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JOURNAL OF EXPERIMENTAL ZOOLOGY 283:573–579 (1999)
Agouti Locus May Influence Reproduction
Under Food Deprivation in the Water Vole
(Arvicola terrestris)
N.M. BAZHAN,* T.V. YAKOVLEVA, AND E.N. MAKAROVA
Institute of Cytology and Genetics, Novosibirsk, Russia, 630090
ABSTRACT
The effect of 16-hr food deprivation on day 3 and again on day 5 of pregnancy on
the fecundity of female water voles homozygous (ae/ae) or heterozygous (A/ae) for, an allele at the
Agouti (A) locus, non agouti extreme (ae) was studied. 63 A/ae females (mated to ae/ae males)
produced 115 food-deprived and 115 control pregnancies, and 52 ae/ae females (mated to A/ae
males) produced 55 food-deprived and 57 control pregnancies. Regardless of the experimental group,
pregnant ae/ae females weighed less than A/ae females. The effect of food deprivation on fecundity depended on the Agouti-locus genotype of the female. In food-deprived A/ae females, fecundity was diminished due to fewer successful pregnancies (P < 0.001) and lower survival of the
young (P < 0.05). In food-deprived ae/ae females, reproductive performance was not changed; a
somewhat reduced rate of successful pregnancies was compensated for by significantly increased
(P < 0.002) postnatal survival of the young. In progeny weaned from food-deprived mothers, the
frequency of A/ae females was diminished. Resistance of ae/ae females to the negative effect of
nutritional stress, and predominance of ae/ae young in progeny produced by food-deprived mothers, may favour the maintenance of polymorphism for the Agouti-locus in natural populations of
the water vole. J. Exp. Zool. 283:573–579, 1999. © 1999 Wiley-Liss, Inc.
Natural water vole (Arvicola terrestris) populations are polymorphic for coat color. There is a
spectrum in which three main coat colors can be
distinguished: brown, black-brown, and black
(Nikolaeva, ’78). Laboratory studies have revealed
that brown individuals are homozygous for the
dominant wild-type allele Agouti (A); black, melanic individuals are homozygous for extreme nonagouti (ae); and black-brown individuals are A/ae
heterozygotes (Nasledova et al., ’80; Prasolova et
al., ’91). In western Siberia, populations of the
water vole exhibit 7–8 year population cycles
(Moshkin et al., ’90). A sharp decrease in population size usually is coincident with drought, decreased availability of green vegetation, and
increased frequency of black voles from 4 to 10%
(Nikolaeva, ’78; Moshkin et al., ’90). We suspect
that malnutrition and short-term food deprivation
occur in nature in the beginning of the breeding
season as a consequence of food stores being
exausted, in the years of high density. Studies by
Gerlinskaya et al. (’94) indicate that malnutrition
can inhibit the reproduction of the water vole.
They found that in the course of the population
cycle there is a positive correlation between blood
levels of fatty free acids (index of undernutrition)
and the number of embryos lost. Under labora© 1999 WILEY-LISS, INC.
tory conditions, food deprivation in late pregnancy
reduces fecundity in the water vole (Bazhan et
al., ’96a). Accordingly, we hypothesize that the increased frequency of black (ae/ae) voles that occurs in years of low population density results
from the fact that ae/ae females are better able
than A/ae females to reproduce successfully when
food is scarce.
The Agouti locus is one of the major coat color
loci in mammals. The Agouti gene of mice has
been cloned and shown to encode a 131-amino acid
protein-signaling factor that dictates melanocyte
pigment production within individual hair follicles
(Bultman et al., ’92; Miller et al., ’93). The mouse
agouti protein also has been shown to be an antagonist of α-MSH on melanocortin-1 receptors
(MC1R) in skin and ACTH and α-MSH on MC4R
in brain tissue (Lu et al., ’94). It also regulates
the concentration of intracellular calcium (Zemel
Grant sponsor: Program of Russian Ministry of Education “Fundamental Researches in Natural Sciences”; Grant number: 3H-20098; Grant sponsor: Russian Foundation for Fundamental Researches;
Grant number: 98-04-49502.
*Correspondence to: Nadezhda M. Bazhan, Institute of Cytology
and Genetics, Lavrentjev Av., 10, Novosibirsk, 630090, Russia. E-mail:
bazhan@bionet.nsc.ru
Received 9 April 1998; Accepted 11 August 1998.
574
N.M. BAZHAN ET AL.
et al., ’95). These findings suggest possible roles
for the agouti protein in a variety of cellular and
physiological functions. In mice, the recessive mutation ae results in a lack of translation initiation,
and hence no agouti protein (Hustad et al., ’95).
Although little is known about the pleiotropic effects of the ae allele in mice, the absence of the
functional agouti protein does not appear to affect
either viability or fertility (reviewed in Manne et
al., ’95). In other species, the ae allele was shown
to effect neuronal and hormonal functions. A pleiotropic effect of the ae allele on brain catecholamines
(Hayssen et al., ’94) and ability to reproduce under stressful conditions (Hayssen, ’98) have been
demonstrated in deer mice (Peromyscus maniculatus). Our earlier studies have revealed a pleiotropic effect of the ae allele on secretion of adrenal
corticosterone and progesterone in immature female water voles (Bazhan and Ivanova, ’89) and
adrenal response to osmotic stress in adults
(Bahzan, ’91). The object of this study was to test
our hypothesis that the increased frequency of
ae/ae animals that occurs during periods of food
deprivation stems from their enhanced ability to
reproduce successfully when nutritionally deprived.
Accordingly, we have compared the reproductive
performances of A/ae and ae/ae females when experimentally deprived of food.
MATERIALS AND METHODS
Animals
Wild water voles were introduced into the laboratory in 1974 from the subtaiga region of West
Siberia (Ubinsk). A colony has since been successfully bred at the vivarium of the Institute of Cytology and Genetics (Siberian Division, Russian
Academy of Science), at an ambient temperature
of 21°C under natural light conditions. Judging
from the frequency of litters and their size, there
appears to have been no diminution in reproductive ability in captivity. Details on the laboratory
breeding of water voles have been described elsewhere (Bazhan et al., ’96a). The breeding season
of the water vole under natural and laboratory conditions lasts from April to August (Panteleev, ’68).
Ovulation is induced and implantation occurs on
the 7th day of pregnancy (Nasledova et al., ’87).
The gestation period is 21 days, animals are
weaned at about three weeks of age (Blake, ’82).
Breeding
Females (150–170 g) and males (170–190 g),
aged 8–9 months, were housed individually before
the beginning of the breeding season. Females
were paired with males, and vaginal smears were
checked daily for spermatozoa. The day that spermatozoa were detected was designated day 0 of
pregnancy. After mating, males were removed
from females. Cages with pregnant females were
checked daily at time of expected parturition. The
day young were found was designated day 1 of
age. Young were separated from mothers on day
21 postpartum, and females were paired for the
second time with another male. Under those experimental conditions each female can produce
3–4 litters during the breeding season. Litter size
is not related to the number of pregnancies.
Crosses
Polymorphism for Agouti locus alleles in our
laboratory population has been maintained by an
outbreeding regime. Inbreeding coefficient does not
exceed 12.5%. The fecundity of heterozygous and
homozygous females has been shown to be the
same in reciprocal crosses A/Ae × ae/ae and ae/ae
× A/ae (Bazhan et al., ’96b). We worked only with
these reciprocal crosses in this study, so litters were
genetically similar (only the mothers differed).
Food deprivation
Females were denied access to food for 16 hours
(from 1700 hr to 0900 hr) on day 3 and again on
day 5 of pregnancy. Paper shavings were substituted for hay, and fresh water was freely available. Females were not food deprived for two
pregnancies in succession. During control pregnancies females received the standard laboratory
diet and food and water ad libitum (Bazhan et
al., ’96a). Sixty-three A/ae females produced 115
pregnancies that were food deprived (experimental group) and 115 pregnancies that were treated
normally (control group). Similarly, 52 ae/ae females produced 55 food-deprived and 57 control
pregnancies. Both A/ae and ae/ae females were
mated 3–4 times in each breeding season. There
were fewer pregnancies in ae/ae females (both control and food-deprived) because it was necessary
to use some of these females for test crosses in
other experiments.
Reproductive measures
For each female we recorded the total number
of pregnancies, the number of successful pregnancies (i.e., that resulted in birth), the number of
young (litter size) at parturition, at 8 days, and
at weaning (day 21). Since mortality is significantly lower in litters that are not disturbed post-
FOOD DEPRIVATION AND AGOUTI LOCUS
partum (Blake, ’82) animals were not handled
(sexed) until they were 8 days old, the youngest
age at which their genotype can be ascertained.
In each litter the number of young homozygous
and heterozygous females and males also was recorded at weaning. The litters in which all young
lived to weaning were designated successful, litters where some did not live, partially successful,
and litters where none lived to weaning, unsuccessful. The total index of observed fecundity, expressed as the mean number of weaned young per
pregnancy, was calculated for each pregnancy. In
the case of pregnancy failure the total index of
fecundity was set equal to zero. Daily weights
were measured during 56 control and 17 food-deprived pregnancies in A/ae females and during 34
control and 12 food-deprived pregnancies in ae/ae
females. The day that female ceased gaining
weight was designated the day of pregnancy failure. The weights of females in which pregnancy
failed were excluded from the consideration.
Statistical analysis
Weight data were analyzed by two-way ANOVA.
The ANOVA includes two factors: female agouti
genotype (A/ae and ae/ae) and group (control and
food-deprived). To compare the rates of successful
pregnancies, weaned litters, weaned young, males
and melanics (ae/ae genotype) in progeny, the χ2test was applied with Yates correction for continuity. Litter size at birth and number of surviving
young at weaning were compared with a t-test.
Nonparametric variances such as relative frequencies of young A/ae and ae/ae females and males
in successful litters and mean number of young
weaned per one pregnancy were analysed with use
of Mann-Whitney U-test. Statistical significance
was defined as P < 0.05. Data are presented as
mean ± 1 S.E.
RESULTS
Food deprivation resulted in failed pregnancies.
Among control pregnancies in which female
575
weights were measured daily 30% (27 of 90) failed,
whereas in food-deprived groups, 48% (14 of 29)
failed (P < 0.05). The risk of pregnancy failure
increased in the second week (Table 1). The rate
of pregnancy failure in this period was significantly greater in food-deprived than in control females. In week two, in food-deprived groups, all
pregnancies failed from 8 to 12 days postcoitum
and most (62%) failed on days 8 and 9 (Table 1),
i.e., just after implantation.
Regardless of experimental group, ae/ae females
weighed less than A/ae females throughout gestation (ANOVA, P < 0.001). Food deprivation for
16 hours on days 3 and 5 of pregnancy induced
weight loss on days 4 and 6. In aa/aa females,
loss (6.9%) was less than in A/ae females (8.2%,
ANOVA, P < 0.001). After the second week of pregnancy the weight was regained. At parturition
weights in food-deprived and control females were
not differed (in A/ae: control 214 ± 6 g, food deprivation 215 ± 10 g; in ae/ae: control 201 ± 5 g,
food deprivation 187 ± 10 g).
There were no significant differences in litter
size by experimental conditions or genotype (overall mean in control groups 4.5 ± 0.2, n = 121; in
food-deprived groups 4.6 ± 0.2, n = 76).
A/ae and ae/ae females differed in the index of
fecundity—the mean number of young weaned per
pregnancy—when they were deprived of food
(Table 2). In the control, there was no difference,
but after food deprivation, the index of fecundity
for A/ae females dropped to 0.55 times that of the
controls (P < 0.001). By contrast, food deprivation
in early pregnancy did not alter the index of fecundity ae/ae females. There were somewhat fewer
successful pregnancies in this group, but survival
of young actually increased after food deprivation
(P < 0.002; Table 2). Breeding under food deprivation, ae/ae females had some advantages over
A/ae females; the rates of litters weaned and of
young surviving were much greater (P < 0.001,
Table 2), and as a result the index of fecundity
tended to be higher (P < 0.1, Table 2).
TABLE 1. Times of pregnancy failures due to food deprivation (for 16 hr on days 3 and 5), as shown by failures in each week
of pregnancy and by distribution of failures in second week of pregnancy (when failures were greatest)
Distribution of failures throughout pregnancy
Group
Control
Food-deprived
P1
1
Total no.
of failures
27
14
Week of pregnancy
Days in second week
1
2
3
8–9
10–12
13–14
4 (15%)
0
n.s.
18 (67%)
13 (93%)
<0.003
5 (18%)
1 (7%)
n.s.
9 (50%)
8 (62%)
n.s.
5 (28%)
5 (38%)
n.s.
4 (22%)
0
n.s.
Probability levels are given for differences between control and food-deprived females.
576
N.M. BAZHAN ET AL.
TABLE 2. Effect of food deprivation during pregnancy (16 hr on days 3 and 5) on reproduction in water voles of two
genotypes, A/ae (n = 65) and ae/ae (n = 52)1
Parental
genotype
(female × male)
A/ae × ae/ae
ae/ae × A/ae
Survival of young
Experimental
group
Reproductive
success
(litters/
pregnancy)
(litters
weaned/litters
born)
(young
weaned/young
born)
Weaning rate
(number of young
per pregnancy
mean ± 1 SE)
Control
Food-deprived
P
Control
Food-deprived
85/115 (74%)
49/115 (43%)
<0.001
36/57 (63%)
27/55 (49%)
71/85 (84%)
35/49 (71%)
<0.1
31/36 (86%)
26/27 (96%)
311/371 (84%)
170/220 (77%)
<0.05
143/179 (80%)
118/127 (93%)
2.7 ± 0.2
1.5 ± 0.2
<0.001
2.5 ± 0.4
2.2 ± 0.3
P
n.s.
n.s.
<0.002
n.s.
P (food-deprived
A/ae vs. ae/ae)
n.s.
<0.001
<0.001
<0.1
1
Probability levels for differences between control and experimental females shown in body of table; probabilities for differences between two
female genotypes shown in last row of table.
Some litters were entirely lost before weaning,
when in other litters a few young were lost. Nevertheless, there were no significant differences in
the losses of partial litters in any categories examined (Table 3). By contrast, the losses of whole
litters varied with time (most of them occurred in
the first 8 days postpartum), experimental conditions and genotype. Food deprivation caused more
losses in A/ae females—more than in controls (P
< 0.05) and more than in ae/ae females (P < 0.001;
Table 3). In food-deprived A/ae mothers, 43% of
lost litters (6/14) consisted of only one infant;
therefore, although fewer litters survived, their
mean size at weaning was significantly greater
than the mean size of litters at birth (5.3 ± 0.3 g,
vs. 4.5 ± 0.3 g, P < 0.05).
Food deprivation also altered the frequency of
gender and genotype of young weaned. There was
a tendency for more males (P < 0.1) and more
ae/ae offspring (P < 0.06) to be weaned from A/ae
mothers following food deprivation (Table 4). There
also were more (55%) ae/ae offspring weaned from
both food-deprived ae/ae and A/ae mothers (P <
0.05). Since we could not distinguish the agouti
genotype before 8 days of age, any differential
elimination of offspring between birth and 8 days
would contribute to, or might totally account for,
the observed frequency differences. Therefore, in
order to estimate the effect of prenatal elimination on litter composition, we compared only successful litters in which all young survived
weaning. In the successful litters born to fooddeprived mothers, there were more males of both
genotypes (28% A/ae and 29% ae/ae) and a decrease (to 19%) in A/ae females.
DISCUSSION
e
e
In A/a and a /ae food-deprived females, gestation was an all-or-none phenomenon, in that either complete litters developed or all implants
were resorbed. Most pregnancy failures (93%) occurred on days 8–12 postcoitum, i.e., just after
implantation (Nasledova et al., ’87). In rats (Berg,
’65) and mice (Archunan, et al., ’94), fasting and
food restriction in early pregnancy also caused the
death of whole litters at the time of implantation.
TABLE 3. Ages at which neonates died in litters in which some died (“partially successful”) and in litters in which all died
(“unsuccessful”), following food deprivation of mothers during pregnancy (for 16 hr on days 3 and 5)
Parental
genotype
(female × male)
A/ae × ae/ae
ae/ae × A/ae
Experimental
group
Control
Food-deprived
P
Control
Food-deprived
P
P (food-deprived
A/ae vs. ae/ae)
No. of
litters
born
85
49
36
27
No. of litters in which young died
Partially successful litters
Unsuccessful litters
Days 1–8
Days 9–21
Days 1–8
Days 9–21
9 (11%)
5 (10%)
n.s.
3 (8%)
1 (4%)
n.s.
n.s.
1 (1%)
1 (2%)
n.s.
2 (6%)
2 (7%)
n.s.
n.s.
10 (12%)
14 (29%)
<0.02
5 (14%)
1 (4%)
n.s.
<0.001
4 (5%)
0
n.s.
0
0
n.s.
n.s.
Young weaned in completely successful litters
Young weaned in all litters
Parental
genotype
(female × male)
A/ae × ae/ae
ae/ae × A/ae
Total
a
Experimental
group
Sex ratio
(males/total)
(%)
Agouti ratio
(ae/ae out
of total) (%)
Control
Food-deprived
P
Control
Food-deprived
P
Control
Food-deprived
P
147/311 (47)a
92/170 (54)
<0.1
80/143 (56)a
67/118 (57)
n.s.
227/454 (50)
159/288 (55)
n.s.
142/311 (46)
93/170 (55)
<0.06
74/143 (52)
65/118 (55)
n.s.
216/454 (48)
158/288 (55)
<0.05
Relative frequencies
No. of
litters
Male
A/ae
Male
ae/ae
61
29
0.26 ± 0.03
0.28 ± 0.04
n.s.
0.26 ± 0.05
0.29 ± 0.06
n.s.
0.26 ± 0.03
0.28 ± 0.03b
n.s.
0.25 ± 0.03
0.28 ± 0.03
n.s.
0.29 ± 0.05
0.31 ± 0.06
n.s.
0.26 ± 0.03
0.29 ± 0.03c
n.s.
26
23
87
52
Female
A/ae
0.27 ± 0.03
0.20 ± 0.03
<0.07
0.22 ± 0.05
0.17 ± 0.04
n.s.
0.25 ± 0.02
0.19 ± 0.02bc
<0.05
P < 0.05, differences between ae/ae and A/ae control females.
P < 0.05, differences between relative fequencies of A/ae young females and males, in litters weaned by food-deprived females.
c
P < 0.05, differences between relative frequencies of A/ae young females and ae/ae young males, in litters weaned by food-deprived females.
b
Female
ae/ae
0.23 ± 0.03
0.25 ± 0.04
n.s.
0.23 ± 0.04
0.23 ± 0.04
n.s.
0.23 ± 0.02
0.24 ± 0.03
n.s.
FOOD DEPRIVATION AND AGOUTI LOCUS
TABLE 4. Effect of 16 hr maternal food deprivation during pregnancy (days 3 and 5) on relative frequencies of male and female young of
different agouti genotypes
577
578
N.M. BAZHAN ET AL.
It may be assumed that food deprivation induced
pregnancy failures by disturbance of progesterone
and corticosterone levels before implantation. In
water voles, 24-hr fasting on day 3 and on day 5
of pregnancy significantly increased blood corticosterone and decreased urinary progesterone concentrations on day 6, (our unpublished data). We
assume that in pregnant voles fasting-increased
corticosterone level may reduce progesterone level.
Corticosterone was shown to inhibit progesterone
secretion in pregnant rats by affecting the corpus
luteum directly, (Sugino et al., ’91). The lack of
progesterone at implantation blocks pregnancies
in the ferret (Rider and Heap, ’86) and in the
mouse (Rider et al., ’87; Heap et al.,’92).
Food deprivation affected the survival of nestlings both in A/ae and ae/ae females. Nevertheless, the effect differed strikingly: in A/ae females,
postnatal survival decreased compared to controls,
whereas in ae/ae females it increased. Nutritional
stress apparently has different effects on maternal behaviour in A/ae and ae/ae females. We believe so because in A/ae females, food deprivation
increased postnatal death of whole litters (rather
than individual pups) within the first week of lactation, when maternal care is of primary importance to neonatal survival. Postpartum maternal
behaviour is regulated by endocrines, including
progesterone (Bridges ’84). In mice, short-term
depletion of active progesterone, induced by a
single administration of anti-progesterone antibody on day 2 of pregnancy, blocks embryo development and implantation (Vinijsanun et al., ’90)
and it also causes aberrant maternal behaviour
towards the neonate within the first 5 days of
lactation (Wang et al., ’95). We suggest that in
food-deprived A/ae females the same hormonal
mechanism—a decreased level of progesterone before implantation—inhibited survival of offspring
both prenatally and postnatally.
It is not clear why the opposite effect—the increased survival of young of ae/ae mothers—occurred. Doubtless this is related to pleiotrophic
effects of the nonagouti allelle ae on the endocrine
system. Adult females of the two genotypes differ
in their adrenal response to stress, for ae/ae
females do not increase their level of serum corticosterone when subjected to osmotic stress
(Bazhan, ’91) or to social stress (Moshkin et al.,
’90), in contrast to A/ae females, which do.
Our data support other observations that there
are differences in fertility associated with the Agouti locus. In deer mice (Peromyscys maniculatus),
reproduction after the stress of transport was sup-
pressed to a different extent in nonagouti and
agouti females, with nonagouti deer mice having
more failures after transatlantic transportation
(Hayssen, ’98). It is not clear how the Agouti locus influences reproduction under stressful conditions, obviously its pleiotropic effect depends on
the nature of the stress and on the species.
In additional to changes in fertility, there were
also differences in gender and genotype of progeny born after food deprivation with fewer A/ae
females born than would be expected. It suggests
that fasting-induced prenatal elimination of embryos was not random; litters with predominance
of A/ae females were more vulnerable and more
likely were eliminated. It indicates that the Agouti
locus is expressed at an early stage of development
in the water vole. This suggestion agrees with observation in mice, in which homozygosity for lethal
Agouti locus alleles (Ay and a(x)) results in embryo
death at about the time of implantation (Duhl et
al., ’94; Miller et al., ’94). In summary, the ae allele
in homozygote has a positive pleiotropic effect on
female fecundity and on embryo viability under nutritional stress.
Our earlier report has demonstrated that maintenance of polymorphism for the Agouti locus in
natural populations of the water vole might be
associated with positive pleiotropic effect of the
ae allele on fecundity of homozygous and heterozygous females (Bazhan et al., ’96b). In nature, in
years of high population density, food deprivation
may occur in populations of the water vole if winter food stores are exhausted (Evsikov et al., ’97).
Present findings show that resistance of ae/ae females to the negative effect of nutritional stress,
and predominance of ae/ae young in progeny produced by food-deprived mothers, may also favour
the maintenance of polymorphism for the Agouti
locus in natural populations of the water vole.
ACKNOWLEDGMENTS
We thank Dr. Tatiana Aksenovich who advised
on statistical analyses. We are grateful to Dr. Barbara Blake, Bennett College, who read various
versions of the manuscript and made many useful suggestions. We thank her also for help with
English.
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