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First published online January 18, 2008
Journal of Experimental Biology 211, 447-458 (2008)
Published by The Company of Biologists 2008
doi: 10.1242/jeb.013649
Cutaneous water loss and sphingolipids in the stratum corneum of house sparrows, Passer domesticus L., from desert and mesic environments as determined by reversed phase high-performance liquid chromatography coupled with atmospheric pressure photospray ionization mass spectrometry
1 Department of Evolution, Ecology, and Organismal Biology, Ohio State
University, 300 Aronoff Lab, 318 W 12th Avenue, Columbus, OH 43210, USA
2 Applied Biosystems, 500 Old Connecticut Path, Framingham, MA 01710, USA
* Author for correspondence (e-mail: munoz-garcia.1{at}osu.edu)
Accepted 27 November 2007
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Summary |
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 TOP Summary INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONReferences |
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In this study, we used high-performance liquid chromatography coupled with atmospheric pressure photoionization-mass spectrometry (HPLC/APPI-MS) to identify and quantify over 200 sphingolipids in the SC of house sparrows from desert and mesic populations. Principal components analysis (PCA) led to the hypotheses that sphingolipids in the SC of desert sparrows have longer carbon chains in the fatty acid moiety and are more polar than those found in mesic sparrows. We also tested the association between principal components and CWL in both populations. Our study suggested that a reduction in CWL found in desert sparrows was, in part, the result of modifications in chain length and polarity of the sphingolipids, changes that apparently determine the interactions of the lipid molecules within the SC.
Key words: cutaneous water loss, house sparrows, mass spectrometry, stratum corneum
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INTRODUCTION |
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TOPSummaryINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONReferences |
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;
Elias, 2004). In mammals and
birds, CWL is mediated primarily by the outer layer of the epidermis, the
stratum corneum (SC), formed by multiple layers of cornified cells embedded in
a matrix of lipids (Menon and Menon,
2000; Lillywhite,
2006); these lipids determine the rate of water permeation through
the skin (Potts and Francoeur, 1991;
Madison, 2003). In mammals,
lipids in the intercellular spaces of the SC are primarily a mixture of
cholesterol, free fatty acids, and ceramides – sphingolipids that
consist of a sphingosine molecule attached to a fatty acid by an ester bond
(Gray and Yardley, 1975;
Bouwstra et al., 2003;
Lillywhite, 2006). In birds,
these same lipid classes are also found in the SC, but cerebrosides, formed by
a ceramide covalently linked to a hexose, also constitute a large fraction of
the intercellular lipid mixture
(Muñoz-Garcia and Williams,
2005).
Lipids within the SC are organized in layers called lamellae, but the
details of how individual lipid molecules are involved in the organization of
the SC remain obscure (Kitson et al.,
1994; Bouwstra et al.,
2003; Hill and Wertz,
2003). It is thought that ceramides form the structural backbone
of the lamellae, and those containing linoleic acid serve to rivet bilayers
together (Bouwstra et al.,
2003; Lillywhite,
2006). Free fatty acids form hydrogen bonds with ceramides,
maintaining the cohesion of the lamellae
(Bouwstra et al., 2003). At the
concentrations found in the SC of mammals, cholesterol promotes the stability
of the lamellae (Norlén,
2001). In mammalian epidermis, cerebrosides are initially extruded
into the extracellular spaces, but are thereafter enzymatically converted to
ceramides and therefore they do not occur in the SC. In contrast, birds have a
high proportion of cerebrosides in the SC. In humans, accumulation of
cerebrosides in the SC results in a pathological state characterized by an
increase in CWL and dry scaly skin, but in birds normal SC function involves
cerebrosides (Holleran et al.,
1993; Muñoz-Garcia and
Williams, 2005). The association between cerebrosides and CWL in
birds remains to be explored in detail.
The biochemical properties of lipid molecules in the SC also seem to be
important in determining water loss through the skin
(Lillywhite, 2006). Longer
carbon chains in ceramides and cerebrosides presumably form a more tightly
packed SC, and therefore a greater barrier to water vapor diffusion
(Schaefer and Rodelmeier,
1996). More polar ceramides tend to form a tighter barrier to
water loss because they will create stronger molecular interactions
(Haugen et al., 2003a).
Cerebrosides contain a sugar molecule and therefore can potentially bind
molecules of water, in contrast to ceramides, cholesterol and free fatty acids
(Norlén, 2001).
Because vertebrates that live in desert environments often face high
ambient air temperatures (Ta), low humidity and little
surface water for drinking, natural selection might have acted on phenotypic
features of the SC that would reduce CWL, thereby promoting an efficient water
economy (Williams and Tieleman,
2005). Although early studies posited that CWL was not an
important source of water loss in birds
(Mount, 1979), later
investigations showed that CWL represented more than 50% of total water efflux
(Bernstein, 1971;
Dawson, 1982;
Webster and King, 1987;
Wolf and Walsberg, 1996;
Williams and Tieleman, 2005).
Mechanisms that decrease CWL will therefore contribute to a reduction in total
water loss in birds that live in deserts. For example, studies on species of
larks across an aridity gradient, from Saudi Arabia to the Netherlands,
revealed that desert birds had a lower CWL than did species from mesic
environments (Tieleman and Williams,
2002; Williams and Tieleman,
2005). This reduction in CWL was associated with an increase in
the proportion of more polar ceramides, and a decrease in the proportion of
free fatty acids (Haugen et al.,
2003a; Haugen et al.,
2003b). However, the skin of some species of birds also plays a
role in thermoregulation, at least when Ta is high
(Bernstein, 1971;
Marder and Ben-Asher, 1983).
So, when Ta is favorable, desert birds should have minimal
water loss through the skin, but when Ta exceeds body
temperature, CWL should be elevated. This dual role of skin might have
implications for the lipid organization within the SC.
We previously reported that CWL of house sparrows, Passer
domesticus, living in the desert of Saudi Arabia was about 25% lower than
that of sparrows living in mesic Ohio
(Muñoz-Garcia and Williams,
2005). To analyze the lipid classes in the SC of sparrows, we used
thin layer chromatography (TLC) with a solvent system that separated lipid
classes based primarily on polarity. Sparrows from Saudi Arabia had a
significantly higher concentration of ceramides and cerebrosides than did
sparrows from Ohio (Muñoz-Garcia
and Williams, 2005).
Although TLC has been a useful tool to separate and quantify lipid classes
in the SC, it cannot unambiguously identify all individual molecules. In
theory, the use of different TLC systems in combination with other techniques,
such as gas chromatography, allows the assessment of many different molecules
of lipid in the SC (Wertz et al.,
1985; Karlsson and Pascher,
1971). However, it is not possible to resolve each individual
molecule of sphingolipids unambiguously
(Raith et al., 2000).
Therefore, each band of lipids on a chromatography plate represents a number
of different molecules that might differ in chain length, or in head groups
that yield similar polarity (Ponec et al.,
2003) (J.R., A.M.-G., J.C.B. and J.B.W., unpublished data). These
uncertainties constitute an important limitation of TLC because knowledge
about individual lipid molecules within the SC is crucial to our understanding
of the molecular underpinnings of barrier function.
To overcome the limitations of TLC in discriminating individual molecules
of lipid, we developed a method to identify individual ceramides and
cerebrosides using reversed phase high-performance liquid chromatography
(HPLC) coupled with atmospheric pressure photoionization-mass spectrometry
(APPI-MS) (Muñoz-Garcia et al.,
2006). Our method uses retention time, molecular weight and
fragmentation patterns to identify sphingolipids and, in most cases,
unambiguously elucidate their biochemical structure. Using this information
permits more detailed insight into the possible arrangement of the lipid
molecules within the SC.
In the present study, we apply our new method of HPLC/APPI-MS to identify and quantify sphingolipids in the SC of house sparrows, one population from the desert known to have a reduced CWL and the other living in a mesic environment. We characterized the biochemical properties of the sphingolipids that we identified and grouped them into families. To search for common themes within the approximately 200 different lipid molecules that we identified in 27 sparrows, we used principal components analysis (PCA). The results of our PCA led to the hypotheses that sphingolipids in the SC of desert sparrows have longer carbon chains in the fatty acid moiety and are more polar than those found in mesic sparrows. To evaluate the functional significance of the differences in lipid composition between desert and mesic sparrows, we tested for associations between principal components and CWL in both populations. Our results suggested that the reduction in CWL found in desert sparrows was the result of modifications in chain length and polarity of the sphingolipids, changes that apparently determine the interactions of the lipid molecules within the SC with attendant modification of water permeation through the SC.
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MATERIALS AND METHODS |
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TOPSummaryINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONReferences |
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). House
sparrows in Saudi Arabia, P. d. indicus, are native to this region,
although they depend on humans for access to water
(Cramp and Perrins, 1994). The
population of house sparrows that we studied in west-central Saudi Arabia had
daily access to water and was subjected to episodes of extreme heat
(>43°C) during the summer. We mist netted 12 house sparrows at the National Wildlife Research Center near Taif, Saudi Arabia (22°15'N, 41°50'E) and 15 sparrows in Columbus, Ohio (40°00'N, 83°10'W), during October–November 2003. During the brief time that sparrows were held captive prior to measurements, we fed them a mixture of seeds, egg yolk and mealworms. Water was provided ad libitum. Experiments were approved by IACUC of Ohio State University (protocol 2003-A0072).
Isolation of the SC and separation of skin lipids
To isolate the SC we used the methods of Wertz et al.
(Wertz et al., 1986) as
modified by Haugen et al. (Haugen et al.,
2003a). We killed birds, removed their skin, pinned it to a Teflon
sheet, and immersed it in a waterbath for 3 min at 65°C. Then, we gently
peeled the epidermis from the dermis. Thereafter, we incubated the epidermis
in a solution of 0.5% trypsin in phosphate-buffered saline (PBS; 1.2%
Na2HPO4 7H2O, 0.06%
NaH2PO4 H2O monobasic, and 0.8% NaCl in
distilled water, pH 7.4, 370 mosmol l–1), at 4°C
overnight, followed by another incubation in fresh 0.5% trypsin solution in
PBS at 38°C, to separate the SC from the epidermis. The SC was
freeze-dried and stored at –20°C in an atmosphere of nitrogen or
argon.
We determined the dry mass of the SC and extracted the lipids with a series
of chloroform–methanol (2:1, 1:1 and 1:2) for 2 h each step
(Law et al., 1995). We
combined the extracts and dried them under a stream of nitrogen gas.
Reversed phase HPLC
Sphingolipids of the SC of sparrows were re-dissolved in a mixture of
isopropyl alcohol, toluene and ethyl acetate (7.4:2:1 v/v/v) with 72.6 ng
ml–1 of synthetic C17:0 ceramide as an internal standard, and
43.5 µg ml–1 butylated hydroxytoluene (BHT) as an
antioxidant. We separated sphingolipids using reversed phase HPLC with a
Phenomenex Luna® C18 column 150 mm x 2.0 mm i.d.,
spherical 5 µm particle size, 100 Å pore size (Phenomenex, Torrance,
CA, USA) at 48°C. We loaded 3 µl of lipid extracts onto our HPLC column
for each run of each sample. We used a gradient solvent system following
Muñoz-Garcia et al.
(Muñoz-Garcia et al.,
2006); the initial solvent was 100% methanol:isopropanol:water
(85.5:10:4.5 v/v/v) run at a flow rate of 180 µl min–1,
and changed gradually to a final concentration of 75%
methanol:isopropanol:water (85.5:10:4.5 v/v/v) and 25% ethyl acetate during
the course of 24 min. The latter solution was changed during the course of 1
min to 100% ethyl acetate at a flow rate of 300 µl min–1.
A final step using 100% methanol:isopropanol:water (85.5:10:4.5 v/v/v) for 5
min was used to precondition the column for the next run.
Detection and identification of sphingolipids
We detected sphingolipids coming from our HPLC system with an Applied
Biosystems Q-TRAP® hybrid quadrupole linear ion trap mass spectrometer
system (Applied Biosystems, Ontario, Canada) fitted with a PhotoSpray® ion
source operated in positive and negative ion mode, with toluene as a dopant.
For parameters that we used on our Q-TRAP system, see Muñoz-Garcia et
al. (Muñoz-Garcia et al.,
2006).
Using AnalystTM 1.4.1 (Applied Biosystems, Ontario, Canada), we generated a contour plot for each sample, with mass/charge (m/z) as the y-axis and retention time as the x-axis. The contour plot yielded series of dark bands, some representing a molecular ion of a sphingolipid, whereas others corresponded to source fragments.
To denominate families of sphingolipids, we followed Motta et al.
(Motta et al., 1993) who
designated the three types of fatty acids in sphingolipids, non-hydroxy acids,
-hydroxy acids and 
-hydroxy acids ester linked to linoleate, as
N, A and EO, respectively. The three types of sphingoid bases, sphingosine,
phytosphingosine and 6-hydroxysphingosine found in ceramides and cerebrosides
were indicated as S, P and H. Thus, a ceramide consisting of a
-hydroxyacid ester linked to a molecule of 6-hydroxysphingosine would
be designated as CER EOH. We have elaborated the structure of ceramides and
cerebrosides that we have found and ordered them from least to most polar
(Fig. 1).
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The polarity of sphingolipids is determined by the number and position of
hydroxyl groups in the molecule as well as the chain length of the hydrophobic
residues (Wertz and Downing,
1983). Based on Wertz and Downing
(Wertz and Downing, 1983), we
ranked families of ceramides and cerebrosides from the least polar, given a
rank of 1, to the most polar. We assumed that a rank step in polarity was
linear over the range that we considered. Because of their intrinsic
differences in polarity, we ranked ceramides and cerebrosides separately.
Quantification of lipid classes in the SC of house sparrows
Previously, we explored the possibility that HPLC/APPI-MS could be used to
quantify sphingolipids in the SC, and found reasonable concordance in our
estimates of total ceramides and cerebrosides (mg lipid g–1
dry SC) using TLC and HPLC-APPI/MS, with a deviation of 0.95% for ceramides
and 2.5% for cerebrosides
(Muñoz-Garcia et al.,
2006). The relative intensity of each individual sphingolipid was
proportional to the amount of that molecule in the lipid mixture. We used
calibration curves with known concentrations of ceramide 17:0 as our standard
to convert intensities to absolute amounts of each lipid molecule
(Muñoz-Garcia et al.,
2006). We have shown an absence of matrix effects using APPI, and
therefore quantification of lipids is possible (J.R., A.M.-G., J.C.B. and
J.B.W., unpublished data).
Because similar masses of lipid molecules with different molecular weights
yield a different number of moles, and molar ratios of the different lipids in
the SC appear to be important in determining water loss through the skin
(Bouwstra et al., 2003), we
also calculated the number of moles of each sphingolipid molecule in our
samples.
Statistics
All statistical tests were performed with SPSS 14.0 (Chicago, IL, USA),
with statistical significance set at P<0.05. When multiple
comparisons were performed, we used the Bonferroni correction
(Zar, 1996). Means are
reported ±1 s.d. We tested for differences between means using
Student's two-tailed t-test for independent samples. Percentages were
logit transformed [ln(Y/1–Y)] prior to analyses to
normalize data (Zar, 1996).
Differences in distributions of data were assessed using the
Kolmogorov–Smirnov test.
To explore underlying themes in our data, we used PCA on the quantities (in
mmol or mg lipid g–1 dry SC) of each family of sphingolipids
(Shaw, 2003). This analysis
yielded uncorrelated composite variables, the principal components. We used
the program `Factor analysis' in SPSS without rotation to extract components
with eigenvalues greater than one as our selection criterion. Interpretation
of the principal components led to the generation of hypotheses that involved
chain length of the fatty acid residues and polarity of the sphingolipids in
the SC. We used Student's two-tailed t-test for independent variables
to test for differences in chain length of the fatty acid moiety and polarity
of the sphingolipids between desert and mesic sparrows. We also determined
associations between CWL and the scores of the principal components for each
individual bird using linear regression.
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RESULTS |
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TOPSummaryINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONReferences |
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The distribution of moles of lipid within each sphingolipid family could be the result of selective pressures that would favor the occurrence of some molecules over others, which in turn could influence the structure and properties of the permeability barrier. Distributions of molecules within ceramide families were unimodal with the main peak located towards short-chain free fatty acids (Fig. 2). The shape of the distributions of ceramides did not differ significantly within each family between sparrows from Saudi Arabia and Ohio (Kolmogorov–Smirnov test, Z<0.926, P>0.36). There were significant differences between desert and mesic sparrows in the distributions of the number of moles of cerebrosides NP, EOH, AS-NH, AH and NH (Kolmogorov–Smirnov, Z>1.423, P<0.035; Fig. 2); distributions in these families of cerebrosides were in general more flat in mesic than in desert sparrows.
PCA on sphingolipid families
To reduce the number of variables in our data, we used PCA on the number of
moles per gram of dry SC of each family of sphingolipids. Three axes accounted
for 82.7% of the variance (Table
1). A plot of scores for individual birds along these three axes
provided clear separation between mesic and desert sparrows
(Fig. 3A). When we added to
this plot the eigenvector loadings of the sphingolipid families, we were able
to sort our variables into three groups
(Fig. 3B). Diosylceramides, the
only sphingolipid type with negative values for principal component 1 (PC 1),
were isolated in the coordinate plane. PC 2 separated ceramides with negative
scores and cerebrosides with positive scores. PC 3 separated ceramides
containing long or short fatty acid chains, and polar from non-polar
cerebrosides (Fig. 3A). These
two plots combined suggest that PC 1 was related to the presence or absence of
diosylceramides, and that this variable separated mesic from desert sparrows.
When we repeated the analysis excluding diosylceramides, we also found that PC
1 discriminated between desert and mesic sparrows and the eigenvector loadings
of the remaining variables for PC 1 were also high (>0.5). Taken together
these results suggest that PC 1 is related to the interaction of sphingolipid
molecules in the SC of house sparrows, and that there are some characteristic
combinations of lipids in desert birds compared with mesic sparrows that yield
the distinct scores in this component. The scores of all desert birds were
positive for PC 2 suggesting that modification of cerebrosides is more
important for these birds than for mesic sparrows. PC 3 scores of the sparrows
from Saudi Arabia tend to cluster close to the loadings of polar cerebrosides
and long ceramides, whereas scores of sparrows from Ohio are scattered
throughout the entire range of values of PC 3.
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|
We also performed PCA on the amount in milligrams of lipid per gram of dry SC of each sphingolipid family present in the SC of sparrows. We extracted two components, accounting for 74.8% of the variance (Table 2). The results were similar to those when we used the number of moles in the analysis. The scores for each individual bird provided a good separation between sparrows from Saudi Arabia and Ohio, as when we used the number of moles in the analyses (Fig. 4A). The scores of desert sparrows tended to cluster closer to the loadings of longer ceramides and more polar cerebrosides (Fig. 4B).
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PCA suggests, then, that long chain length and sphingolipids with greater polarity distinguish desert sparrows from those living in Ohio, leading to the idea that the decrease in CWL observed in desert sparrows is in some way related to the chain length of the fatty acid moiety and to the polarity of sphingolipid molecules.
Biochemical properties of the sphingolipids in the SC of desert and mesic sparrows
To test the hypothesis that desert sparrows have longer chain lengths in
the fatty acid moieties of sphingolipids, we grouped sphingolipids that we
extracted from the SC into decades, based on carbon chain length of the fatty
acid (Fig. 5). We found that
desert sparrows had more sphingolipids in their SC with fatty acid tails
ranging between 51 and 60 carbons than did sparrows from Ohio
(t=2.62, P<0.02), whereas Ohio individuals had a
significantly higher proportion of sphingolipids with fatty acid tails
21–30 carbons long (t=4.09, P<0.001). Therefore,
desert sparrows had a larger proportion of long sphingolipids, whereas mesic
birds had proportionally more sphingolipids with short chain lengths in
support of the hypothesis.
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Relationship between PC scores and CWL
Our data suggested that a decrease in CWL in desert sparrows could be the
result of longer free fatty acid moieties and more polar sphingolipids in
their SC. To test this idea, we explored the association between principal
components, dominated by chain length and polarity of sphingolipids, and CWL
in each population of sparrows. Using PCA on both the number of moles and the
amount (mg lipid g–1 dry SC) of each sphingolipid family, we
found a negative association between PC 1 and CWL, with sparrows from the two
populations combined (P<0.05).
In some cases, the relationship between CWL and PC scores differed between desert and mesic sparrows. Using PCA on the number of millimoles of each sphingolipid family, we found that CWL was significantly correlated with PC 2 in desert birds (P<0.015), and with PC 3 in mesic sparrows (P<0.03; Fig. 6). After performing PCA on the amount (mg lipid g–1 dry SC) of each sphingolipid class in SC, we found that CWL was positively associated with PC 1 (P<0.05) and negatively correlated with PC 2 (P<0.005), but only in desert individuals in both cases (Fig. 7).
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DISCUSSION |
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TOPSummaryINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONReferences |
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;
Haugen et al., 2003b).
However, the most polar sphingolipid classes in sparrows were more abundant in
individuals from Ohio, but their overall abundance was low compared with other
lipids. Diosylceramides were found only in the SC of mesic sparrows, but the
significance of this finding is unclear; diosylceramides may have the
potential to attract more molecules of water than other cerebroside classes,
which might influence water permeation.
In the sandwich model for the organization of the SC of mammals
(Bouwstra et al., 2000), the
polar heads of ceramides line up facing each other in the lamellae, whereas
non-polar tails orient inward. This creates a highly ordered lattice of lipids
in the crystalline phase, much like the membrane of a cell, which does not
allow movement of water through the lamellae. However, in spaces between
lamellae, fatty acid residues of the ceramides interact with cholesterol
creating a fluid state between lamellae. Ceramide EOS, with its long carbon
chain consisting of two fatty acids, one of them linoleic acid, spans the
bilayers affixing them one to another and contributes to the fluidity of the
central region between lamellae.
|
). Here we incorporate knowledge about the molecular
structure of lipids in the SC of sparrows obtained in this study. With this
information, we propose a plausible model for the organization of the lipids
in the intercellular spaces of the SC in house sparrows
(Fig. 8). Like the `sandwich
model', our model suggests that the more polar regions of ceramides line up
together whereas non-polar tails orient inwardly forming a bilayer in the
crystalline phase. However, we think that in the central fluid layer,
cholesterol, as found in mammals, will be replaced by cerebrosides in SC of
birds, which will form a gel phase. This central layer then would contain
cerebrosides, free fatty acids, small amounts of cholesterol and ceramides
with short fatty acid chains. Because of the scarcity of the appropriate lipid
species able to form a fluid phase in the mammalian SC, the fluid phase of the
central layer of the lamellae is probably discontinuous in this taxon
(Bouwstra et al., 2000).
However, in birds this phase would be continuous within lamellae in the SC and
would allow higher rates of water diffusion, explaining why birds have higher
CWL rates than mammals.
Relative proportions of different classes of lipids in SC seem to be
important in the formation of lamellar structures that are responsible for
reduced CWL rates (Bouwstra et al.,
2003) (Muñoz-Garcia et
al., in press). In mesic species of larks and mesic populations of
house sparrows, increases in free fatty acid content may alter the free fatty
acid to ceramide ratio, and affect the formation of the lamellae in the
intercellular spaces of the SC (Haugen et
al., 2003a;
Muñoz-Garcia and Williams,
2005). If we assume that the average free fatty acid molecule in
the SC is 26 carbons long, free fatty acids, ceramides and cerebrosides are
present in roughly equimolar amounts in sparrows from Ohio, but cholesterol is
present in far lower quantities. The molar ratio between cholesterol and
ceramides was 0.05–0.1. This cholesterol to ceramide ratio prevents the
formation of a lamellar phase in mixtures of cholesterol, ceramides and free
fatty acids (Bouwstra et al.,
2000). In desert house sparrows the molar ratio of free fatty
acids, ceramides and cerebrosides is approximately 1:1:2, and cholesterol
still contributes little to the total, with a ratio of 0.08:1 to ceramides.
This result suggests that cerebrosides, and not ceramides, might be the key
lipid class to explain differences in CWL between desert and mesic sparrows.
Carbon chains of free fatty acids will be longer in mesic sparrows in our
model, which will explain why amounts of free fatty acids are higher in mesic
birds, but molar ratios with ceramides do not change, at least within species.
In this case, the periodicity between lamellae should be the same in desert
and mesic birds. On the other hand, if free fatty acids are on average the
same length in the two populations, then periodicities should be longer in
desert birds.
From an evolutionary perspective, what is the significance of the preponderance of some lipid classes over others in organisms from different environments? Birds have higher CWL rates than mammals, a feature presumably related to thermoregulation, and the substitution of cholesterol by cerebrosides would provide a less tight permeability barrier. At the same time, adjustments in the lipid ratios in the SC will make the barrier more competent in species that live in xeric environments. It is worth noting, though, that CWL of desert birds is still higher than that of the average mammal. On the other hand, the thermoregulatory needs of mammals are satisfied in different ways from those of birds, and CWL is not an important process in this context. Free from the thermoregulatory function, the mammalian SC has evolved towards the creation of a highly efficient barrier, where cerebrosides have no part, except as ceramide precursors. However, few species of free-living mammals have been studied and therefore conclusions for this taxon are tentative.
At the population level, the lipid composition of the SC and the interactions among lipid classes are important to reduce CWL in desert house sparrows. Consistent with this idea, the coefficient of variation of the amounts of all the lipid classes that we identified in this study is larger in mesic sparrows than in desert sparrows, suggesting that selection pressures have been stronger towards the occurrence of particular combinations of lipids in the SC in birds that live in the desert. Moreover, the association between CWL and PC scores was stronger in desert individuals, a sign of tighter regulation of the composition and interactions of the lipids in the SC in sparrows from desert environments. We cannot exclude, though, the role of phenotypic plasticity; current work is addressing the relative importance of natural selection and phenotypic plasticity in the formation of the permeability barrier in birds.
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Acknowledgments |
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References |
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TOPSummaryINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONReferences |
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Bernstein, M. H. (1971). Cutaneous water loss in small birds. Condor 73,468 -469.
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