically that DGCs are an adaptation to reduce respiratory
water loss under conditions of either hypoxia or hypercapnia or both; and the
oxidative damage hypothesis – DGE is a consequence of the need to remove
accumulated CO2, followed by the demand for reduced oxygen toxicity
(Hetz and Bradley, 2005).
Although many tests of these hypotheses have been undertaken, they remain
controversial, largely because of considerable differences in the approach
adopted. For example, in the case of the significance of respiratory
modulation of water loss, including the modulation of elements of the DGE [the
hygric hypothesis originally proposed by Buck and Keister
(Buck and Keister, 1955)], much
of the work has been experimental, examining one or a few species. Frequently,
these studies have concluded either that respiratory water loss is too
insignificant to be the subject of selection, that DGE is abandoned just when
it is most required, or that the predictions of the hygric hypothesis are not
supported (e.g. Hadley and Quinlan,
1993; Lighton and Berrigan,
1995; Chown and Holter,
2000; Hetz and Bradley,
2005; Lighton and Turner,
2008) (reviewed by Lighton,
1998; Chown, 2002;
Quinlan and Gibbs, 2006). By
contrast, explicitly comparative studies, examining either respiratory water
loss more generally, or the likelihood that DGE may be responsible for
modulating it, have tended to support the idea that respiratory water loss is
sufficiently significant to be the focus of natural selection
(Zachariassen et al., 1987;
Addo-Bediako et al., 2001) and
that DGE may contribute to its modulation (e.g.
Duncan and Byrne, 2000;
Duncan and Dickman, 2001;
Chown and Davis, 2003). In
consequence, considerable controversy continues to surround the significance
of respiratory water loss in insects and the extent to which DGE may be
responsible for its modulation (e.g. Gibbs
and Johnson, 2004; Lighton and
Turner, 2008).
Although this controversy must partly be based on genuinely dissimilar ways
in which insects from different taxa respond to various environmental
conditions (Lighton, 1998),
and the likelihood that several factors may select simultaneously for DGE
(Chown, 2002), it is probably
also the consequence of the way in which tests of the alternative hypotheses
have been undertaken: more specifically, the predominant, single hypothesis
testing approach. While the importance of a strong inference approach to
testing alternative hypotheses in evolutionary physiology is now widely
appreciated (e.g. Huey et al.,
1999; Angilletta et al.,
2006; Deere and Chown,
2006; Bozinovic et al.,
2007), and has been repeatedly called for in the context of the
hypotheses proposed to explain the origin and maintenance of DGE
(Chown et al., 2006;
Quinlan and Gibbs, 2006), only
two studies have adopted such an approach, with only one of these controlling
for phylogenetic non-independence (Marais
et al., 2005; White et al.,
2007). Both investigations have been comparative, rather than
experimental, illustrating the ongoing difficulty of adopting a strong
inference approach in experimental work, despite the acknowledged importance
of experimental manipulation for addressing questions in this field
(Lighton, 2007). Indeed, most
experimental studies tend to address only a single hypothesis and are
frequently characterized by the manipulation of only a single environmental
variable thought to be significant in influencing DGE (e.g.
Chown and Holter, 2000;
Lighton and Turner, 2008),
often neglecting potential confounding effects of the treatment. For example,
since temperature has a positive, non-linear relationship with saturation
deficit, experimental manipulation of temperature may inadvertently alter
relative humidity (RH), thus confounding the interpretation of the effects of
temperature on DGE modulation. Insect spiracles are sensitive to the hydration
state of both the animal and the surrounding atmosphere. Partially dehydrated
tsetse flies (Glossina fuscipes; Diptera, Glossinidae) show increases
in spiracular control (Bursell,
1957), whereas in Aedes (Diptera, Culicidae) spiracles
can respond to the relative humidity of ambient air
(Krafsur, 1971). Consequently,
it seems possible that insects that have been shown to abandon DGE at high
temperatures (e.g. Chappell and Rogowitz,
2000) may have responded to humidity changes or to interactions
between humidity and temperature, rather than to temperature alone (see also
Chown, 2002).
Here, to address this notable absence of a strong inference, multi-factorial experimental approach in the field we examine the predictions of several alternative hypotheses proposed to explain DGE by manipulating ambient oxygen partial pressure, carbon dioxide partial pressure and relative humidity. Specifically, we test simultaneously the predictions of the hygric, chthonic, hybrid and oxidative damage hypotheses using a three-way study design, separating the effects of atmospheric humidity, partial pressure of carbon dioxide (PCO2) and oxygen (PO2) on the modulation of DGE. We make four predictions for whole-organism responses to address the relative importance of each of the primary adaptive hypotheses (Table 1). (1) If the hygric hypothesis is the key driver of DGE, DGE should be present under low humidity but should be abandoned at high humidity since it is no longer required for conservation of respiratory water. Moreover, if present, the DGE should be modulated such that a positive relationship should be found between DGE cycle frequency and ambient relative humidity (i.e. longer closed phases in drier conditions). Specifically, DGE should be tightly regulated under dry conditions. (2) If the chthonic hypothesis accounts for DGE, the effects of experimental manipulation of PO2 or PCO2, but not ambient humidity, should be evident. Specifically, under high PCO2 and/or low PO2 DGE should be evident, but under low PCO2 and/or high PO2 DGE should be abandoned. Humidity should make little difference to DGE prevalence. (3) The chthonic–hygric hypothesis makes the prediction that PCO2 and PO2 as well as atmospheric humidity changes will influence DGE modulation. Specifically, under low relative humidity conditions and high PCO2 and/or low PO2 DGE will occur. In all other cases DGE should be abandoned. (4) The oxidative damage hypothesis makes the prediction that the alteration of atmospheric water and PCO2 should have little effect on DGE prevalence. When ambient PO2 is reduced below intra-tracheal PO2, however, DGE should be abandoned. Varying ambient humidity should also have little or no effect on DGE occurrence or on the modulation of cycle frequency. Furthermore, DGE cycle frequency should decline at higher PO2.
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MATERIALS AND METHODS |
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). Two
weeks after pupation they were stored in a refrigerator at 6°C and
70–80% RH for the duration of the experiments. Diapausing pupae were
used in respirometry experiments 8–12 weeks after pupation.
To overcome problems associated with manipulations of CO2, such
as potential failure of the infra-red gas analyzer to detect DGE under high
experimental CO2 treatment conditions, and also to distinguish
accurately flutter (F) from open (O) and closed (C) phases
(Wobschall and Hetz, 2004), we
make use of simultaneous CO2 and intratracheal pressure
recordings.
Respirometry
Flow-through respirometry of carbon dioxide release rates was undertaken in
Samia cynthia pupae using a two channel respirometry system equipped
with a two channel differential infra-red carbon dioxide analyzer (URAS 14,
range 0 to 100 p.p.m., ABB, Frankfurt, Germany). The air was first driven
through the reference cell before it entered the analyzer cell of the URAS 14.
The output signal of the CO2 analyser was converted into nmol
min–1 g–1 fresh live body mass using the
following equation:
 | (1) |
CO2 is the
carbon dioxide release rate in nmol min–1
g–1; fCO2 is the fractional
concentration of carbon dioxide (signal from the analyzer);
Phyd is the hydrostatic pressure within the air stream
(kPa); 
gas is the flow rate
of gas in ml min–1 STPD (standard temperature and pressure
dry); R is the gas constant (8.314 J mol–1
K–1); T is temperature (283.16 K); and
manimal is the animal fresh live body mass (g).
Because pilot studies showed that the duration of respiratory cycle at
6°C was 4–8 h, 10°C was chosen as a compromise between a
temperature that maintained diapause and one that gave a shorter cycle
duration (Hetz, 2007). The
temperature of the respirometry chambers were therefore maintained at 10°C
by a custom-made Peltier-cooling unit with a computer-controlled feedback
(accuracy ±0.1°C). Two individuals were measured simultaneously
using 144 ml min–1 flow rates produced by two Woesthoff pumps
(5KM3O3/a-F; H. Woesthoff GmbH, Bochum, Germany). The pump action of the
gas-mixing pumps generated slight pressure fluctuations in the flow-through
setup that were recorded by the pressure transducers. Therefore, pressure
dampers (silicon gloves located inside 5 l glass jars) were inserted after the
Woesthoff pumps to buffer the baseline of the differential pressure
transducers. One pump was used to control the gas composition while the second
pump was used to split the airstream and drive it at equal flow rates through
the two respirometry chambers (see Fig.
1). The internal dead space of the respirometry chamber (after
factoring in the average pupal size) was estimated to be 
12 ml. Thus, at
the flow rate used the chamber would be flushed approx 12 times per minute
resulting in adequate temporal resolution.
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 | (2) |
is the
fractional concentration of oxygen in gas1 (0 for N2, 0.209 for air
and 1 for O2);
gas1 is the flow rate of
gas1 (ml min–1);
is the hydrostatic
pressure [air pressure and perfusion pressure (usually less than 0.01 kPa)] of
air flowing through the respirometer (in kPa; readout from pressure sensor in
URAS 14); and PH2O is the water vapour pressure
(kPa) calculated from the Magnus equation
(Magnus, 1844) for the
saturation water partial pressure above a water surface with given temperature
T (°C):
 | (3) |
For setting hypoxic and hyperoxic gas mixtures for gas1 usually air was used whereas for gas2 pure oxygen was used to set hyperoxic and pure nitrogen was used to set hypoxic gas mixtures. To set hypercapnic carbon dioxide mixtures while leaving the oxygen partial pressure at ambient level the above gas mixture was fed into another Woesthoff pump in order to set the partial pressures of two gases (e.g. oxygen and carbon dioxide) independently.
The oxygen partial pressure was verified by directing the airstream from the outlet of the carbon dioxide analyzer into an Ametek S-3II/A oxygen analyser (Applied Electrochemistry, USA). All data were sampled at 0.25 s frequency and recorded at 1 s intervals to a computer hard disk using a customized recording program in TurboLab 4.03 (Bressner Technology, Germany).
Tracheal pressure
Tracheal pressure is a tool to monitor the action of the spiracles
(Wobschall and Hetz, 2004),
especially if no carbon dioxide release can be recorded (e.g. at high ambient
CO2 concentrations). We therefore recorded the tracheal pressure
with the help of a differential pressure transducer. One port of the pressure
transducer was connected to a short piece of polyethylene tubing inserted into
the tracheal system via a single spiracle
(Fig. 2). The surrounding of
the spiracle was sealed with wax. The other port of the transducer opened into
the respirometry chamber. Pressure was measured with a precision micro
pressure transducer (SenSym SDXL010, SensorTechnics, Puchheim, Germany). The
pressure sensor electrically consisted of a Wheatstone bridge that was driven
with a precision voltage reference of 6.000V (REF02, Burr Brown, Texas
Instruments, Dallas, TX, USA) buffered with a precision operational amplifier
(OPA177, Burr Brown). Differential voltage output of the pressure sensor
bridge was amplified with a differential amplifier (INA131, Burr Brown). This
corresponded to a pressure difference of 504.6 Pa V–1. An
offset voltage was added to set the pressure range for measuring
sub-atmospheric pressure in the tracheal system from –2200 to +317Pa.
Accuracy and stability over 24 h was within 5 Pa (<0.25%) of full scale.
This low baseline drift was mainly achieved by attaching the pressure sensor
device to the Peltier-cooled respirometer. The pressure recordings were used
to verify presence or absence of DGE under altered gas conditions.
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All electrical signals from the devices (temperature, tracheal pressure, carbon dioxide release rate, ambient oxygen partial pressure) were amplified with custom-made voltage amplifiers and sampled to the hard disk of a computer via an A/D-board (DT2821, Data Translation, Marlboro, MA, USA) and the use of the TurboLab data acquisition software (Stemmer, Puchheim, Germany).
Analyses
Experimental runs were started with a control period in which pupae were
held under controlled standard gas conditions consisting of normoxia (20.9
kPa), normocapnia (<0.1 kPa) and normal rearing humidity (70% RH). The
control part of the experiments lasted for at least 10 to 12 h. Subsequently,
the gas conditions were switched to treatment conditions (see
Table 2 for range of
treatments) and gas exchange measured for an additional 6–10 h. The raw
data were processed offline using a custom made script running under the
TurboLab data acquisition software (Stemmer, Puchheim, Germany) (for details,
see Hetz, 2007). Individual
pupae were assessed for DGE presence or absence based on the rate of
CO2 release
(CO2) and
intratracheal pressure recordings under control and several experimental
treatment conditions. The number of individual pupae that showed DGE under
control and treatment conditions relative to the total number of individuals
recorded was then used to generate contingency tables which were tested for
significance using Fisher's exact test which is robust for small and unequal
sample sizes (Zar, 1997). The
contingency tables were used to compare DGE presence/absence for predictions
of the hygric, oxidative, chthonic and hybrid hypotheses by inference on the
basis of the predictions made in the Introduction and
Table 1. Thereafter, for all
treatments in which individual pupae maintained DGE, we undertook analyses
examining potential modulation of individual components of the DGE. Analyses
of CO2 variables
were undertaken as pair-wise responses of treatment versus control
conditions within individuals. For these analyses, data from individuals for
which three consecutive bursts were available under both control and treatment
conditions were used. We extracted phase duration, average
CO2 and the
average maximum
CO2 for each
phase from each of the treatment groups. All parameters were averaged over
three bursts within a control or treatment period, and these were compared
using a paired t-test between control and treatment periods.
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RESULTS |
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CO2 suggested
little modification of any phase of the DGE during hyperoxia, high moisture or
low moisture, although there was a substantial though non-significant
reduction in the O-phase during hyperoxia
(Table 3).
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It does appear, however, that some of the marginal effects in our study
were probably a consequence of relatively small sample sizes and relatively
large inter-individual variation in DGE parameters under treatment conditions.
For example, in the hyperoxic treatment
(Table 4), the only significant
effect was an increase in O-phase duration. However, although duration of the
F-phase decreased by nearly 50%, this was non-significant and should probably
be attributed to a type II statistical error. Similarly, C-phase duration,
although 1.5 times longer during hyperoxia, was marginally
non-significant (Table 4;
specifically, total DGE duration increased nearly twofold yet this variation
was statistically non-significant). A post-hoc power test for the
C-phase duration result suggests that β=0.401 (
2-tailed=0.05). If
this effect size remained constant as more individuals were added, at least 30
individuals would be required to raise the power to 0.8, which might be
considered a suitably rigorous, although somewhat arbitrary, level of
statistical power (Di Stefano,
2003).
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For average duration of each phase, hyperoxia resulted in non-significantly longer C-phases, non-significantly shorter F-phases, and significantly longer O-phases (Table 4). Across other treatment groups, no effects were detected on average duration which might be indicative of DGE burst modulation (Table 4). For the total DGE duration (i.e. the time taken for C, F and O phases combined), wet and dry treatments appeared to have little significant influence on S. cynthia. By contrast, hyperoxia appeared to increase total DGE duration substantially, but with considerable variation among bursts and among individual pupae, and hence this effect was statistically non-significant (Table 4).
Maximum CO2
during each phase suggested a possible marginal effect of hyperoxia indicating
an increase in peak burst volumes during the O-phase relative to normoxic
conditions (Table 4), probably
as a consequence of remaining in a C-phase for longer and thus increasing
intratracheal PCO2. By contrast, the dry
treatment clearly resulted in a significant reduction in peak O-phase
CO2, although
other phases appeared unaffected (Table
5). This small, but significant reduction was accompanied by a
significant decline in O-phase burst volume (area under the
CO2 curve) in
dry compared with normal rearing (70%) humidity conditions, from
741.4±160.8 to 690.1±176.9 nmol CO2
g–1 (t5=3.326; P<0.021).
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). Although somewhat surprising, the increased O-phase
duration detected in the pupae under hyperoxic conditions
(Table 4) can be explained by
increased CO2 build-up during extended C-phases, thereby requiring
more time to restore haemolymph pH balance when spiracles open under
hyperoxia. Owing to the maintenance of DGE in pupae when switched from normal
to high ambient moisture conditions the predictions of the hygric and the
hybrid hypotheses also appear to have been falsified. If water conservation is
an important factor favouring the maintenance of DGE, the abolition of DGE
under high moisture conditions might be expected, yet even after increasing
moisture content in the air stream, DGE was maintained. However, the reduction
in maximum CO2
observed under dry conditions is indicative of some support for the hygric
hypothesis, especially if it is assumed that respiratory water loss is
positively related to
CO2 during the
O-phase. This is not an altogether unreasonable assumption based on previous
simultaneous recordings of
CO2 and
H2O in DGE and
non-DGE insects [in particular, see Fig.
3 in Lighton et al. (Lighton
et al., 1993)] (see also
Lighton and Berrigan, 1995;
Chown and Davis, 2003;
Gibbs and Johnson, 2004;
Lighton et al., 2004;
Schilman et al., 2005;
Gray and Chown, 2008).
Moreover, O-phase burst volume showed a significant reduction under dry
conditions, in keeping with the finding that reduced respiratory water loss
rates are associated with simultaneous declines in both O-phase duration and
CO2 in other
insect species (e.g. Chown and Davis,
2003). Unfortunately, in this study the declines in water loss
could not be confirmed by direct measurement because in pilot trials we
struggled to distinguish the animal's water loss from the background signal
under high humidity conditions. Nonetheless, a range of studies has examined
the conditions thought to promote DGE using
CO2 data only
(e.g. Lighton, 1990;
Lighton, 1991;
Duncan and Dickman, 2001;
Duncan et al., 2002a).
Of course, it would be remiss of us not to point out that several of the
marginal effects in our study were probably a consequence of relatively small
sample sizes, which, in turn, were constrained by equipment and experimental
duration. Similarly, relatively large inter-individual variation in DGE
parameters under treatment conditions, which was not obvious in earlier pilot
trials, may have compounded these effects. Experimental design and future
tests of adaptive hypotheses should attempt to account for, and understand
factors contributing to, this variation at the individual and species level.
However, it is clear that statistical power is always going to be a limiting
factor in such multi-factor experimental designs. If, for example, a
full-factorial experimental design were to be used in which four levels are
investigated within each of four main experimental treatments (e.g. humidity
manipulation which includes humidities of 5, 20, 60 and 80%), based on the
present statistical power (Results) in which approximately 30 individuals
might be required per treatment group, a total of 480 individuals would
be required. This obviously presents a considerable logistic constraint.
Bearing the above caveats in mind, it appears that avoidance of oxidative
damage, and to a lesser extent, retention of water, play significant roles in
determining the presence and form of DGE in the pupae of this moth species.
Obtaining support for the oxidative damage hypothesis in S. cynthia
is perhaps unsurprising considering that a related species, Attacus
atlas, has provided rigorous experimental data in favour of the oxidative
damage hypothesis (Hetz and Bradley,
2005). Nonetheless, it does indicate that, at least for pupae of
this group of insects, the oxidative damage hypothesis has support, especially
since a strong inference, experimental approach has been unable to discount
it, but has proved able to reject predictions of most of the competing
hypotheses. Moreover, since the predictions of this hypothesis were also
partially supported in the comparative study of White et al.
(White et al., 2007), it is
clear that the extent to which avoidance of oxidative damage might select for
current maintenance of DGE requires further, careful scrutiny in other
groups.
Similarly, the hygric hypothesis has received considerable support across a
wide range of species and studies (e.g.
Kestler, 1985;
Lighton et al., 1993;
Vogt and Apple, 2000;
Duncan et al., 2002b;
Chown and Davis, 2003;
Marais et al., 2005;
White et al., 2007), including
the pupae of other moth species (Levy and
Schneiderman, 1966). Thus, finding support for it here is not
surprising either, although this outcome contrasts strongly with some recent
statements concerning the demise of the hygric hypothesis
(Lighton and Turner,
2008).
Importantly, the finding that the predictions of both the hygric and
oxidative damage hypotheses cannot be rejected re-emphasizes the idea that DGE
and the modulation of its underlying components may be a response to several
different environmental factors (Chown,
2002). The independent origin of this gas exchange pattern in
several higher taxa (Klok et al.,
2002; Lighton and Joos,
2002; Marais et al.,
2005) also provides some support for this idea, although
conceivably DGE could have arisen as an independent response to a single
factor across all of the taxa. In consequence, besides the variety of
investigative approaches adopted, multiple pathways to DGE might also
constitute a sound reason why most of the hypotheses proposed to explain its
maintenance continue to engender support, and why this support seems to vary
so consistently from one group to the next (e.g. in ants, predictions of the
chthonic hypothesis and its variations cannot be rejected, whereas in moth
pupae the predictions of the oxidative damage hypothesis cannot be rejected).
Nonetheless, on present evidence the hygric and oxidative damage hypotheses
clearly have the broadest support, especially since neither comparative nor
experimental strong-inference approaches have been able to reject their
predictions (see also White et al.,
2007). However, it should be noted that these tests all examine
hypotheses for the present maintenance of DGE
(Chown et al., 2006). Given
that several of the hypotheses continue to engender support, it may well be
the case that a variety of conditions select for maintenance or further
modulation of DGE, but that its origin is non-adaptive. Chown and Holter
(Chown and Holter, 2000)
proposed that DGE may be a non-adaptive consequence of the interactions
between the CO2 and O2 regulation systems that together
determine spiracle opening and gas exchange under conditions of minimal
demand. Their hypothesis has yet to be the subject of careful scrutiny.
The present data also provide some grounds for questioning a number of the
conclusions reached by Sláma et al.
(Sláma et al., 2007).
In particular, they claim that the DGEs identified by Lighton
(Lighton, 1996) could be an
artefact resulting from exposure of insects to a dry air stream. Clearly, the
full DGE pattern (C-, F- and O-phases) is maintained in S. cynthia
under dry and humid conditions, although some modulation of its form takes
place. In consequence, Sláma et al.'s
(Sláma et al., 2007)
conjecture is not supported here, although whether this is the case also for
other species, besides the termites they discuss, is not clear. Second, they
claim that continuous adjustment of the form of CO2 release,
dependent on environmental conditions, means that previous hypotheses
concerning the maintenance of DGE have poor physiological substantiation. Our
simultaneous measurements of
CO2 and tracheal
pressure provide little support for the notion that different conditions, and
especially dry vesus humid conditions, result in very different
CO2 sequestration and release patterns.
In conclusion, to date, the majority of experimental investigations have
not adopted a strong inference approach and have most commonly found support
for the oxidative damage (Hetz and
Bradley, 2005), chthonic-hygric
(Lighton and Berrigan, 1995)
and chthonic hypotheses (Lighton and
Turner, 2008). By contrast, recent comparative studies have
typically found support either for the hygric hypothesis or the oxidative
damage hypothesis (e.g. Marais et al.,
2005; White et al.,
2007). The results of the present study are therefore significant
for several reasons. First, the study constitutes the first strong
inference-based, experimental assessment of the predictions of the adaptive
hypotheses proposed to explain the maintenance of DGE. Second, by employing
intratracheal pressure measurement, this study is also the first to assess DGE
presence or absence under varying PCO2,
PO2 and PH2O
conditions. Such an approach has not previously been adopted most probably
because of the technical limitations of specific gas analysers, e.g. operating
against high gas concentration (`background') conditions [though see
Sláma et al. (Sláma et al.,
2007) for an alternative approach]. Third, support for the
oxidative damage hypothesis was found with some limited support for the hygric
hypothesis. Clearly, several questions remain, most notably the likely effect
of short-term plasticity on the DGE responses of organisms to altered
conditions, the likely role of CO2 sequestration and release in
affecting DGE, and the role played by interacting setpoints in giving rise to
DGE. These questions have been only either discussed briefly or investigated
in a small set of taxa (Chown and Holter,
2000; Sláma et al.,
2007; White et al.,
2007), and therefore not given nearly sufficient consideration in
experimental and theoretical work to date. Nonetheless, the present results,
in conjunction with those of Hetz and Bradley, Marais et al. and White et al.
(Hetz and Bradley, 2005;
Marais et al., 2005;
White et al., 2007), suggest
that the death of the hygric and oxidative damage hypotheses has been declared
prematurely by Lighton and Turner (Lighton
and Turner, 2008). Neither is ready for interment and the funeral
must be called off.
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Acknowledgments |
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References |
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TOPSummaryINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONReferences |
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