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Nervous system function: Maternal effects on conduction velocity in mice
BEHAVIORALBIOLOGY,8, 815-818 (1973), Abstract No. 277R
N e r v o u s S y s t e m F u n c t i o n : Maternal Effects on C o n d u c t i o n V e l o c i t y in Mice
JOSEPH P. HEGMANN 1 and JOSEPH E. WHITE
Zoology Department, University of lowa lowa City, Iowa 52240
Backcross mice with inbred mothers display lower peripheral nerve conduction velocity than age-matched offspring of hybrid mothers. Also, reciprocal hybrid offspring groups show age-stable differences in conduction velocity. These maternal effects are not removed with cross fostering and may be associated with modified developmental rates.
The ubiquity with which genetic variance influences individual differences in behavior (McCleam, 1966) and the immediacy of nervous system involvement in behavior make determination of gene-imposed alterations in nervous system function a promising approach to integrated studies of behavior and physiology (Benzer, 1967). A recent detailed genetic analysis of caudal nerve conduction velocity in mice (Hegmann, 1972) demonstrated the presence of genetic variance and heterosis for the character and suggested modification of this nervous system function by differences in maternal environment. Because of important implications of maternal effects on nervous system function for research in psychology, neurobiology, animal behavior and behavioral genetics, we have examined this apparent maternal effect in detail and found that it is substantial, long lasting, and dependent on prenatal environmental factors (broadly interpreted). Caudal nerve conduction velocity was assessed (Hegmann, 1972) in anesthetized mice by piercing their tails with three pairs of stainless steel electrodes mounted in Plexiglas. A supramaximal stimulating pulse (.025 msec in duration) was applied to the electrode pair nearest the base of the tail and electrical activity was monitored at the sites of penetration by the other two electrode pairs using the dual traces of a storage oscilloscope. Compound action potentials were amplified and stored at a sweep speed of 0.5 msec per cm. The distance between the recording electrodes (15 mm) and the time between compound action potential peaks at the two recording points provided conduction velocity estimates. Mice were reared in stainless-steel 1Supported by NINDS Grant NS09536. 815 Copyright Q 1973 by Academic Press, Inc. All rights of reproduction in any form reserved.
816
HEGMANNAND WHITE
cages in 16:8 light-dark photoperiod at temperatures of 23 +- 2°C with food and water available ad lib. Body weight and tail length measurements were obtained for all animals tested. Backcross generation offspring (N = 201) from an initial cross of the C3H/HeJ (C) and DBA/1J (D) inbred strains were tested at 4 5 - 6 days (range). Within each backcross four mating groups were established and maintained contemporaneously: two groups with inbred female parents (C or D) and males of the two reciprocal cross hybrids (CD or DC where leading symbol represents genotype of female parent) and two groups with reciprocal cross hybrid female, and inbred male parents. In Table 1 we present mean conduction velocities for female and male offspring of the eight groups. TABLE 1 Reciprocal Backcross Group Mean Conduction Velocities (m/sec +-SE; subclass numbers are indicated in parentheses) Backcross mating
Female offspring
Male offspring
C9 X CDdd CQ X DCd
13.9 _+0.2 (6) 15.3 -+ 0.6 (9)
14.3 +- 0.3 (13) 14.7 -+ 0.4 (7)
Cd X CD9 Cd X DC~
16.1 +_0.4 (19) 17.0 -+ 0.2 (15)
16.4 +- 0.3 (17) 17.2 -+ 0.4 (12)
D9 X CDd D9 X DCd
15.3 -+ 0.6 (4) 15.5 +_0.4 (9)
14.4 +-0.7 (4) 15.8 -+ 0.4 (19)
Dd X CD9 Dd X DC9
17.0 -+ 0.2 (16) 17.0 -+ 0.5 (17)
16.7 -+ 0.4 (11) 16.3 -+ 0.3 (23)
dc and D symbolize the C3H/HeJ and DBA/1J parental genotypes. For hybrids, genotype of female parent is symbolized first, followed by symbol for male parent genotype. Conduction velocity by the D strain exceeds that of age-matched C animals (Hegrnann, 1972) so that the pattern of differences displayed by backcrosses with inbred female parents and those with inbred male parents is expected. The substantial difference between backcross groups with hybrid and inbred female parents (an average difference of 1.8-+ .2 m/sec, calculated by subtracting corresponding subclass means) is not expected. This represents a maternal effect of more than 10% of the maximum conduction velocity displayed by these groups. In addition to these hybrid-inbred maternal differences we have observed maternal effects associated with differences between inbred mothers. Mean conduction velocities for female and male offspring of reciprocal hybrid mating groups are displayed in Table 2 for independent samples assessed at different ages. Age, environmental influence, and test day were unconfounded
MATERNAL EFFECTS ON CONDUCTION VELOCITY
817
TABLE 2 Reciprocal Cross Hybrid Group Mean Conduction Velocities (m/sec -+ SE, with N's in parentheses) for Independent Samples at Three Test Ages Genotype of parents 9
¢~
c
D
Sex of offspring
?
? D
C c~
Age at testing 50 days
100 days
150 days
20.5 __. 1.1 (6) 20.3 _+0.4 (10) 16.9 _+0.4 (5) 17.9 + 0.5
26.0 _+0.9 (5) 24.0 +_0.4 (5) 22.7 +- 1.2 (5) 22.4 _+ 1.1
26.0 _+0.8 (5) 24.5 _+ 1.8 (5) 24.3 _+ 0.6 (7) 23.1 +- 0.3
(5)
(5)
(6)
to the extent possible by staggering matings, rearing animals similarly, and including samples from all age groups on any specific test day. Females in these offspring groups are genetically identical and males differ only with regard to the source of sex chromosomes. The obvious differences in conduction velocity with age represent developmental changes which are not unexpected (Birren and Wall, 1956). The pattern of differences in conduction velocity displayed by animals at each age gives no suggestion that the character is influenced by sex-linked loci (a situation requiring differences between reciprocal groups shown only by males) but clearly indicates a maternal effect on nervous system function which persists through 150 days of age. While the effect does not tend to increase offspring resemblance to their maternal parent (in this sense, it is a negative maternal effect) it is associated with maternal phenotype and makes offspring of the same mother look alike with regard to conduction velocity. Mediation of maternal effects may be either prenatal or postnatal by differences in nutrition and transmitted pathogens and antibodies (prenatal blood or postnatal milk supply) or by cytoplasmicaUy inherited differences (prenatal), learning, or common environmental influences (postnatal). We performed a cross-fostering experiment to determine whether the maternal influence on conduction velocity is associated with pre- or postnatal differences. Offspring in the experiment were transferred at birth from their natural mother (C or DC genotype) to a mother of the same or different genotype. Male parents of offspring of C mothers were DC hybrids and males mated to DC females were C inbreds so that similar genotype distributions are expected for offspring. Differences in expectation for sex chromosomes should be irrelevant in light of previous results. Conduction velocities assessed at 50 days
818
HEGMANN AND WHITE
of age were considered the result of a two-squared factorial arrangement of treatmenls (inbred vs hybrid prenatal female parent and inbred vs hybrid postnatal female parent). Orthogonal contrasts were employed to estimate least- squares constants for main effects and their interaction (which represents a contrast of animals transferred within female genotypes to those placed with a mother of different genotype). Mean conduction velocity for animals with C prenatal mothers was 15.9 m/sec and that of offspring of DC mothers was 16.7 m/sec (t = 3.7, d f = 62). In contrast to these differences, offspring reared by C mothers and those reared by DC mothers averaged 16.4 and 16.1 m/sec, respectively (t = 1.4, d f = 62). Thus, we conclude that the maternal effect observed is of prenatal origin. Additional research is required to localize the source of this prenatal effect and to determine the mode of its action on conduction velocity. However, three observations of importance regarding the nature of the influence are available. First, animals from the backcross groups with inbred female parents (and low conduction velocity) display consistently lower body weights and shorter tail lengths than offspring with hybrid mothers. Second, this difference in characters known to index developmental level exists in spite of the fact that average litter sizes (and hence, postnatal competition levels) for offspring with inbred mothers are lower than those of hybrid mothers. The same differences are observed in comparing litter sizes for D and C mothers. Finally, examination of Table 2 indicates that differences between individuals with identical genotypes (hybrids) but different inbred female parents, though persisting through 150 days of age, are decreasing in magnitude as a result of greater increase in conduction velocity with age for individuals with D mothers. A similar pattern of decreasing differences with age through greater increase by offspring with D mothers is displayed by the morphological measures from these animals. These observations suggest that the presence of maternal influence on conduction velocity may reflect general effects of uterine environmental differences on developmental rate.
REFERENCES Benzer, Seymour (1967). Behavioral mutants of Drosophila isolated by countercurrent distribution. Proc. Nat. Acad. Sci. U.S.A. 58, 1112. Bitten, J. E. and Wall, P. D. (1956). Age changes in conduction velocity, refractory period, number of fibers, connecting tissue space and blood vessels in sciatic nerve of rats. J. Comp. Neurol. 104, 1-16. Hegmann, J. P. (1972). Physiological function and behavioral genetics. I. Genetic variance for peripheral conduction velocity in mice. Behav. Genet. 2, 55-67. McClearn, G. E. and Meredith, W. (1966). Behavioral genetics. Ann. Rev. Psychol. 17, 515-550.
N e r v o u s S y s t e m F u n c t i o n : Maternal Effects on C o n d u c t i o n V e l o c i t y in Mice
JOSEPH P. HEGMANN 1 and JOSEPH E. WHITE
Zoology Department, University of lowa lowa City, Iowa 52240
Backcross mice with inbred mothers display lower peripheral nerve conduction velocity than age-matched offspring of hybrid mothers. Also, reciprocal hybrid offspring groups show age-stable differences in conduction velocity. These maternal effects are not removed with cross fostering and may be associated with modified developmental rates.
The ubiquity with which genetic variance influences individual differences in behavior (McCleam, 1966) and the immediacy of nervous system involvement in behavior make determination of gene-imposed alterations in nervous system function a promising approach to integrated studies of behavior and physiology (Benzer, 1967). A recent detailed genetic analysis of caudal nerve conduction velocity in mice (Hegmann, 1972) demonstrated the presence of genetic variance and heterosis for the character and suggested modification of this nervous system function by differences in maternal environment. Because of important implications of maternal effects on nervous system function for research in psychology, neurobiology, animal behavior and behavioral genetics, we have examined this apparent maternal effect in detail and found that it is substantial, long lasting, and dependent on prenatal environmental factors (broadly interpreted). Caudal nerve conduction velocity was assessed (Hegmann, 1972) in anesthetized mice by piercing their tails with three pairs of stainless steel electrodes mounted in Plexiglas. A supramaximal stimulating pulse (.025 msec in duration) was applied to the electrode pair nearest the base of the tail and electrical activity was monitored at the sites of penetration by the other two electrode pairs using the dual traces of a storage oscilloscope. Compound action potentials were amplified and stored at a sweep speed of 0.5 msec per cm. The distance between the recording electrodes (15 mm) and the time between compound action potential peaks at the two recording points provided conduction velocity estimates. Mice were reared in stainless-steel 1Supported by NINDS Grant NS09536. 815 Copyright Q 1973 by Academic Press, Inc. All rights of reproduction in any form reserved.
816
HEGMANNAND WHITE
cages in 16:8 light-dark photoperiod at temperatures of 23 +- 2°C with food and water available ad lib. Body weight and tail length measurements were obtained for all animals tested. Backcross generation offspring (N = 201) from an initial cross of the C3H/HeJ (C) and DBA/1J (D) inbred strains were tested at 4 5 - 6 days (range). Within each backcross four mating groups were established and maintained contemporaneously: two groups with inbred female parents (C or D) and males of the two reciprocal cross hybrids (CD or DC where leading symbol represents genotype of female parent) and two groups with reciprocal cross hybrid female, and inbred male parents. In Table 1 we present mean conduction velocities for female and male offspring of the eight groups. TABLE 1 Reciprocal Backcross Group Mean Conduction Velocities (m/sec +-SE; subclass numbers are indicated in parentheses) Backcross mating
Female offspring
Male offspring
C9 X CDdd CQ X DCd
13.9 _+0.2 (6) 15.3 -+ 0.6 (9)
14.3 +- 0.3 (13) 14.7 -+ 0.4 (7)
Cd X CD9 Cd X DC~
16.1 +_0.4 (19) 17.0 -+ 0.2 (15)
16.4 +- 0.3 (17) 17.2 -+ 0.4 (12)
D9 X CDd D9 X DCd
15.3 -+ 0.6 (4) 15.5 +_0.4 (9)
14.4 +-0.7 (4) 15.8 -+ 0.4 (19)
Dd X CD9 Dd X DC9
17.0 -+ 0.2 (16) 17.0 -+ 0.5 (17)
16.7 -+ 0.4 (11) 16.3 -+ 0.3 (23)
dc and D symbolize the C3H/HeJ and DBA/1J parental genotypes. For hybrids, genotype of female parent is symbolized first, followed by symbol for male parent genotype. Conduction velocity by the D strain exceeds that of age-matched C animals (Hegrnann, 1972) so that the pattern of differences displayed by backcrosses with inbred female parents and those with inbred male parents is expected. The substantial difference between backcross groups with hybrid and inbred female parents (an average difference of 1.8-+ .2 m/sec, calculated by subtracting corresponding subclass means) is not expected. This represents a maternal effect of more than 10% of the maximum conduction velocity displayed by these groups. In addition to these hybrid-inbred maternal differences we have observed maternal effects associated with differences between inbred mothers. Mean conduction velocities for female and male offspring of reciprocal hybrid mating groups are displayed in Table 2 for independent samples assessed at different ages. Age, environmental influence, and test day were unconfounded
MATERNAL EFFECTS ON CONDUCTION VELOCITY
817
TABLE 2 Reciprocal Cross Hybrid Group Mean Conduction Velocities (m/sec -+ SE, with N's in parentheses) for Independent Samples at Three Test Ages Genotype of parents 9
¢~
c
D
Sex of offspring
?
? D
C c~
Age at testing 50 days
100 days
150 days
20.5 __. 1.1 (6) 20.3 _+0.4 (10) 16.9 _+0.4 (5) 17.9 + 0.5
26.0 _+0.9 (5) 24.0 +_0.4 (5) 22.7 +- 1.2 (5) 22.4 _+ 1.1
26.0 _+0.8 (5) 24.5 _+ 1.8 (5) 24.3 _+ 0.6 (7) 23.1 +- 0.3
(5)
(5)
(6)
to the extent possible by staggering matings, rearing animals similarly, and including samples from all age groups on any specific test day. Females in these offspring groups are genetically identical and males differ only with regard to the source of sex chromosomes. The obvious differences in conduction velocity with age represent developmental changes which are not unexpected (Birren and Wall, 1956). The pattern of differences in conduction velocity displayed by animals at each age gives no suggestion that the character is influenced by sex-linked loci (a situation requiring differences between reciprocal groups shown only by males) but clearly indicates a maternal effect on nervous system function which persists through 150 days of age. While the effect does not tend to increase offspring resemblance to their maternal parent (in this sense, it is a negative maternal effect) it is associated with maternal phenotype and makes offspring of the same mother look alike with regard to conduction velocity. Mediation of maternal effects may be either prenatal or postnatal by differences in nutrition and transmitted pathogens and antibodies (prenatal blood or postnatal milk supply) or by cytoplasmicaUy inherited differences (prenatal), learning, or common environmental influences (postnatal). We performed a cross-fostering experiment to determine whether the maternal influence on conduction velocity is associated with pre- or postnatal differences. Offspring in the experiment were transferred at birth from their natural mother (C or DC genotype) to a mother of the same or different genotype. Male parents of offspring of C mothers were DC hybrids and males mated to DC females were C inbreds so that similar genotype distributions are expected for offspring. Differences in expectation for sex chromosomes should be irrelevant in light of previous results. Conduction velocities assessed at 50 days
818
HEGMANN AND WHITE
of age were considered the result of a two-squared factorial arrangement of treatmenls (inbred vs hybrid prenatal female parent and inbred vs hybrid postnatal female parent). Orthogonal contrasts were employed to estimate least- squares constants for main effects and their interaction (which represents a contrast of animals transferred within female genotypes to those placed with a mother of different genotype). Mean conduction velocity for animals with C prenatal mothers was 15.9 m/sec and that of offspring of DC mothers was 16.7 m/sec (t = 3.7, d f = 62). In contrast to these differences, offspring reared by C mothers and those reared by DC mothers averaged 16.4 and 16.1 m/sec, respectively (t = 1.4, d f = 62). Thus, we conclude that the maternal effect observed is of prenatal origin. Additional research is required to localize the source of this prenatal effect and to determine the mode of its action on conduction velocity. However, three observations of importance regarding the nature of the influence are available. First, animals from the backcross groups with inbred female parents (and low conduction velocity) display consistently lower body weights and shorter tail lengths than offspring with hybrid mothers. Second, this difference in characters known to index developmental level exists in spite of the fact that average litter sizes (and hence, postnatal competition levels) for offspring with inbred mothers are lower than those of hybrid mothers. The same differences are observed in comparing litter sizes for D and C mothers. Finally, examination of Table 2 indicates that differences between individuals with identical genotypes (hybrids) but different inbred female parents, though persisting through 150 days of age, are decreasing in magnitude as a result of greater increase in conduction velocity with age for individuals with D mothers. A similar pattern of decreasing differences with age through greater increase by offspring with D mothers is displayed by the morphological measures from these animals. These observations suggest that the presence of maternal influence on conduction velocity may reflect general effects of uterine environmental differences on developmental rate.
REFERENCES Benzer, Seymour (1967). Behavioral mutants of Drosophila isolated by countercurrent distribution. Proc. Nat. Acad. Sci. U.S.A. 58, 1112. Bitten, J. E. and Wall, P. D. (1956). Age changes in conduction velocity, refractory period, number of fibers, connecting tissue space and blood vessels in sciatic nerve of rats. J. Comp. Neurol. 104, 1-16. Hegmann, J. P. (1972). Physiological function and behavioral genetics. I. Genetic variance for peripheral conduction velocity in mice. Behav. Genet. 2, 55-67. McClearn, G. E. and Meredith, W. (1966). Behavioral genetics. Ann. Rev. Psychol. 17, 515-550.