The effect of mycoplasmosis on carotenoid plumage coloration in male house finches
Department of Biological Sciences, 331 Funchess Hall, Auburn University, Auburn, Alabama 36849, USA
* Author for correspondence (e-mail: ghill{at}acesag.auburn.edu)
Accepted 23 March 2004
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Summary |
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Key words: sexual selection, plumage coloration, carotenoid, parasite, house finch, Carpodacus mexicanus, Mycoplasma gallicepticum
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Introduction |
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Carotenoid pigmentation is one of the most widespread mechanisms for
ornamental coloration, particularly in fish and birds
(Goodwin, 1984), and carotenoid
pigmentation has become a text-book example of a condition-dependent display
trait (e.g. Gill, 1995
;
Alcock, 2001
). Carotenoids are
a class of molecules that cannot be synthesized by vertebratesthey must
be ingested to be used as integumentary pigments (Völker,
1934
,
1938
). Expression of
carotenoid-based ornamental coloration is thus partly a function of the type
and quantity of pigments that are ingested
(Hill, 2002
). Once carotenoids
are ingested, however, they still have to be properly utilized to create
ornamental coloration, and parasites can disrupt this utilization process.
Previous experimental studies have shown that monogenean parasites and the
protozoan parasite, Ichthyophthirius multifillis, can affect
expression of carotenoid pigmentation in guppies Poecilia reticulata
and three-spined stickleback Gasterosteus aculeatus, respectively
(Milinski and Bakker, 1990
;
Houde and Torio, 1992
). Field
correlational studies of yellowhammers Emberiza citrinella
(Sundberg, 1995
) and
greenfinches Carduelis chloris
(Merila et al., 1999
) suggest
that parasites can affect expression of carotenoid-based plumage coloration in
birds. Controlled aviary experiments with American goldfinches Carduelis
tristis also showed that coccidiosis
(McGraw and Hill, 2000
) and
mycoplasmosis (Navara and Hill,
2003
) can affect expression of ornamental carotenoid
coloration.
In this study we tested the effects of the bacterium Mycoplasma
gallicepticum (MG) on expression of plumage coloration in the house finch
Carpodacus mexicanus, a species in which males have carotenoid-based
ornamental coloration that varies from pale yellow to bright red
(Hill, 1993). Several studies
have been conducted on the effects of parasites on expression of plumage
coloration in the house finch. Thompson et al.
(1997
) found that males that
that were infected with avian pox prior to molt grew feathers that were less
brightly pigmented compared to males that did not have pox infection prior to
molt. They found a similar relationship between feather mites and coloration
(Thompson et al., 1997
).
Brawner et al. (2000
) conducted
an experiment to test the effect of Isospora coccidia on plumage
coloration. They found that males that they infected with Isospora
during molt grew drabber plumage than males that were maintained free of
coccidial infection. In this experiment, some birds contracted mycoplasmal
conjunctivitis and birds with MG grew drabber plumage than birds that were
free of MG.
Although these studies represent a substantial body of work on the effects
of parasites on carotenoid-based plumage coloration, and the house finch has
been the focus of more research than any other passerine bird, fundamental
questions remain regarding the effects of parasites on carotenoid-based
plumage coloration in the house finch. First, the only carefully controlled
experiment looking at the effects of parasites on plumage coloration was
conducted with isosporan coccidia. Coccidia are parasites of the
gastrointestinal tract. They are known to directly inhibit carotenoid
absorption and the production of carotenoid carrier proteins (Allen,
1987a,b
).
Thus, coccidiosis is a disease that is expected to have direct effects on
expression of carotenoid-based plumage coloration. The effects on plumage
coloration of diseases that are more systemic and that are not known to
directly inhibit carotenoid uptake have not been tested experimentally. In the
Brawner et al. (2000
) study, MG
broke in cages of birdsindividuals were not assigned to treatment
groups. Whether or not individual birds became infected was likely to have
been related to their ability to resist the pathogen. Thus, the effect of
parasites is confounded by the overall health and condition of individual
birds. Finally, in previous aviary studies looking at the effects of parasites
on plumage coloration in house finches, males were maintained on diets
supplemented with the red carotenoid canthaxanthin
(Brawner et al., 2000
).
Canthaxanthin is used by male house finches directly as a plumage pigment
without being modified (Inouye et al.,
2001
; Hill, 2002
).
The dominant red pigment in the plumage of wild male house finches, however,
is 3-hydroxy-echinenone, which is the metabolic derivative of the dietary
pigment ß-cryptoxanthin (Inouye et
al., 2001
). By feeding males a red pigment and bypassing metabolic
pathways, previous feeding experiments may have underestimated the effects of
parasitism on plumage coloration.
In the present study we tested the effects of Mycoplasma gallicepticum on expression of plumage coloration in male house finches. Two groups of males were maintained on a diet supplemented with ß-cryptoxanthin. One group was experimentally infected with MG while the other group was maintained free of MG through the molt period. This study was designed as an experimental test of the effects of a systemic infection on expression of carotenoid pigmentation when birds are utilizing a natural dietary precursor for plumage pigments.
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Materials and methods |
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Males were housed in small cages with 2 birds per cage for the duration of the experiment. Birds within a cage received the same experimental treatmenteither infected or not infected (see below). They were held near windows so they experienced a natural light cycle. All birds had ad libitum access to a canary pellet diet (canary maintenance, Avi-Sci Inc., St Johns, Michigan, USA). Through the molt period, we added one part tangerine juice (100% pure juice, not from concentrate, never frozen) to one part drinking water for all birds. Tangerine juice tended to spoil at room temperature so tangerine juice/water was changed every 24 h.
To ensure that none of the birds in our experiments had been previously
exposed to MG, we tested the serum of each bird for antibodies to MG using a
serum plate agglutination assay as described in Roberts et al.
(2001). We also tested birds
for the presence of MG by polymerase chain reaction (PCR;
Roberts et al., 2001
). We
collected samples for analysis by PCR by swabbing the choanal cleft using a
micro-tip swab (Becton Dickinson and Co. Maryland, USA). Any birds that were
found to have antibodies to MG or that were PCR-positive were excluded from
the study. Coccidiosis is another widespread disease of house finches that is
known to affect expression of carotenoid coloration
(Brawner et al., 2000
). To be
certain that coccidiosis did not confound the effects of mycoplasmosis in this
experiment, we added sulfadimethoxine to the water of all birds to ensure that
they remained free of coccidiosis (Brawner
et al., 2000
)
We cultured MG from symptomatic wild house finches caught in Auburn,
Alabama. We infected the birds in the MG treatment group by dropping 10 µl
of SP4 medium containing 1x106 color-changing units
ml1 into each eye for a total dose of 2x104
color-changing units. This dose of MG has been effective in previous studies
in inducing a modest infection among captive house finches
(Roberts et al., 2001). Birds
in the uninfected treatment group were sham infected with the same amount of
sterile SP4. We monitored the birds daily for onset of disease. Disease was
measured for each eye on a five-point scale, where 0=normal eye and
4=blindness caused by swelling (Roberts et
al., 2001
). We captured all birds three weeks post-inoculation to
collect blood for serology and swabs for MG detection by PCR.
Following molt, we measured plumage coloration of all males using a
Colortron reflectance spectrophotometer
(Hill, 1998). Male house
finches display carotenoid-based plumage coloration on their crown, breast and
rump, and a technician with no knowledge of the experiment scored plumage
color by taking three measurements in each of these areas. We averaged these
measurements to obtain an overall hue, saturation and brightness for each
male. We photographed the breast patch of each male along with a size standard
and used Sigma Scan 5.0 to measure breast patch size. We calibrated each
picture using the size standard and then traced the breast patch three times
and used the average size in all analyses.
All infection protocols carried out in this study were approved by the Institutional Animal Care and Use Committee (IACUC) at Auburn University. We used the smallest sample of birds that would give us reasonable power to detect differences among groups.
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Results |
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All birds that were inoculated with MG developed conjunctivitis in both eyes within 10 days of inoculation. All birds in this group developed antibodies to MG and all tested positive for MG by PCR. For most birds the infection lasted throughout the 8-week molt period, with the most severe clinical symptoms occurring 26 weeks after infection.
After molt on the pellet diet supplemented with tangerine juice (as a
source of ß-cryptoxanthin; Hill,
2000), all males grew pale orange plumage, much drabber than the
average wild male in the Auburn, Alabama population. There was a significant
effect of MG infection on ornamental plumage coloration. Males that were
infected with MG during molt grew breast feathers that were more yellow/less
red (Z=1.90, N=15,16, P=0.03), less
saturated (Z=1.86, N=15,16, P=0.03), and
less bright (Z=1.78, N=15,16, P=0.04) than
males that had no MG infection (MannWhitney U Tests)
(Fig. 1). There was no
significant effect of the cage in which males held on any component of plumage
coloration (hue: F-ratio=1.62, P>0.17; saturation:
F-ratio=1.84, P>0.12; brightness: F-ratio=1.33,
P=0.29).
|
Patch size was measured on males several months after molt and by that time several males had died or been used in other experiments. Therefore we had only 12 infected and 13 control males for patch size comparison. We found no significant difference in the patch sizes of male that were infected with MG during molt and the patch sizes of males that were not infected (Z=0.65, P=0.51).
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Discussion |
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Previous research had shown that in controlled infection experiments,
coccidiosis caused male house finches to grow less red and less saturated
plumage coloration (Brawner et al.,
2000). In these infection experiments that controlled the exposure
of males to coccidiosis, some males contracted mycoplasmal conjunctivitis, and
it was found that males with mycoplasmal conjunctivitis grew less red, less
saturated, and less bright plumage than males that were not infected
(Brawner et al., 2000
). Because
males in the Brawner et al.
(2000
) study were not infected
with MG by researchers, but rather either contracted or resisted contracting
the disease, there is the chance than male health and condition affected both
plumage coloration and disease state.
In the present study we eliminated the uncertainty of previous studies and
show definitively that mycoplasmal conjunctivitis during molt depressed
expression of ornamental plumage coloration in male house finches. In our
experiment, we randomly assigned males to treatment groups eliminating any
confounding effects of condition or health. We also fed males the precursor to
the red pigment that is the most abundant red pigment in the plumage of wild
house finches. By feeding the metabolic precursors to red feather pigments, we
forced birds to include more steps in the utilization of pigments.
Interestingly, even though we forced birds to metabolically modify dietary
pigments to produce red ornamental coloration, and thus added at least one
step to the process of carotenoid utilization, we saw no greater effect of MG
on plumage coloration in this study, compared to the previous study in which
males were fed red feather pigments directly
(Brawner et al., 2000).
We found a significant negative effect of mycoplasmosis on plumage
coloration, but no significant effect of mycoplasmosis on the size of
carotenoid breast patches. This observation is consistent with observations
from feeding experiments in which access to carotenoid pigments had a large
effect on plumage coloration but a small effect on patch size (Hill,
1992,
1993
). This finding is also
consistent with the idea that the quality of ornament pigmentation
(coloration) and the area of the body with pigment (patch size) are under
distinct developmental control, with largely independent responses to
environmental challenges and different signaling function
(Badyaev et al., 2001
).
As a source of ß-cryptoxanthin, we fed males in this experiment
tangerine juice (Hill, 2000).
ß-cryptoxanthin is the predominant carotenoid in tangerine juice
(Mangels et al., 1993
), but
the amount of ß-cryptoxanthin ingested by male house finches in this
study was probably still small compared to the amount of the pigment that is
likely to be ingested by wild finches feeding on fruits. Males in both
treatment groups appeared to have fewer pigments available than they needed
for maximum expression of ornamental coloration, and all males were drab at
the end of the feeding experiment. Experimentally infected males were simply
drabber on average than uninfected males. We assume that we may have seen
larger differences in plumage coloration between control and infected groups
if males had been fed larger doses of ß-cryptoxanthin, but it is also
conceivable that access to more ß-cryptoxanthin may have masked the
effects of mycoplasmosis.
Previous experimental studies of parasites and plumage coloration in house
finches have focused on coccidiosis. In one sense, these studies are
particularly valuable because they focus on a parasite for which a mechanism
for direct inhibition of carotenoid utilization is known
(Brawner et al., 2000). On the
other hand, studies of coccidia leave open the question of the effects of
parasites that do not directly inhibit carotenoid uptake or transport, but
that have more general systemic effects on the bird and would have indirect
effects on pigment utilization. MG is an upper respiratory disease
(Jordan, 1996
). It does not
infect gastro-intestinal tissues and thus it is unlikely to directly affect
carotenoid absorption. Nevertheless, we found that MG depressed expression of
ornamental coloration. The present study, coupled with previous experiments on
the effects of disease on house finches, shows that both parasites that
specifically target gastro-intestinal tissues and directly disrupt carotenoid
utilization as well as parasites that infect tissues outside the
gastro-intestinal track can have a significant effect on how carotenoid
pigments are utilized. The relative importance of these two types of parasites
on expression of plumage coloration among males in wild populations remains to
be determined.
One intriguing possibility is that, in birds infected with MG, carotenoids
that could have been used for ornamental display were instead diverted to
bolster the immune system against the infection
(Lozano, 1994;
Møller et al., 2000
).
We cannot assess this hypothesis with the data at hand, but it has yet to be
shown for any species that there is a trade-off between use of carotenoids for
ornamental plumage coloration and use of carotenoids for immune defense
(Hill, 1999
). A recent study
on American goldfinches failed to find such a trade-off in a carefully
controlled infection experiment (Navara
and Hill 2003
). A simpler explanation is that the house finches in
this study that were infected with MG diverted energy away from pigment
utilization to immune defense and this diversion of energy caused the loss of
plumage coloration in infected males.
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Acknowledgments |
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