Maternal and direct effects of the intestinal nematode Heligmosomoides polygyrus on offspring growth and susceptibility to infection
Department of Biology, University of California, Riverside, CA 92521,
USA
Present address: Department of Biological Sciences, University of Idaho,
Moscow, ID 83844, USA
(e-mail: kristand{at}uidaho.edu)
Accepted 16 September 2002
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Summary |
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Key words: parasites, maternal effects, offspring growth, susceptibility, morphology, phenotypic plasticity, Heligmosomoides polygyrus, nematode
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Introduction |
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Maternal condition can also affect offspring growth, immunocompetence and
survival (Sorci et al., 1994;
Sorci and Clobert, 1995
;
Merino et al., 1996
;
Saino et al., 1997
;
Hõrak et al., 1999
).
For example, maternal ectoparasite load in the lizard Lacerta
vivipara was positively correlated with offspring growth rate in the
first year but negatively correlated with offspring growth rate in the second
year (Sorci and Clobert, 1995
)
and differentially affected the survival of male and female offspring
(Sorci et al., 1994
). Most
research on the effects of parasitism on host life history has studied the
effects of ectoparasites, but fewer studies have examined large endoparasites
such as helminths (Kyriazakis et al.,
1996
). Many rodents encounter helminth parasites as young, growing
animals but the effects on their growth are unknown. Importantly, some rodents
have compensatory growth after weaning [e.g. eastern woodrats (Neotoma
floridana) and northern grasshopper mice (Onychomys
leucogaster); Sikes 1996
,
1998
], whereas mass at weaning
for other rodent species can influence adult size and affect reproduction
[e.g. deer mouse (Peromyscus maniculatus;
Myers and Master, 1983
),
Levant vole (Microtus guentheri;
German, 1993
) and prairie voles
(Microtus ochrogaster; Solomon,
1994
)]. Therefore, perturbations of the growth process may have
far-reaching implications for some rodent species.
Laboratory mice (Mus musculus) reallocate resources during a
sublethal infection with Heligmosomoides polygyrus (an intestinal
nematode), as shown by increased metabolic rate associated with increased lean
mass and by changes in organ size and function (Kristan and Hammond,
2000,
2001
). Chronic sublethal
parasite infection with H. polygyrus was also associated with changes
in host reproductive output, whereby parasitized mothers produced 6% smaller
female offspring at weaning than unparasitized mothers
(Kristan, 2002
). Although
female offspring of infected mothers were smaller at weaning, the effects of
parasites (both direct and maternal) on subsequent offspring growth rates and
adult size and body composition are unknown.
In the present study, I measured direct effects of parasite infection on
offspring growth by experimentally infecting pups at weaning with H.
polygyrus. I also examined the consequences of maternal infection on
offspring growth by measuring uninfected pups from either parasitized or
unparasitized mothers. In both cases, I examined differences in growth between
male and female offspring to account for potential sex differences in parasite
susceptibility (e.g. Dobson,
1961; Dobson and Owen,
1978
; Zuk and McKean,
1996
) and growth rates. I first predicted that parasitized mice
would grow more slowly than unparasitized mice because resources normally used
for growth would be used to respond to parasites. Second, I predicted that
maternal parasite infection would not affect offspring growth trajectories but
would affect adult size (based on smaller size of pups from parasitized
mothers; Kristan, 2002
),
assuming no compensatory growth. Third, I predicted that greater infection
intensity would differentially affect body composition and that mice with more
parasites would have greater increases in organ mass associated with
parasitism than mice with fewer worms. Finally, I predicted that the offspring
of parasitized mothers would have a greater infection intensity than the
offspring of unparasitized mothers because offspring would be in a poorer
condition at weaning (assuming smaller size as an index of overall
quality).
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Materials and methods |
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On days 15, 17 and 20, and every 5 days thereafter until the pups were 60
days old, mass (measured in g) and two variables of size right
hindfoot length (from the heel to tip of the claw on the longest toe; measured
in mm) and tail length (from the base to the tip of the tail; measured in mm)
were measured. At 60 days, growth rates have begun to plateau
(Poiley, 1972) and mice are
sexually mature; therefore, measurements were stopped at this time.
At 20 days of age, pups were weaned (i.e. removed from the parental cage) and individually marked with an ear punch. From each litter (if litter size and sex ratio permitted), two male and two female pups were infected with parasites (see below) and two male and two female pups were given tapwater. For smaller litters or litters with uneven sex ratios, approximately half of the offspring were parasitized and half were unparasitized for each sex. This design made it possible to discern maternal effects (by examining unparasitized pups from parasitized mothers) and direct effects (by examining parasitized pups from unparasitized mothers) of parasitism.
Mouse infection procedures
H. polygyrus infective-stage larvae (L3) were
cultivated from non-experimental mice. For adult females used in mated pairs
(50-90 days old), mice were given 300±1 to 300±11 L3
(number of worms ± 1 S.D.; range of S.D. reflects different parasite
cultures used during the experiment) by anesthetizing mice and administering
the L3 with a feeding tube
(Kristan and Hammond, 2000).
Females were mated 14 days post-infection (PI) after H. polygyrus
eggs were detected in the feces, indicating that adult worms occupied the
small intestine lumen.
For pups, on the day of weaning parasitized mice were infected either with
90±1 L3 (low-infection-intensity group) or 200±1 to
200±7 L3 (high-infection-intensity group) suspended in
tapwater. Parasites were administered using a 200 µl pipette tip
(Fisherbrand Redi-Tip on a Pipetman) that was placed at the back of the
mouse's throat. Larvae that were suspended in water were dispensed into the
throat, and mice swallowed to complete the infection procedure. This method
eliminated the use of anesthesia and feeding tubes and produced a similar
final infection intensity to the more invasive method. All unparasitized mice
were given an approximately equal volume of tapwater only. These two infection
intensities (high and low) resulted in final worm burdens that approximated
either a naturally occurring infection intensity (approximately 80-100
L3) for mice at their age at the end of the experiment for the
high-infection-intensity group or resulted in half of the naturally occurring
worm burden (Scott, 1988). The
infection status of all pups was checked using a modified McMaster technique
(Bowman, 1995
) when they were
35 days old (15 days PI).
Body composition, organ morphology and hematocrit measures of
offspring
On days 60-62, mice were anesthetized by intraperitoneal injection of 0.07
ml sodium pentobarbital (65 mg ml-1). Two 25-50 µl blood samples
were collected using retro-orbital puncture with a 75 µl heparinized
capillary tube. Samples were spun for 10 min on a microhematocrit centrifuge,
and the average hematocrit of the two samples was used in the analysis. Mice
were then euthanized by cutting the diaphragm, and the small intestine,
stomach, cecum, large intestine, heart, liver, spleen, kidneys and lungs were
subsequently removed. Excess fat and connective tissue were removed from each
organ and returned to the mouse carcass. The pancreas and associated mesentery
were also removed for another experiment, and data for this organ were not
included in the calculation of fat and lean body mass.
For stomach, cecum and large intestine, each organ was weighed with and
without contents (flushed clean with mammalian Ringer's solution: for
composition, see Karasov and Diamond,
1983). The small intestine was divided into three regions of equal
length (proximal, mid and distal), the wet mass of each region was measured,
and the three masses were added together to determine the total small
intestine mass (corrected for mass of the parasites as described below).
Mucosal and submucosal tissue (hereafter called `mucosa') was separated from
muscularis and serosal tissue (hereafter called `serosa') for two 1.5-cm
segments per region (Diamond and Karasov,
1984
). The dry mass:wet mass ratio was calculated for each
segment, and the mean of these ratios was used to calculate the mucosal and
serosal wet mass and dry mass of the entire small intestine
(Diamond and Karasov,
1984
).
Small intestine mass of infected mice was corrected for the mass of the
parasites. All adult H. polygyrus found in the small intestine during
rinsing, from intestinal segments used for mucosa and serosa measures, and
from remaining unused tissue were collected and counted. The number of worms
was counted using a stereoscope to determine the final infection intensity.
The wet mass of H. polygyrus
(Kristan and Hammond, 2001)
was subtracted from the small intestine wet mass so that calculations of small
intestine dry mass used in analyses did not include worm mass. The dry mass of
all organs and the carcass after drying to a constant mass at 55-60°C for
2 days and 2 weeks, respectively, was measured.
The dried carcass was ground and lipids were extracted using petroleum ether (Goldfische apparatus; Labconco, Kansas City, MO, USA) and the percentage fat and mass of fat were calculated for each mouse. The fat content of all organs, except the small intestine, was measured. Organ fat was removed by soaking organs in 10 ml aliquots of petroleum ether for six 24-h periods (pouring off ether at the end of 24 h and replacing it with fresh ether) and the mass of fat (in g) and percentage fat were calculated from mass loss. Total fat mass of each mouse was the sum of the masses of body and organ fat. Therefore, lean mass was calculated as the initial whole body mass minus the total fat mass.
Statistics
This experiment consists of three independent variables [maternal parasite
status (parasitized, unparasitized), pup parasite status
(parasitized-low-intensity, parasitized-high-intensity, unparasitized) and pup
sex] and numerous dependent variables (body growth, body mass and composition,
organ masses, hematocrit and infection intensity). Data were analyzed with a
split-plot general linear model [either analysis of variance (ANOVA) or
multivariate analysis of variance (MANOVA)] where the main plot was maternal
parasite status and the subplot was pup parasite status.
Because the Gompertz growth equation:
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To examine morphological variables of pups at day 60, a MANOVA was first used to test for significant differences between treatments [pup parasite status (unparasitized, low infection intensity, high infection intensity), maternal parasite status and pup sex] for all dependent variables together (except mucosa and serosa variables because of missing data for some individuals). Because this MANOVA was significant (P<0.05), independent post-hoc ANOVAs were used to determine which treatments and which dependent variables were statistically significant. For all ANOVAs of organ masses, effects of body mass were tested for. If the relationship between the dependent variable and body mass was significant, ANCOVA and present least-squares means ± 1 S.E.M. were used; otherwise, ANOVA and present arithmetic means ± 1 S.E.M. were used. For infection-intensity data, the low- and high-infection-intensity levels were examined separately, and the effects of body mass on this variable were examined as described above.
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Results |
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Offspring growth rate
Growth curve coefficients.
Overall, when examining the three coefficients of the Gompertz growth curve
together, body mass growth differed with pup parasite status
(F1,18=5.6, P=0.008) and pup sex
(F1,18=8.2, P=0.002) but not with maternal
parasite status (Fig. 2), and
tail length growth varied with pup parasite status
(F1,18=3.6, P=0.037) but not pup sex or maternal
parasite status (Fig. 3). There
was a significant interaction between pup and maternal parasite status
(F1,18=3.6, P=0.037) for tail length because the
slope of the curves differed with maternal parasite status but not pup
parasite status. Coefficients for the growth curves of foot length did not
differ among treatment groups (Fig.
4).
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Body mass growth
An overall examination of the three calculated variables of the growth
curve (maximum growth rate, size at maximum growth rate and age at maximum
growth rate) showed that body mass growth varied with pup sex
(F1,19=6.4, P=0.004) and pup parasite status
(F1,19=4.9, P=0.012) and that litter size was a
significant covariate (F1,17=5.7, P=0.008).
Maternal parasite treatment (F1,17=2.6, P=0.091)
and the interaction between pup and maternal parasite treatments
(F1,19=2.9, P=0.063) were marginally
significant.
When I examined the three dependent variables separately, I found that parasitized pups grew 5% faster than unparasitized pups (F1,19=4.5, P=0.048; parasitized, 1.041±0.018 g day-1; unparasitized, 0.988±0.017 g day-1; least squares-means ± 1 S.E.M.) and males grew 23% faster than females (F1,19=4.3, P=0.053; male, 1.121±0.018 g day-1; female, 0.908±0.017 g day-1) with litter size as a significant covariate (F1,17=7.7, P=0.013). There was no maternal effect on the maximum rate of body mass gain.
When pups were growing at their fastest, males were 20% heavier than females (F1,19=8.5, P=0.009; male, 12.03±0.092 g; female, 10.00±0.096 g) and pups from parasitized mothers were 4% heavier than pups from unparasitized mothers (F1,17=6.9, P=0.018; parasitized mother, 11.10±0.102 g; unparasitized mother, 10.93±0.089 g) with litter size as a significant covariate (F1,17=17.7, P=0.0006). There was no effect of pup parasite status on the mass of a pup during maximum body mass growth.
Parasitized pups reached their maximum growth rate 0.5 days earlier than unparasitized pups (F1,19=13.5, P=0.002; parasitized, 20.2±0.183 days; unparasitized, 20.6±0.179 days) and females reached their maximum growth rate 1.7 days earlier than males (F1,19=7.6, P=0.013; males, 21.3±0.177 days; females, 19.6±0.185 days) with litter size as a significant covariate (F1,17=5.4, P=0.033). There was a significant interaction between maternal parasite status and pup parasite status (F1,19=8.8, P=0.008) because the age at which parasitized and unparasitized pups achieved maximal growth was similar for parasitized mothers but differed for unparasitized mothers. There was also a marginally significant interaction between maternal parasite status and offspring sex (F1,19=3.7, P=0.070) because the difference between males and females was greater for unparasitized mothers than for parasitized mothers.
Tail growth
When I examined all three calculated variables together, pups differed in
tail length according to their parasite status (F1,17=4.7,
P=0.015) but not as a function of maternal parasite status or sex.
Litter size was a significant covariate (F1,17=5.2,
P=0.011) and there was a significant interaction between maternal and
pup parasite status (F1,17=4.2, P=0.022). If the
mother was unparasitized, her parasitized offspring had tails that grew 9%
faster than her unparasitized offspring. However, if the mother was
parasitized, her parasitized, offspring had tails that grew 6% slower than her
unparasitized offspring.
Post-hoc analyses revealed no effect of pup parasite status, maternal parasite status or pup sex on maximal rate of tail growth, but litter size was a significant covariate (F1,17=14.2, P=0.002). Parasitized pups had 1% shorter tails at their time of maximal growth compared with unparasitized pups (F1,19=6.3, P=0.022; parasitized, 32.2±0.17 mm; unparasitized, 32.6±0.17 mm), but there was no effect of maternal parasite status or pup sex on the size of the tail when it was at maximum growth rate. Females reached their maximum tail growth rate 1 day earlier than males, which was marginally significant (F1,19=4.3, P=0.053; males, 10.0±0.284 days; females: 9.0±0.297 days), and parasitized pups reached their maximum tail growth rate 1 day earlier than unparasitized pups (F1,19=10.1, P=0.005; parasitized, 9.1±0.294 days; unparasitized, 9.9±0.287 days). There was a significant interaction between maternal parasite status and pup sex (F1,19=4.5, P=0.047) because the difference between males and females was greater for parasitized than for unparasitized mothers. The age when a pup achieved maximum tail growth did not differ with maternal parasite treatment alone or with litter size.
Foot growth
There were no overall differences in foot growth among treatment groups,
but there was a significant interaction between maternal and pup parasite
status (F1,17=7.2, P=0.003). Univariate
post-hoc analyses showed that this occurred because maximum growth
rate differed, but only marginally so (F1,19=4.1,
P=0.058). For parasitized mothers, maximum foot growth was greater
for their unparasitized than their parasitized pups, but, for unparasitized
mothers, foot growth was less for their unparasitized than parasitized
pups.
Morphology and hematocrit
Body mass regressions were significant for all morphological variables but
not for hematocrit. Therefore, I present least-squares means ± 1 S.E.M.
for all morphological variables (to show the effects of treatments after body
mass effects were removed) except hematocrit, where I present the arithmetic
mean ± 1 S.E.M. The MANOVA for morphology variables showed significant
effects of pup parasite status (using all three levels of unparasitized,
low-intensity and high-intensity infection; F2,13=6.4,
P=0.042) and pup sex (F1,13=127.6,
P=0.008) and there was a significant interaction between pup and
maternal parasite status (F2,13=5.4,
P=0.026).
Body mass and composition
At 60 days old, males were 19% heavier than females
(F1,15=6.2, P=0.025; males, 30.86±0.34 g;
females, 25.830±0.35 g) but body mass did not differ either with pup or
maternal parasite treatment (Fig.
5). Mass differences between males and females reflect differences
in lean mass, whereby males had 21% greater lean mass
(F1,15=6.32, P=0.024) but similar total fat mass
and percentage fat mass as females. There were no maternal effects or direct
effects of parasites on body composition (fat versus lean mass) and
all mice had approximately 10% fat mass (range 8-12%).
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Organ masses and hematocrit
Because fat was not extracted separately from each organ, I used whole
organ dry masses (including fat content) to examine treatment effects.
Compared with pups from unparasitized mothers, pups from parasitized mothers
had 2% larger livers (F1,13=5.6, P=0.035), 5%
larger stomachs (F1,13=13.8, P=0.003), marginally
heavier small intestines (by 4%; F1,13=4.5,
P=0.055), longer small intestines (by 2%;
F1,13=3.9, P=0.069) and heavier serosa (by 8%;
F1,11=4.1, P=0.067;
Table 2). Maternal parasite
treatment did not affect the masses of heart, lung, kidney, spleen, cecum or
large intestine.
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Males had larger livers (by 3%; F1,14=13.9, P=0.002) and kidneys (by 21%; F1,14=19.5, P=0.0006) but marginally smaller spleens (by 19%; F1,144.3, P=0.057) than females (Table 2). There was a significant interaction between pup sex and pup parasite status for cecum mass (F2,14=4.4, P=0.032) because males had a 12% smaller cecum than females if they were unparasitized, but, if mice had parasites (regardless of infection intensity), males had only a 7% smaller cecum than females. Males and females had similar organ masses for stomach, small intestine, large intestine, heart and lung.
Overall, parasitized pups had 5% larger livers (F2,14=12.6, P=0.0008) than unparasitized pups (Table 2), but the difference between unparasitized and parasitized pups is significant only for the high-intensity-infection treatment (based on 95% confidence interval around the least-squares mean). Mice with high infection intensity had kidneys that were 5% smaller than those of unparasitized mice (F2,14=4.0, P=0.043), but the kidneys of parasitized mice in the low-infection-intensity group did not differ from those of uninfected mice. Spleen mass varied with pup parasite infection (F2,14=42.2, P<0.0001) and was smallest for unparasitized mice, larger for low-infection-intensity mice and largest for high-infection-intensity mice. Small intestine length increased with pup parasite status (F2,14=17.1, P=0.0002) because pups in the high-infection-intensity group had longer small intestines than those of pups in either the low-infection-intensity group or unparasitized pups, which did not differ from each other. Similarly, small intestine mass differed with parasite treatment (F2,12=35.3 P<0.0001), such that mice in the high-infection-intensity group had a greater small intestine mass than either unparasitized or low-infection-intensity pups, which did not differ from each other. The increase in small intestine mass was a result of increases in both mucosa (F2,12=32.6, P<0.0001) and serosa (F2,12=17.3, P=0.0003; Table 2). Mucosa mass was greater for the high-intensity-infection group compared with the others but did not differ between unparasitized and low-infection-intensity treatments. Serosa mass, however, did not differ between the two infection intensity groups, although parasitized mice had a greater serosa mass than unparasitized mice. Pup parasite treatment did not affect the masses of heart, lung, stomach, cecum or large intestine.
Hematocrit was recorded only for pups in the high-intensity group and unparasitized mice; therefore, pup parasite status has two levels for this analysis (parasitized, unparasitized). Pups from parasitized mothers had 3% lower hematocrit than pups from unparasitized mothers, which was marginally significant (F1,7=5.5, P=0.051), and parasitized pups had a 2% greater hematocrit than unparasitized pups (F1,9=5.6, P=0.043; parasitized pup, parasitized mother: 48.8±0.428%; parasitized pup, unparasitized mother: 50.2±0.363%; unparasitized pup, parasitized mother: 47.8±0.436 units; unparasitized pup, unparasitized mother: 49.7±0.430 units).
Parasite infection intensity
The number of adult worms averaged 52% of the number of larvae administered
for the low-intensity group and 49% for the high-intensity group. The
regressions of infection intensity with body mass were not significant for
mice in either the high- or low-infection-intensity groups; therefore, I
present arithmetic means (number of worms) ± 1 S.E.M. of final
infection intensity. Nine pups that were confirmed to be infected at 35 days
old cleared their infection by 60 days old. All of these pups were born to
parasitized mothers [1 pup (female) had received low-intensity treatment, and
eight pups (three males and five females) had received high-intensity
treatment; Fig. 6]. I excluded
mice that cleared their infection from the analysis of final infection
intensity.
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Low-intensity treatment
For the low-intensity treatment, one female had 10 worms, whereas all other
pups (N=20) had 23-91 worms (46±4 worms; mean ± 1
S.E.M.). Therefore, I analyzed the effect of pup sex and maternal parasite
status first for all individuals and then only for mice with >10 worms to
determine if the outlier affected the results. Final infection intensity did
not differ with pup sex or maternal parasite treatment when all infected mice
were included or when only mice with >10 worms were included
(Fig. 6).
High-intensity treatment
For mice in the high-intensity treatment, three pups had <10 worms (two
worms, N=1 female; nine worms, N=2 males), whereas all other
pups (N=49) had 54-201 worms (110±6 worms; mean ± 1
S.E.M.). When all pups were included in the analysis, there was no effect of
either pup sex or maternal parasite treatment. However, when only pups with
>10 worms were included, pups born to parasitized mothers had a 36% greater
infection intensity than pups born to unparasitized mothers
(F1,12=4.8, P=0.049; parasitized mother,
131.8±9.1 worms; unparasitized mother, 99.2±6.3 worms;
Fig. 6).
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Discussion |
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Direct effects of parasites on growth and morphology
Growth
Overall growth curves of mass and tail length differed with parasitism, but
foot length was similar between parasitized and unparasitized pups. Unlike
other growth measures, foot length may not have varied with parasite treatment
because it already averaged 80% of adult size when pups were first measured
(compared with tail length, which was 63% of adult size, and body mass, which
was 31% of adult size). Therefore, there may have been little distinction
among treatment groups because there was relatively little growth left for
this structure. Final adult body mass did not vary with pup parasite
infection, indicating that the slight differences in growth trajectories with
parasitism were not biologically significant under laboratory conditions.
Morphology
Parasitized pups had similar body mass to unparasitized pups but different
body composition, which may suggest a change in energy allocation to organs
during growth or may simply reflect systemic morphological changes owing to
parasite pathology. Specifically, parasitized pups had larger livers, kidneys,
spleens and small intestines. Mice have both a cell- and antibody-mediated
immune response to H. polygyrus that involves both the spleen and
small intestine (Liu, 1965;
Panter, 1969
;
Dehlawi and Wakelin, 1988
;
Scott and Koski, 2000
), and
presumably changes in size of these organs may, at least partially, be a
result of their function in the immune response. Some intestinal parasites
elicit complement responses by their hosts
(Schmidt and Ruppel, 1988
;
Shin et al., 2001
) and it may
be possible that H. polygyrus also elicits this response in M.
musculus. An increased liver size may result from increased function
during complement production while the mouse is still growing. Other organs
show an increased size during times of increased function (e.g. small
intestine, Hammond et al.,
1994
; kidney, Hammond and
Janes, 1998
) and, although kidney size can increase with increased
protein intake rate (Hammond and Janes,
1998
), it is puzzling that kidney size changed with parasitism,
especially because previous studies of this hostparasite system do not
show changes in kidney mass for infected mice (Kristan and Hammond,
2000
,
2001
). These previous studies
used adult mice, however, and allocation of resources to kidneys may differ
for growing mice and adult mice.
Differences between morphological responses of the low-and
high-infection-intensity groups indicate that infection intensity modulated
the phenotypic plasticity of organ sizes in growing mice such that a greater
infection intensity produced a greater increase in organ mass. Hematocrit (one
indicator of the blood's capacity to carry oxygen) was slightly greater for
parasitized mice than for unparasitized mice. Parasitized mice may use greater
hematocrit to enhance oxygen delivery to enlarged organs or to help supply
oxygen during greater resting metabolism, which can occur with H.
polygyrus infection (Kristan and Hammond,
2000,
2001
).
Maternal effects of parasites on offspring growth and morphology
Growth
Growth curves of body mass but not size measures (tail and foot length)
were affected for offspring from parasitized compared with unparasitized
mothers. Although maternal parasite status did not affect growth trajectories
directly for size measures, maternal effects did modulate the direct effects
of parasites on growth. Previous research has shown that parasitized mothers
wean smaller female pups (Kristan,
2002), but data in the present study show no effects of maternal
parasite status on final adult size of male or female offspring, indicating
that laboratory mice female pups fed ad libitum have some
compensatory growth. Pups may use energy for fast growth potentially to reach
reproductive ability early and thereby modulate possible negative effects of
parasites (Sorci and Clobert,
1995
). This use of energy for fast growth may come at a cost of
using energy for other purposes, such as activity or immune response.
Morphology
A change in energy allocation by offspring during growth occurred as a
result of maternal parasite infection, as indicated by offspring of
parasitized mothers having similar overall body mass to offspring of
unparasitized mothers, despite some greater organ masses (stomach, small
intestines and livers). Importantly, the greater small intestine mass resulted
from an increase in length and also an increase in the mass of the serosal
layer, but not the mucosal layer. The greater mass of the serosal layer may
result from increases in muscularis layers or layers of connective tissue,
which could be determined by histological analysis. Why such morphological
changes occur in small intestines of offspring born to infected mothers is
unclear but may suggest altered digestive functions related to peristalsis
(which utilizes the muscularis layers). Finally, pups born to parasitized
mothers had lower hematocrit values than pups born to unparasitized mothers,
indicating that maternal parasitism infection affects blood characteristics of
offspring, which may influence overall offspring health.
Effects of sex on growth and morphology
In general, males grew faster, weighed more at their time of maximal
growth, and reached their maximal growth rate earlier than females, but
overall differences in growth between males and females were mainly related to
body mass and not size (foot or tail length). Sex differences in body
morphology were modified by parasite infection for only one organ, the cecum.
The difference in cecae size between males and females was more when mice were
unparasitized than when they had parasites. This is surprising because
parasite infection alone did not affect cecum size. It is apparent, however,
that H. polygyrus infection produces variable effects in the cecum,
because some studies show no effect of parasites
(Kristan and Hammond, 2000;
present results) whereas another study showed greater cecae size for infected
mice compared with uninfected mice
(Kristan and Hammond, 2001
).
Changes in nutrient composition and the density of ingesta with parasitism may
affect cecal function and warrant future investigation.
Infection intensity as a function of sex and maternal parasite
status
Host sex
Although male mice are expected to be more susceptible to infection with
H. polygyrus than female mice (Dobson,
1961,
1962b
;
Dobson and Owen, 1978
),
infection intensity did not differ between males and females for either the
low-infection-intensity or high-infection-intensity groups. Importantly, a
number of researchers have shown that host susceptibility to H.
polygyrus is variable for M. musculus. For example,
susceptibility varies with host age and infection duration such that young
mice with 5-day-old infections showed no sex differences in worm burden but
both young and mature males with 10-day-old infections had more worms than
similar-aged females (Dobson,
1962a
). Given that many aspects of this hostparasite system
vary with mouse strain (Liu,
1965
; Monroy and Enriquez,
1992
; Scott and Tanguay,
1994
; Su and Dobson,
1997
), it is not altogether surprising that susceptibility of
Swiss-Webster mice to H. polygyrus is similar for males and females
given the infection intensities and duration used in my study.
Maternal effects
Only pups from parasitized mothers cleared their previously mature
infections by 60 days of age. Moreover, whereas some infected mothers had all
their infected pups clear the infection, other mothers had only some of their
infected pups clear the infection. To try to understand this result, consider
the host response to H. polygyrus. In an initial infection, the
infective-stage larvae first enter the small intestine muscularis layer and
elicit a cell- and antibody-mediated immune response
(Liu, 1965;
Panter, 1969
;
Monroy and Enriquez, 1992
).
However, before the host's immune response is fully effective, some worms
migrate to the intestinal lumen and mature into adults that release an
immunosuppressive factor (Dehlawi and
Wakelin, 1988
; Monroy et al.,
1989
; Monroy and Enriquez,
1992
; Scott and Koski,
2000
). This immunosuppressive factor protects the adult worms and,
presumably, any additional larvae that emerge into the intestinal lumen. In a
second infection, mice mount a rapid cell-mediated immune response that
attacks larvae and delays larval development, and any larvae that do reach
maturity to enter the small intestine lumen are expelled within several weeks
(Monroy and Enriquez, 1992
;
Scott and Koski, 2000
).
Because of the initial immune response, mothers with H. polygyrus
infections have circulating levels of antibodies [e.g. immunoglobulin G (IgG)
and immunoglobulin E (IgE); Scott and
Koski, 2000] that can be transferred to pups via milk
(Greenberg, 1971
;
Jansen et al., 1994
and
references therein). This transfer of antibodies may act similarly to a
secondary infection where parasites could easily be expelled by 60 days of age
(i.e. 5 weeks PI). In this scenario, the antibodies passed from mother to pup
act in place of the typical cell-mediated response of secondary
infections.
So why don't all pups of a litter from parasitized mothers clear their
infections? The effects of the host immune response in secondary infections
with H. polygyrus are variable among mouse strains
(Scott and Tanguay, 1994), and
presumably among individuals, and are at least partially genetically based
(Scott and Tanguay, 1994
).
Therefore, different parasitized mothers potentially have variations in
circulating antibody levels that can be transferred to pups, who will also
show different capacities to respond to infection. Furthermore, at 20 days old
(when pups were experimentally infected), some pups may have a lot of
antibodies from milk whereas others may have relatively little, depending both
on circulating maternal levels and on the amount of nursing done by pups.
There may be a threshold level of antibodies needed before an infection can be
cleared, given that this is not the typical pathway of the immune response to
secondary infections, and this threshold may differ among individual pups.
These proposed mechanisms of the effects of maternal antibodies on offspring
susceptibility to H. polygyrus remain to be tested.
When high-infection-intensity pups that cleared their infections were excluded from the analysis, offspring from parasitized mothers had greater infection intensities than offspring from unparasitized mothers, suggesting that the overall immune capacity of pups from parasitized mothers was compromised. Therefore, maternal condition and, possibly, subsequent differences in maternal effort (e.g. nursing, brooding) may change the susceptibility of offspring to H. polygyrus and possibly other parasite species as well.
Ecological and evolutionary implications
Although research has linked parasite infection with empirical and
theoretical changes in host and parasite life history
(Minchella, 1985;
Hochberg et al., 1992
;
Forbes, 1993
;
Richner and Heeb, 1995
;
Koella et al., 1998
;
Gustafsson et al., 1994
;
Perrin and Christe, 1996
;
Thomas et al., 2000
), current
models of hostparasite associations do not adequately account for the
cost of a long-term demand of chronic, sublethal infections that may
ultimately affect host reproduction, metabolism and life history
(Roberts et al., 1995
).
Measuring the effects of parasites on adult physiology and reproduction and
the direct and maternal effects of parasites on juvenile growth will provide a
better picture of the importance and time course of life history traits
affected by these types of parasites. We still need to know if offspring of
parasitized mothers have different reproductive success than offspring of
unparasitized mothers to determine if H. polygyrus has evolutionary
implications for its host.
Conclusions
This study examined how a host responded to sublethal parasite infection as
a young, growing animal and how an infected mother can affect her offspring
growth and final body condition even after weaning. I provided evidence of
limited effects of both direct and maternal effects of parasite infection on
offspring growth and adult body condition in a controlled setting, which can
provide the basis for future comparisons in nature. Because observations of
captive animals can show different growth patterns compared with their wild
counterparts (Morrison et al.,
1977), it will be important to determine whether the patterns
exhibited by these laboratory mice can be extrapolated to wild house mice.
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