Am J Physiol Regulatory Integrative Comp Physiol
281: R1817–R1824, 2001.
Genetic variation in photoperiodism among naturally
photoperiodic rat strains
ANNAKA M. LORINCZ, M. BENJAMIN SHOEMAKER, AND PAUL D. HEIDEMAN
Department of Biology, College of William and Mary, Williamsburg, Virginia 23187-8795
Received 12 June 2001; accepted in final form 17 August 2001
reproduction; seasonal breeding; critical day length; photoperiod
in the temperate zones
use environmental cues such as photoperiod to time
sexual maturity and reproductive attempts (2, 4). This
increases the probability that offspring will be born in
conditions that are optimal for survival. However, occasional reports of wild rodents that do not suppress
reproduction in winter have been followed by studies
that have demonstrated genetic variation in responses
to photoperiod both among and within populations of
photoperiodic rodents (6, 11, 18, 20, 27, 29, 33). It has
been hypothesized that genetic variation in photoresponsiveness may be typical of temperate zone populations of rodents (13).
Variation in photoresponsiveness has also been observed in laboratory rats. Recent studies have shown
that the Fischer 344 (F344) inbred rat strain exhibits
robust obligate photoresponsiveness, repressing reproduction, somatic growth, and food intake in short photoperiods (SD) (9, 10, 14, 15, 19). In contrast, other
SEASONALLY BREEDING RODENTS
Address for reprint requests and other correspondence: P. D.
Heideman, Dept. of Biology, College of William and Mary, Williamsburg, VA 23187-8795 (E-mail: [email protected]).
http://www.ajpregu.org
strains of laboratory rats have not been considered
functionally photoresponsive because unmanipulated
rats of these strains do not show marked differences in
body mass, gonad size, or food intake (22) compared
with traditional photoresponsive rodents such as Phodopus sungorus, Mesocricetus auratus, or Peromyscus
leucopus (3). However, even in rat strains considered
nonphotoperiodic, such as the Wistar and SpragueDawley outbred strains, procedures such as neonatal
androgen treatment and food reduction have been
shown to unmask facultative photoresponsiveness (26,
30, 31). Those strains of rats that can be induced to
become photoperiodic also differ from F344 rats in
having a much shorter critical photoperiod, the photoperiod below which reproduction is inhibited (24, 25,
30). The presence of this variation among rat strains
allows comparisons of rat strains to identify the neuroendocrine and genetic basis for photoperiodism and
also raises the question of how widespread photoresponsiveness might be among strains of rats.
Two hypotheses have been proposed to account for
the existence of photoperiodic and nonphotoperiodic
laboratory strains of rats. First, it has been hypothesized that wild Rattus norvegicus, ancestral to laboratory rats, was not photoperiodic but retained in vestigial form some of the elements of the complex
neuroendocrine pathway for photoperiodism (21–23,
31). Under this hypothesis, functional photoresponsiveness in particular rat strains has arisen due to a
mutation(s) that restored partial function to the pathway. Such mutations should be rare, and, therefore, we
would predict that photoresponsiveness in rats should
be based on changes at a single locus or a small set of
loci. If so, photoresponsive strains should have a common genetic basis for their photoresponsiveness; in
other words, the allele(s) that causes photoresponsiveness should be identical in each photoresponsive strain
of rats. In addition, such rare mutant alleles should
have occurred in only a single ancestor or a few of the
ancestors of laboratory rats. Therefore, one would predict also that photoresponsiveness should be present in
very few strains, perhaps only a single strain or a
single clade of related inbred strains derived from the
same founder population. The second hypothesis is
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Lorincz, Annaka M., M. Benjamin Shoemaker, and
Paul D. Heideman. Genetic variation in photoperiodism
among naturally photoperiodic rat strains. Am J Physiol
Regulatory Integrative Comp Physiol 281: R1817–R1824,
2001.—Rattus norvegicus has been considered nonphotoperiodic, but Fischer 344 (F344) rats are inhibited in growth and
reproductive development by short photoperiod (SD). We
tested photoresponsiveness of the genetically divergent
Brown Norway (BN) strain of rats. Peripubertal males were
tested in long photoperiod or SD, with or without 30% food
reduction. Young males were photoresponsive, with reductions in testis size, body mass, and food intake in SD and with
enhanced responses to SD when food restricted. Photoperiods
ⱕ11 h of light inhibited reproductive maturation and somatic
growth, whereas photoperiods of 12 h or more produced little
or no response. F344/BN hybrids differ from both parent
strains in the timing, amplitude, and critical photoperiod of
photoperiodic responses, indicating genetic differences in
photoperiodism between these strains. This is consistent
with the hypothesis that ancestors of laboratory rats were
genetically variable for photoperiodism and that different
combinations of alleles for photoperiodism have been fixed in
different strains of rats.
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GENETIC VARIATION IN PHOTOPERIODISM
MATERIALS AND METHODS
General. Breeder rats of the strain BN/RijHSD and F344/
NHsd were obtained from Harlan Sprague Dawley (Indianapolis, IN). All rats were held individually in polyethylene
cages (36 ⫻ 24 ⫻ 19 cm) with stainless steel wire tops and
pine shavings for bedding. Food (LM-485 Mouse/Rat Diet
7012; Harlan Teklad, Madison, WI) and tap water were
provided ad libitum. Breeder rats were kept on a 16:8-h
light-dark [long photoperiod (LD)] photoperiod with lights on
at 0500. SD treatment was an 8:16-h light-dark photoperiod
with lights on at 0900. Light intensity was 80–250 lx.
Experiment 1: Test for obligate and facultative photoperiod
responsiveness. This experiment tested the somatic growth
and reproductive maturation of male BN rats in response to
LD or SD photoperiods. In addition, this experiment tested
whether BN rats have the capacity for stronger photoperiod
response when faced with food restriction (FR). Male BN rats
were gestated, born, and raised to weaning at 21 days of age
in LD. At the age of 21 days, rats were caged individually and
assigned to weight-matched treatment groups, with individuals from each litter distributed as evenly as possible among
treatment groups. Rats were placed in one of four treatments: LD with ad libitum food; SD with ad libitum food; LD
with FR; or SD with FR (n ⫽ 10 per treatment). FR rats were
given 30% less food than same age, ad libitum-fed rats in the
same photoperiod. This level of mild FR was chosen because
in a similar experiment on F344 rats, 30% food reduction
induced facultative photoperiodic responses to SD but did not
significantly affect reproduction in LD (14). Temperatures in
the LD and SD rooms were 24 ⫾ 2°C, but the LD room was
consistently ⬃1°C cooler than the SD room.
After 2.5 and 5 wk of treatment, body mass and testis size
were measured. Measurements were taken from each rat on
the day it achieved the target age, as we have found that
measurements from peripubertal rats of slightly different
ages cause unacceptable increases in variability of the data.
For testis measurements, rats were lightly anesthetized with
isoflurane (Ohmeda Caribe, Guayama, Puerto Rico), the skin
covering the left testis was moistened with 70% ethanol, and
testis length and width were measured with dial calipers.
Testes of live mammals are somewhat compressible, which
results in some subjectivity in measurement. To minimize
variability and eliminate potential biases, testes were measured by a single person (P. D. Heideman) blind with respect
to treatment. Testis volume was estimated using the formula
for the volume of a prolate spheroid (width2 ⫻ length ⫻
0.523), which is highly correlated with testis mass of rats in
our hands (15).
At the end of 10 wk of treatment, rats were euthanized by
carbon dioxide gas, body mass was recorded, testes were
excised and weighed, and paired seminal vesicles, including
their fluid contents, were excised and weighed.
Experiment 2: Critical photoperiod of BN rats. After finding significant effects of SD on BN rats, we conducted a
second experiment to test whether the critical photoperiods
for reproductive development and for somatic growth of BN
rats would result in functional photoperiodism for a rat in
natural conditions in the wild. Male BN rats were gestated,
born, and raised in LD until weaning at 21 days of age, at
which time they were caged individually and assigned to
weight-matched treatment groups. Rats were transferred
from LD into one of five photoperiod treatments: 10:14-h
light-dark photoperiod (L10; n ⫽ 6); 11:13-h light-dark photoperiod (L11; n ⫽ 5); 12:12-h light-dark photoperiod (L12;
n ⫽ 6); 13:11-h light-dark photoperiod (L13; n ⫽ 6); or
14:10-h light-dark photoperiod (L14; n ⫽ 5). Rats were held
in fan-ventilated photoperiod chambers with light provided
by two standard 20-W fluorescent light bulbs (F20T12 Cool
White; General Electric) dimmed with neutral density filters
to 100–300 lx as measured 5 cm above the floor. Photoperiod
treatments were terminated after exactly 5 wk because in
experiment 1, this treatment duration produced the maximal
effects of SD on testicular growth in terms of percent difference from LD. At the end of treatment, rats were euthanized
with carbon dioxide gas, body mass was recorded, both testes
were excised and weighed, and paired seminal vesicles, including their fluid contents, were excised and weighed.
Experiment 3: Photoperiodic response of male F344/BN
hybrids. This experiment tested the obligate photoperiodic
response of F1 male F344/BN hybrids. F344/BN hybrid males
produced in our breeding colony by mating male F344 rats
with female BN rats were gestated and raised in LD. At the
age of 21 days, rats were weaned, caged individually, weight
matched, and placed into either LD (n ⫽ 16) or SD (n ⫽ 15).
Testis width and length were measured for each rat at exact
2-wk intervals by one person (M. B. Shoemaker) blind with
respect to treatment. Testis volume was estimated using the
formula for the volume of a prolate spheroid (width2 ⫻
length ⫻ 0.523). Food intake and body mass of all rats were
also measured at precise 2-wk intervals. At week 12, data
were obtained on only eight rats in each group.
Experiment 4: Photoperiodic response of female F344/BN
hybrids. This experiment tested the obligate photoperiodic
response of F1 female F344/BN hybrids. F344/BN hybrid
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that the ancestors of laboratory rats were photoperiodic, but photoresponsiveness has been lost in some
strains of laboratory rats due to artificial selection or
genetic drift during domestication (21). This hypothesis implies that the ancestral founders of different
strains of laboratory rats were variable at multiple loci
for photoresponsiveness, as appears to be typical of
temperate-zone rodent species (4, 13). According to the
latter hypothesis, the process of domestication and
inbreeding might have fixed different combinations of
alleles for photoresponsiveness in different strains of
rats (9). If this hypothesis is correct, then one would
predict that photoresponsiveness might be common
among distantly related strains of rats and that photoresponsive strains would differ in alleles that affect
parameters such as critical photoperiod, specific nonreproductive and reproductive responses to photoperiod, and the amplitude and duration of photoperiodic
responses. Thus this hypothesis predicts complex genetic variation in photoresponsiveness among strains.
The goals of this study were to test these hypotheses
by examining photoresponsiveness in an inbred rat
strain that is genetically distant from most other
strains, identifying the critical photoperiod of this
strain, and performing a test for genetic variation in
photoresponsiveness among strains. We chose the
Brown Norway (BN) inbred strain of rats because phylogenetic studies comparing genetic and biochemical
markers of laboratory rats have consistently characterized BN rats as the most genetically different from
most other strains (1, 5, 7). In addition, a pilot experiment on F344/BN rat female hybrids suggested that
the strains differ in genes for photoresponsiveness and
that the BN strain might be either nonphotoperiodic or
only very weakly photoperiodic.
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GENETIC VARIATION IN PHOTOPERIODISM
RESULTS
Experiment 1: Photoperiodic response of BN rats. SD
caused reproductive inhibition of BN rats from 2.5 wk
of treatment until the end of this experiment. Male BN
rats in SD had significantly smaller testis volume than
rats in LD after 2.5 and 5 wk of treatment (F ⫽ 61.3,
P ⬍ 0.0001 and F ⫽ 111.9, P ⬍ 0.0001, respectively;
Fig. 1A). At 10 wk of treatment, rats in SD had significantly smaller paired testis mass than rats in LD (F ⫽
38.1, P ⬍ 0.0001; Fig. 1B) and significantly smaller
paired seminal vesicle mass than rats in LD (F ⫽ 45.8,
P ⬍ 0.0001; Fig. 1C). BN rats in SD also had lower body
mass than rats in LD (F ⫽ 12.8, P ⬍ 0.001; Fig. 2A) and
Fig. 1. Means ⫾ SE of estimated testis volume (A), paired testis
mass (B), and paired seminal vesicle mass (C) for male Brown
Norway (BN) rats given ad libitum (Adlib) food or 30% food restriction (FR) and held in long photoperiod (LD) or short photoperiod (SD)
from weaning. There was significant inhibition of reproductive
growth by SD and FR for all 3 measures. Only for paired testis mass
at 10 wk was there a significant interaction between SD and FR,
with greater inhibition by the combined treatment than expected by
the addition of independent effects of the 2 treatments (statistical
details are provided in text).
lower food intake than rats in LD (F ⫽ 21.79, P ⬍
0.005; Fig. 2B).
FR caused reproductive inhibition of BN rats from
2.5 wk of treatment until the end of this experiment.
Food-restricted male BN rats had significantly smaller
testis volume than ad libitum-fed rats after 2.5 and 5
wk of treatment (F ⫽ 50.3, P ⬍ 0.0001 and F ⫽ 142.8,
P ⬍ 0.0001, respectively; Fig. 1A). At 10 wk of treatment, FR rats had significantly smaller paired testis
mass (F ⫽ 45.8, P ⬍ 0.0001; Fig. 1B) and paired
seminal vesicle mass than ad libitum-fed rats (F ⫽
11.3, P ⬍ 0.01; Fig. 1C). FR BN rats had lower body
mass than ad libitum-fed rats (F ⫽ 144.0, P ⬍ 0.0001;
Fig. 2A).
During the course of this experiment, there were
significant interactions between photoperiod and FR
treatments for reproductive measures but not for body
mass. After 2.5 wk of treatment, however, there was no
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females were taken from the same litters as in experiment 3.
At the age of 21 days, rats were weaned, caged individually,
weight matched, and placed into either LD or SD (n ⫽ 15 for
each). Beginning at 21 days of age, rats were checked daily
for vaginal opening by swabbing the vaginal area with distilled water followed by visual inspection. Food intake was
measured at precise 2-wk intervals through 8 wk of treatment. Body mass measurements were recorded at weeks 4, 6,
and 8, with one additional measurement at week 12 in an
extension of the experiment to test the persistence of body
mass differences.
Statistical analysis. In this study, negative results were
potentially as valuable as positive results for experiments 1,
3, and 4. Therefore, in these three experiments, sample sizes
were relatively large to provide high statistical power to
detect subtle effects and to provide high confidence in any
negative results. Power analyses were used to estimate sample sizes necessary to provide statistical power (1 ⫺ ␤) of 0.8
to detect 10% differences between group means. In other
words, if there were true differences between group means of
at least 10%, tests using these sample sizes would be expected to result in P ⬍ 0.05 in 80% of such tests. Statistical
tests were carried out using Statview 4.5 (Abacus Concepts,
Berkeley, CA) with significance set at P ⬍ 0.05. Attained
levels of significance are presented to provide maximal information on probabilities.
For experiment 1, data on reproduction and body mass
from each time point were analyzed using two-factor
ANOVA. Data on food intake for each time point were analyzed using t-tests. For experiment 2, all data were analyzed
using one-factor ANOVA, followed by paired comparisons
using the Bonferroni-Dunn method. For these paired comparisons with the Bonferroni-Dunn method, the experimentwise significance level (the probability that any of the multiple pairwise comparisons is falsely significant) was set at P ⬍
0.05, equivalent in this particular experiment to a significance level of P ⬍ 0.005 for each individual paired comparison. Although this method is conservative in the probability
of type I error for the entire set of comparisons, it has a fairly
high probability of type II error. In other words, it is likely
that some pairs of groups that would be considered significantly different if compared by t-test were not identified as
significantly different using this method. This conservative
approach was chosen because we felt it was particularly
important for our hypotheses to avoid a potential risk of
incorrectly concluding that critical photoperiods are different
for the measures of reproductive development or somatic
growth. For experiments 3 and 4, data on testis volume, body
mass, and food intake were analyzed at each time point with
unpaired t-tests. Data on vaginal opening were analyzed
using an unpaired t-test.
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GENETIC VARIATION IN PHOTOPERIODISM
Fig. 2. Means ⫾ SE of body mass (A) and average daily food intake
(B) for male BN rats given ad libitum (AL) food or 30% FR and held
in LD (L) or SD (S) for 10 wk from weaning. For body mass, there was
a significant inhibition by SD and FR but no significant interaction
between treatments (statistical details are provided in text). For food
intake, * indicates significant differences (P ⬍ 0.05).
interaction between photoperiod and food treatment on
testis volume (F ⫽ 0.19, P ⬎ 0.50). By 5 wk of treatment, there was a statistical trend (F ⫽ 3.28, P ⫽
0.079) for an interaction between FR and SD to suppress testis growth below the additive effects of SD and
FR (Fig. 1A). After 10 wk of treatment, the interaction
between FR and SD to further enhance the effects of
SD on testis mass was statistically significant (F ⫽
4.48, P ⬍ 0.05; Fig. 1B). This interaction was not
significant for seminal vesicle mass (F ⫽ 2.56, P ⫽
0.11). The combined effect of SD and FR on body mass
was purely additive, with no significant interaction
(F ⫽ 0.01, P ⬎ 0.90; Fig. 2A).
Experiment 2: Critical photoperiod. BN rats treated
with different photoperiods for 5 wk differed significantly in testis mass (F ⫽ 9.86, P ⬍ 0.0001; Fig. 3A),
seminal vesicle mass (F ⫽ 9.11, P ⬍ 0.002; Fig. 3B),
and body mass (F ⫽ 4.18, P ⫽ 0.01; Fig. 3C). After 5 wk
of treatment, BN rats in L11 were reproductively inhibited and showed significantly smaller paired testis
mass compared with rats in all longer photoperiod
treatments (P ⬍ 0.005 for all; Fig. 3A) and significantly
smaller paired seminal vesicle mass compared with
rats in L13 and L14 (P ⬍ 0.05; Fig. 3B). At the final
week of treatment, BN rats in L11 weighed significantly less than rats in L14 (P ⬍ 0.005; Fig. 3C).
Experiment 3: Photoperiodic response of male
F344/BN hybrids. SD caused reproductive inhibition of
F344/BN F1 rats from 2 wk of treatment until the end
of this experiment (Fig. 4A). Male F344/BN F1 rats in
Fig. 3. Means ⫾ SE of paired testis mass (A), paired seminal vesicle
mass (B), and body mass (C) for male BN rats held in a range of
natural photoperiods for 5 wk from weaning. a, b, c, And d indicate
significant differences (P ⬍ 0.05). L10, 10:14-h light-dark photoperiod; L11, 11:13-h light-dark photoperiod; L12, 12:12-h light-dark
photoperiod; L13, 13:11-h light-dark photoperiod; and L14, 14:10-h
light-dark photoperiod.
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SD had significantly smaller testis volume after 2, 4, 6,
8, and 10 wk of treatment (P ⬍ 0.005 for all), but testis
volume in the two groups did not differ after 12 wk of
treatment (P ⫽ 0.53). Male F344/BN F1 rats in SD and
LD were similar in body mass after 2 wk of treatment
(P ⫽ 0.11), but rats in SD had lower body mass than
rats in LD after 4, 6, 8, 10, and 12 wk of treatment
(P ⱕ 0.01 for all; Fig. 4B). Male F344/BN F1 rats in SD
and LD had similar food intake during the first 2 wk of
treatment (P ⫽ 0.81), but rats in SD had lower food
intake than rats in LD at 4, 6, 8, and 10 wk of treatment (P ⱕ 0.05 for all; Fig. 4C). During the last 2 wk of
treatment, the two groups did not differ in food intake
(P ⫽ 0.48).
Experiment 4: Photoperiodic response of female BN/
F344 hybrids. Female F344/BN F1 rats in LD and SD
underwent vaginal opening at the same age (34.07 and
34.13 days, respectively; t ⫽ 0.07, P ⫽ 0.93; Fig. 5A).
However, SD inhibited somatic growth of female hybrids (Fig. 5B). There was no difference in body mass
after 4 wk of treatment (t ⫽ 1.49, P ⫽ 0.15), but body
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GENETIC VARIATION IN PHOTOPERIODISM
Fig. 4. Means ⫾ SE of estimated testis volume (A), body mass (B),
and daily food intake (C) for male Fischer 344 (F344)/BN hybrid rats
held in LD or SD for 12 wk from weaning. *Significant differences
(P ⬍ 0.05) for particular weeks.
mass was significantly lower in SD after 6, 8, and 12
wk of treatment (t ⫽ 3.08, P ⬍ 0.005; t ⫽ 3.54, P ⬍
0.005; and t ⫽ 4.81, P ⬍ 0.0001, respectively) (Fig. 5B).
SD had not affected food intake significantly after 2
and 4 wk of treatment (t ⫽ 0.26, P ⫽ 0.82 and t ⫽ 1.12,
P ⫽ 0.27, respectively), but food intake was significantly lower in SD at 6 and 8 wk of treatment (t ⫽ 2.16,
P ⬍ 0.05 and t ⫽ 2.26, P ⬍ 0.05, respectively) (Fig. 5C).
DISCUSSION
Our results show that BN rats are photoresponsive.
Young males repressed reproductive development, somatic growth, and food intake in response to SD alone.
The amount of suppression of reproduction and somatic growth in BN rats by SD was similar to that seen
in F344 rats. BN rats in SD for 5 wk had testes that
weighed 40% less than those of LD controls (Fig. 3A),
within the range of the 30–60% decrease reported for
F344 rats in several experiments (9, 14, 15). In F344
rats, a slower rate of testicular growth in SD corresponded to slowed, but not halted, spermatogenesis
(15). In BN rats, seminal vesicle mass was much lower
in SD than in LD after 10 wk of treatment (Fig. 1C).
Fig. 5. Means ⫾ SE for age at vaginal opening (A), body mass (B),
and daily food intake (C) for female F344/BN hybrid rats held in LD
or SD from weaning. *Significant differences (P ⬍ 0.05) for particular
weeks.
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This suggests that androgen levels were still low and
fertility would be impaired in SD-treated BN rats, even
though testis size was no longer depressed (Fig. 1B).
BN rats in SD were 20% lower in body mass, and
decreased food intake by 15%; responses very similar
to those of F344 rats (9, 14, 15). The duration of
photoperiodic responses of young male BN and F344
rats to SD was very similar (Figs. 1 and 2) (14, 15).
As in F344 rats (14), the inhibition of reproduction by
SD was not due simply to smaller body size. The much
smaller body size of FR BN rats in LD did not reduce
testis mass or seminal vesicle mass proportionately.
After 2.5 and 5 wk of treatment, food-restricted BN
rats in LD weighed much less than ad libitum-fed rats
in SD (Fig. 2A), but they had testes similar in size (Fig.
1, A and B).
Unlike F344 rats, BN rats apparently possess only a
moderate capacity for enhanced facultative photoperiod response due to mild FR. Although a statistical
trend showing an interaction between FR and SD was
present after 5 wk of treatment (P ⫽ 0.079; Fig. 1),
statistically significant interactions between FR and
SD to further repress reproduction were not evident
until 10 wk of treatment, and even then the affect was
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GENETIC VARIATION IN PHOTOPERIODISM
founders. However, the data available thus far are not
sufficient to firmly reject either hypothesis. Even so,
our results suggest that a functional photoperiod pathway may be present in other inbred strains of rats as
well. Thus photoperiod should be given more attention
in studies that measure reproductive function or feeding and body mass in rats. For example, in recent
publications on the BN rat strain to test testicular
regression during aging (17, 28), photoperiod was not
specified.
Our findings that F344 and BN rats respond differently to FR in SD, that their critical photoperiods are
slightly different, and that the offspring of crosses
between F344 and BN rats differ in photoresponsiveness from parent strains indicate that the two parent
strains differ in the alleles and/or loci of genes that
affect photoresponsiveness. In experiment 3, testis
growth of the first generation male BN/F344 hybrids
was less inhibited by SD (15% smaller testes in SD
than LD) than either parent strain (30–60% smaller
testes in SD than LD). However, inhibition of body
mass and food intake were similar to both parent
strains. Female hybrids were inhibited in food intake
and body mass, but they did not delay vaginal opening
in SD. In contrast, peripubertal F344 females showed a
1- to 2-wk delay in vaginal opening when blinded (19).
If the two strains are fixed for the same alleles at all
loci that affect photoresponsiveness, then these F1 hybrids should have the same photoperiodic responses,
with similar amplitude and timing of response, as the
two parent strains. The differences between F1 hybrids
and the parent strains provide evidence that BN and
F344 rats differ from each other in the alleles for at
least one locus that can modify photoresponsiveness.
In addition, these genetic differences must be sufficient
to cause differences in the critical photoperiod between
the strains. It is possible that all of these life history
traits are controlled by different loci and therefore may
be subject to independent selection in rats and other
species of rodents.
Within-species genetic variation in photoresponsiveness has been found in every test conducted on rodents
from natural populations in the temperate zone (6, 11, 13,
18, 20, 27, 29, 33). The differences in genetic basis for
photoresponsiveness between BN rats (this study) and
F344 rats (9, 14, 15), along with evidence of weaker
photoresponsiveness in other strains, especially when
induced by the seminatural treatment of FR (26), indicate that there is substantial genetic variation for photoresponsiveness among strains of laboratory rats. This
variability is consistent with the hypothesis that rats
ancestral to domesticated strains were also variable in
their responses to photoperiod and that different combinations of alleles for photoresponsiveness have been fixed
in different strains (9). The alternative, that mutations
for photoresponsiveness arose spontaneously and independently during or after domestication in these two
different strains of rats, appears less likely.
We are not aware of any inbred strains of rats that
have been tested thoroughly for photoresponsiveness
and found to have no response to photoperiod. In fact,
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slight. In F344 rats, in contrast, the combination of SD
and FR at 70% of ad libitum intake has a rapid and
strong effect, further enhancing reproductive inhibition starting at 1 wk of treatment and continuing
through 13 wk of treatment (14).
Here we use the term “critical photoperiod” for photoperiods below which inhibitory effects occur, but it is
important to state two caveats. First, the critical photoperiod concept implies a narrow photoperiod range
above which photoperiod treatment is not inhibitory,
and below which there is maximal inhibition, a situation which may not hold for BN rats, especially in the
effects of SD on somatic growth (Fig. 3C). Second, the
concept of a single “critical photoperiod” is only meaningful in the context of some specific treatment or class
of treatments, because in photoresponsive rodents, inhibitory responses to particular photoperiods can vary
with light intensity, the direction of photoperiod
change (16), and gradual or abrupt changes in photoperiod (8, 9, 12).
In BN rats, inhibitory photoperiods for reproductive
effects were ⬃11 or fewer hours of light. Rats in L11 for
5 wk had significantly smaller testis mass and seminal
vesicle mass than those of any longer photoperiod
treatment. The L11 and L10 groups had reproductive
responses similar to that of BN rats in SD in experiment 1. This indicates that, under these conditions, the
critical photoperiod for reproduction is less than 12 h of
light. In contrast, the critical photoperiod for reproductive effects in young male F344 rats in similar conditions is 13.5 h of light (9), and the critical photoperiod
of adult male Harlan Sprague-Dawley rats with photoresponsiveness induced by androgen treatment is 9 h
of light (30). Our data indicate that the critical photoperiod for somatic growth in BN rats is between 11 and
14 h of light (Fig. 3C), but the data do not allow us to
resolve the critical photoperiod more precisely. In fact,
the data suggest the possibility that shorter photoperiods may have progressively stronger effects over that
range of photoperiods. If so, then for somatic growth in
BN rats, there may not be a critical photoperiod above
which there are no effects, but below which there are
complete effects.
The BN inbred strain of rats is the second reported to
have both robust responses to SD alone and to have
critical day lengths that would make the trait functional in nature. In published estimates of genetic
relatedness among inbred rat strains, F344 rats have
been positioned genetically within a large group of
fairly closely related strains. In contrast, BN rats have
been found to be the most genetically divergent from
other strains, consistent with the derivation of the BN
strain from an independent domestication of rats (1, 5,
7). The finding of photoresponsiveness in unmanipulated individuals of these two genetically distant rat
strains might be due to independent mutations uncovering a vestigial photoperiod pathway in these two
strains. We feel that it is more likely that the founders
of these and other laboratory rat strains were variable
for photoresponsiveness and that some strains inherited a functional photoperiodic pathway from these
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GENETIC VARIATION IN PHOTOPERIODISM
we began this study with the hope of identifying a
nonphotoperiodic inbred strain that we could use as a
comparison with photoperiodic F344 rats in studies on
variability in the photoperiod pathway. Most studies
on photoperiodic responses in rats have used outbred
strains, especially Wistar or Sprague-Dawley, two
strains that have very weak responses to photoperiod
and in which photoperiodism is usually considered a
vestigial trait (22, 31). This genetic variation for the
presence or absence of photoperiodic responses, as well
as variation in critical day length and other parameters of photoperiodism, indicates that the photoperiod
pathway in rats should be a useful model for the study
of within-species variation in the brain.
One major hypothesis in evolutionary physiology is
that complex physiological systems have been optimized by the process of evolution by natural selection.
The competing hypothesis is that in physiological systems controlled by complex interactions among multiple genes, considerable genetic variation will persist
because selection on many of the alleles affecting a
complex trait is too weak to eliminate all alleles contributing to nonoptimal phenotypes (see papers collected in Ref. 32). The mammalian genome projects are
currently initiating major studies on individual variation at the level of gene sequences in a “bottom-up”
approach (gene to phenotype). However, complexity
theory suggests that, particularly for phenotypes with
complex genetic bases for variation, bottom-up approaches are likely to fail to find many important
sources of variability because individual genes have
undetectably small effects on the phenotype. Thus topdown approaches will be a necessary part of this
emerging field. Inbred strains of rats and mice can be a
useful tool in top-down studies of normal, nonpathological within-species variation. Because individuals
within a particular inbred strain are nearly genetically
identical, each inbred strain can be treated as a genetic
individual, providing the power of replication in studies of variation. In the current study, we found evidence for genetic variation in the regulation of photoresponsiveness, consistent with the second hypothesis
above. We speculate that variation may be common in
complex physiological pathways in wild mammals and
retained in domesticated mammals. Persistent variation in complex physiological pathways may explain
why artificial selection is so effective during domestication.
We thank M. Rightler, C. Finneran, and A. N. Meyers for assistance with animal care and data collection.
Work was supported by National Institutes of Health Grant R15
DK-51334 to P. D. Heideman, by an Arnold and Mabel Beckman
Foundation 2000 Beckman Scholar Program grant to A. M. Lorincz,
and by a Minor Research grant to M. B. Shoemaker from a Howard
Hughes Medical Institute Undergraduate Biological Sciences Education Program grant to the College of William and Mary.
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