Genetic influences on the timing of puberty in mice

Genetic influences on the timing of three pubertal events--vaginal opening, first vaginal cornification, and onset of cyclicity--were studied in C57BL/6J, DBA/2J, and C3H/HeJ mice and in two F1 hybrid strains (B6D2F1 and B6C3HF1). Marked genotypic differences were found. Among inbred strains, differences in the onset of vaginal opening and first vaginal cornification (C3H less than DBA less than C57) did not parallel those for the onset of cyclicity (C3H much greater than DBA = C57). Compared to parental strains, F1 hybrid strains were intermediate for times of vaginal opening and first vaginal cornification, consistent with the model in which the genetic effects on the timing of these events are additive. By contrast, onset of cyclicity occurred significantly earlier in the F1 hybrids than in their parent strains, indicating heterosis for one or more genes specifying this event. Body weights also differed among the genotypes from weaning onward, but these differences were only partially correlated with the differences in the timing of the pubertal events. Thus, genetic influences other than those affecting body weight contribute to the differential timing of pubertal events in these mouse strains. These results reveal marked genetic variation in the timing of puberty, and indicate that the set of genes specifying the timing of vaginal opening and first vaginal cornification differs from those specifying the onset of cyclicity.     

A reproductive study of Weiser-Maples guinea pigs]

The Weiser-Maples (WE) guinea pig strain was introduced by Backshire Co., Ltd. (USA) in 1977. We have been breeding WE strain guinea pigs for skin melanization research. The WE guinea pig colony produced 1271 pups in 417 litters from May 1978 through December 1983. Breeding date are shown below. The mean litter size was 3.05, the stillborn rate was 15.2%, the weaning rate for live-born pups was 93.5% and the sex ratio was 1.01. The average age at first vaginal membrane rupture was 31.4 days at which time body weight was 290.5g. The mean length of the first 7 estrous cycles was about 17 days, with no cyclical variation in length. The mean duration of gestation was 67.9 days. Duration of pregnancy varied with litter size. There was an inverse relationship between litter size and duration of pregnancy. Most of the pups were delivered alive in mid-pregnancy with a parturition range of 56 to 76 days. The probability of pup death depends on gestational length: the lowest incidence of mortality was seen in litters born at 70 days. The mortalities were related to litter size but not to parity. There was an inverse relationship between birth weight and litter size. In WE guinea pigs, the mean weight for a litter of 1 was 120 g; for a litter of 5, the mean body weight was 58g. Male body weights were slightly heavier than female at birth and at weaning age. The mean body weights are shown below, date of birth: female 88.3g, male 93.3g, weaning age (2 weeks): female 181.1g, male 198.8g and 30 weeks: female 758.7g, male 1018.0g. These date for WE guinea pigs are comparable to those of other strains.     

Factors affecting weaning weight in Antarctic fur seals Arctocephalus gazella at South Georgia

Summary Relationships between size, body condition, age and feeding-attendance patterns during pup rearing of female Antacrtic fur seals Arctocephalus gazella and their effects on the timing of birth and weaning, pup weight, growth and condition were studied at South Georgia in 1981–1982. Twenty-seven (6 male, 21 female) mother-pup pairs were followed from birth to weaning. The analysis of maternal effects was limited to female pups because of the small sample size of male weaners. High weaning weight was associated with those female pups whose mothers spent more time ashore attending their offspring. Weaning weight showed no relationship with perinatal duration, number of feeding trips to sea, days at sea or date of weaning. A further 63 mother-pup pairs were analysed for the effects of maternal body condition (weight/length), age and timing of birth on offspring body weight and condition. Pup weight and condition were weakly correlated with maternal age in female pups. Male pups born earlier in the season were heavier and in better condition. Maternal and offspring body weight and condition were unrelated. For the Antarctic fur seal population at South Georgia where the food supply was apparently not limiting in summer, maternal condition and foraging time were subordinate to maternal care on land (as expressed by attendance duration) in determining offspring weight at weaning.     

Genetic aspects of experimental tuberculosis in mice

Susceptibility of inbred mouse strains, their F1 hybrids, offsprings from BC1 and RI strains from CXB pannel to tuberculosis infection was studied. Mice were infected with virulent strain of M. tuberculosis H37Rv. All F1 hybrids, with the exception of (BRSUNT X I/St)F1, showed superresistance, due apparently to heterosis effect; (BRSUNT X I/St)F1 demonstrated an intermediate level of resistance, as compared with parental strains. Analysis of [(BALB/c X B6)F1 X B6]BC1, [(BALB/c X B6)F1 X BALB/c]BC1 offsprings and RI strains indicated that strains BALB/c and B6 have different alleles of at least two genetic loci mediating susceptibility to tuberculosis.     

Milk ejection in mice LG/J x SM/J

In mammals, milk provision is crucial to offspring survival and growth from birth to weaning. Milk deficiency early in life may cause death or changes in the progeny metabolism that later may lead to obesity and metabolic disorders. This study investigates milk ejection (ME) the first day after birth (D1) in F₂ females from the intercross of LG/J and SM/J inbred mice strains. The absence of milk in F₃ pups?? stomach at D1 is directly associated with their survival (p?<?0.001) and growth pattern (p?<?0.001) in the early stages of life. Furthermore, late growth pattern is also affected by this lack of nutrients at D1 because pups that survive this absence, mostly males, are heavier at weaning (p?<?0.001) which, after necropsy, is shown to be due to significant higher total fat deposition (p?<?0.01). We performed QTL analysis for ME at D1 in these F₂ females. Maternal performance of ME revealed a complex genetic architecture which even though it contains only a single QTL (accounting for 8?% of the variation in ME), it is totally context-dependent on the genetic background. We discovered many regions involved in epistatic interactions that together with the single QTL explain 19?% of the genetic variation for this trait. Milk ejection is an important component of maternal care, and understanding the mechanisms modulating its variation, along with other maternal features, may help to disentangle the complexity that is the mother/offspring relationship.     

Oocyte activation after intracytoplasmic injection with sperm frozen without cryoprotectants results in live offspring from inbred and hybrid mouse strains

Re-establishment of mouse strains used for mutagenesis and transgenesis has been hindered by difficulties in freezing sperm. The use of intracytoplasmic sperm injection (ICSI) enables the production of embryos for the restoration of mouse lines using sperm with reduced quality. By using ICSI, simplified sperm-freezing methods such as snap freezing can be explored. We examined the capacity of embryos from the inbred C57Bl/6J and 129Sv/ImJ mouse strains, commonly used for transgenic and N-ethyl-N-nitrosourea mutagenesis purposes to develop to blastocysts in vitro and to term following ICSI with sperm frozen without cryoprotectant. The results were compared to F1 (C57BlxCBA) hybrid embryos. Following freezing, sperm were immotile but could fertilize oocytes at similar rates to fresh sperm. However, embryo development in vitro to the blastocyst stage was reduced in all three strains. No pups were born from C57Bl/6J or 129Sv/ImJ embryos obtained from frozen sperm following transfer to foster females, and only a limited number of F1 embryos developed to term. Activation of oocytes injected with frozen sperm with 1.7 mM Sr2+ (SrCl2) did result in the birth of pups in all three strains. We conclude that the inability of sperm frozen without cryoprotectants to effectively activate oocytes may affect embryo development to term and can be overcome by strontium activation. This may become an effective strategy for sperm preservation and the restoration of most popular strains used for genetic modifications.     

Growth rates of Antarctic fur seal Arctocephalus gazella pups at South Georgia

Large numbers of pups of the Antarctic fur seal were weighed at regular intervals through their rearing period in six seasons between 1972 and 1981 at South Georgia. Growth rates and birth and weaning weights were estimated from linear regressions of mean weight on mean age. Female pups had significantly lower growth rates and weaning weights than males, but higher variances for all growth parameters. This may reflect the importance to males of polygynous species of attaining high body weight. Except for an inverse relation between growth rate and birth weight there were no systematic relationships between growth parameters. Seasonal differences in growth rates were substantial and there was no indication that the continuing rapid population increase was affecting pup growth rates. The use of these data in detecting seasonal variations in local food availability is discussed. Preliminary estimates of the energy costs of pup rearing indicate that pup existence metabolism consumes c. 80% and mass gain to weaning only c. 15% of the energy provided by the female.     

Milk ejection in mice LG/J x SM/J

In mammals, milk provision is crucial to offspring survival and growth from birth to weaning. Milk deficiency early in life may cause death or changes in the progeny metabolism that later may lead to obesity and metabolic disorders. This study investigates milk ejection (ME) the first day after birth (D1) in F₂ females from the intercross of LG/J and SM/J inbred mice strains. The absence of milk in F₃ pups’ stomach at D1 is directly associated with their survival (p < 0.001) and growth pattern (p < 0.001) in the early stages of life. Furthermore, late growth pattern is also affected by this lack of nutrients at D1 because pups that survive this absence, mostly males, are heavier at weaning (p < 0.001) which, after necropsy, is shown to be due to significant higher total fat deposition (p < 0.01). We performed QTL analysis for ME at D1 in these F₂ females. Maternal performance of ME revealed a complex genetic architecture which even though it contains only a single QTL (accounting for 8 % of the variation in ME), it is totally context-dependent on the genetic background. We discovered many regions involved in epistatic interactions that together with the single QTL explain 19 % of the genetic variation for this trait. Milk ejection is an important component of maternal care, and understanding the mechanisms modulating its variation, along with other maternal features, may help to disentangle the complexity that is the mother/offspring relationship.

     

Genetics of Survival in Mice: Subregions of the Major Histocompatibility Complex

In this study of murine survival, 422 F1 hybrids between DBA/2J (D2) female mice and C57BL/10 (B10) background H-2 congenic male mice (11 strains), 88 F1 hybrids between B10 female mice and B10 background H-2 congenic male mice (3 strains), and 532 control mice from the 11 parental B10 background H-2 congenic mice were bred over a period of 2 yr. Toward the end of the breeding period there was documentation of Sendai infection in the mouse rooms. All analyses were done separately for the two sexes. Although it did not appear that an unusually high number of mice died during the time the colony was infected with Sendai, there was a highly significant tendency for mice who were younger at the time of the Sendai infection to have shorter survival than mice who were older at that time point. The effect of birth date on survival was approximately as significant as the effect of strain on survival. Hence all analyses of genetic effects on survival were either done within subsets of mice born in the same quarter of a particular year or else included date of birth variables in survival models. Of the 18 possible comparisons of pairs of strains which overlapped in birth dates and differed only in the D end of H-2, five were associated with highly significant survival differences. Of the 11 pairs of strains which overlapped in birth date and differed only in the K end of H-2, none was associated with significant survival differences.(ABSTRACT TRUNCATED AT 250 WORDS)     

Growth and reproduction of two species of Akodon and their hybrids

Growth and reproduction of Akodon molinae (A.m.), A. dolores (A.d.) (Rodentia: Cricetidae), and their hybrids are described. A.d. showed less fertility under animal room conditions, fewer "successful" matings, similar litter sizes, and an altered sex proportion as compared to A.m.; A.d. ovulatory index is supposed to be higher than mean litter size. X-linked lethal gene(s) and chromosome polymorphisms are possible causes of these variations. Interspecific crosses showed a marked seasonal (summer) tendency in parturitions. Intraspecific matings also produced young during winter. F1 hybrids showed a reversed tendency, while F2 matings returned to the parental pattern. A.d. male X A.m. female and F1 crosses presented larger litter sizes than the other pairings. A.m. exhibited sexual differences in weight after 60 days of age. A.d. and F1 hybrids did not show significant differences in weight of both sexes at any age. A.m. males and females were heavier than A.d. individuals except 12 months after birth. F1 hybrids born to A.m. male were heavier at birth than the A.d. female s interspecific offspring, and may be interpreted as a maternal influence of the A.m. females. Hybrids were heavier than their parents at birth and some of them were also heavier at 21 days, and probably is due to pseudoheterosis. The comparison of body measurements between sexes of both species did not give conclusive results. A.m. females were larger than females of A.d. Males of both species showed similar total lengths; A.m. males had larger body lengths and shorter tails. Body measurements were not related to weight variations. However, differences in weight and measurements between sexes appeared simultaneously at 2-6 months. This probably reflected the attainment of maturity.     

Estimates of (co)variance components due to direct and maternal effects for body weights in Jamunapari goats

Estimates of (co)variance components were obtained for weights at birth, weaning and at 6, 9 and 12 months of age in Jamunapari goats maintained at the Central Institute for Research on Goats, Makhdoom, Mathura, India, over a period of 23 years (1982 to 2004). Records of 4301 kids descended from 204 sires and 1233 does were used in the study. Analyses were carried out by restricted maximum likelihood (REML), fitting an animal model and ignoring or including maternal genetic or permanent environmental effects. Six different animal models were fitted for all traits. The best model was chosen after testing the improvement of the log-likelihood values. Direct heritability estimates were inflated substantially for all traits when maternal effects were ignored. Heritability estimates for weights at birth, weaning and at 6, 9 and 12 months of age were 0.12, 0.18, 0.13, 0.17 and 0.21, respectively. Maternal heritability of body weight declined from 0.19 at birth to 0.08 at weaning and was near zero and not significant thereafter. Estimates of the fraction of variance due to maternal permanent environmental effects were 0.09, 0.13 and 0.10 for body weights at weaning, 6 months and 9 months of age, respectively. Results suggest that maternal additive effects were important only in the early stages of growth, whereas a permanent environmental maternal effect existed from weaning to 9 months of age. These results indicate that modest rates of genetic progress appear possible for all weights.     

Lasting effects of early life stress in mice: interaction of maternal environment and infant genes

In the mouse, a powerful paradigm of early life stress, infant maternal separation (IMS), can trigger emotional and cognitive dysfunctions in adulthood similar to those found in humans with a history of childhood adversity. The magnitude of IMS effects differs among diverse inbred strains suggesting an interaction between the genetic background of pups and the maternal care they received. Here, we investigated this interaction with studies on reciprocal F1 hybrid mice of the stress‐susceptible Balb/c and the resilient C57Bl/6 strains that were either raised by Balb/c mothers (low maternal care) or by C57Bl/6 mothers (higher maternal care) with or without IMS exposure. The ultrasonic vocalization response to isolation was recorded from infant F1 pups, and their emotional, executive cognitive and epigenetic phenotypes were assessed in adulthood. These studies showed that, regardless of the maternal care received, the emotional phenotype of F1 hybrids was not significantly affected by IMS exposure. However, F1 pups raised by Balb/c (but not C57Bl/6) mothers during IMS exposure exhibit deficits in working memory and attention‐set‐shifting in adulthood. They also exhibit reduced histone deacetylase 1 levels at promotors of brain‐derived neurotrophic factor and early growth response 2 genes, and abnormally high induction of expression of these genes during cognitive testing. As one of affected genes was previously shown to associate with the Balb/c and the other with the C57Bl/6 genetic background, these findings indicate that both parental alleles interact with the maternal environment to modulate the cognitive and epigenetic phenotypes of F1 mice exposed to the IMS.     

Effects of non-genetic factors on production traits of Inner Mongolia cashmere goats in China

The effects of age, sex, age of dam, year of production, herd, and type of birth on cashmere weight, fiber diameter, fiber length, body weight at birth, weaning and yearling in goats, were based on 12 years data from an Inner Mongolia Albas cashmere goat stock farm. The year of production had significant influence on cashmere weight, fiber diameter and length, and body weight of yearlings and adult goats. Year had no significant effect on weaning weight. In the yearling, males produced more cashmere and were heavier in body weights at birth, weaning and 1 year of age than the females. Adult males were significantly heavier than adult females in cashmere weight, fiber diameter and length, and body weight. In the yearling, age of dam had a significant effect on cashmere weight, fiber diameter and length, body weights at birth, weaning and 1 year of age. In the adult, age of dam also had a significant effect on fiber diameter and body weight. Dams aged 2–4 years produced offsprings with a higher cashmere production than those at older ages. In the yearling, type of birth had significant effects on body weight at birth, weaning and 1 year of age, and fiber length. The herd-within-year effects were significant for all traits studied whereas sex×year effects were significant for cashmere weight and body weights at birth, weaning and 1 year of age. These results indicate that the estimation of breeding value in a selection scheme for cashmere production would be more precise following adjustment for age structures, age of dam, sex, type of birth, herd and significant interactions.     

Reproductive performance in grey seals: age-related improvement and senescence in a capital breeder

1. Three hypotheses have been advanced to account for age-related improvement in performance: the selection hypothesis predicts improved due to the loss of lower quality phenotypes, the constraint hypothesis predicts individuals improve function, and the restraint hypothesis predicts younger individuals forego or reduce effort because of mortality risks. A decline in age-related performance (i.e. senescence) is predicted by mutation accumulation, antagonistic pleiotropy and disposable soma (wear and tear) hypotheses. 2. Using five measures of performance - birth rate, maternal and pup birth mass, pup weaning mass, weaning success and lactation length - we tested these hypotheses concerning age-related change in reproduction in 279 female grey seals (Halichoerus grypus), ages 4-42 years, over a 23-year period between 1983 and 2005 on Sable Island, Nova Scotia. These females produced 2071 pups. 3. Although body mass of primiparous females increased with age (4-7 years) birth mass of their pups did not, but pup weaning mass did. Second- and third-parity females of the same age as primiparous females gave birth to and weaned heavier pups. However, parity and age were dropped from models when maternal body mass was included. 4. The proportion of females giving birth varied significantly with maternal age, increasing in young females and then declining late in life. Weaning success rate also increased rapidly to about 8 years and subsequently declined in females > 32 years. 5. Generalized additive models indicated nonlinear changes in 3 day body mass (i.e. approximately birth mass) and weaning mass of pups as a function of maternal age, after accounting statistically for the effects of maternal body mass. Mixed-effects, repeated-measures models fitted to longitudinal data further supported the conclusion that pup birth mass and weaning mass vary nonlinearly with maternal age and indicated nonlinear changes in lactation duration. 6. We found some support for the constraint hypothesis, but our findings were not consistent with the selection hypothesis or the restraint hypothesis as the basis for improvement in reproductive performance. 7. Senescence was evident in multiple female and offspring traits, indicating the degeneration in function of several physiological systems as predicted by the disposable soma hypothesis.     

Genetic influences on oestrous cyclicity in mice: evidence that cycle length and frequency are differentially regulated.

Studies in C57BL/6J, DBA/2J and C3H/HeJ mice and in two F1 hybrid strains (B6D2F1 and B6C3HF1) 2-5 months old revealed marked genotypic differences among inbred strains. C57 mice had three times as many regular (3-6 days) cycles as DBA and C3H mice, due largely to fewer pseudopregnant-like (7-14 day) cycles. C57 had longer regular cycles than DBA and C3H mice. Although the frequencies of regular cycles of DBA and C3H mice were similar, the cycles of C3H mice were shorter than those of DBA mice. The results indicated that the genetic determinants of the frequency of regular cycles differ from those specifying cycle length. Frequency of regular cycles of F1 hybrids was either intermediate between the parent strains (B6D2F1) or similar to the C57 strain (B6C3HF1), suggesting that regular cycle frequency shows additive genetic variation in the former crosses, but mostly dominant variance in the latter background. Regular cycles were either shorter than in both parent strains (B6D2F1) or similar to one of them (B6C3HF1), indicating heterosis and dominance for genes specifying short cycles. Although the lack of reciprocal crosses meant that maternal effects and possible genomic imprinting effects could not be assessed, these results reveal marked genetic influences on cycle length and frequency and suggest that some of the genes specifying these two traits differ.     

Two-generation study of neuroleptic isofloxythepin in rats

The neuroleptic isofloxythepin was tested in a two-generation study in rats in oral doses of 0.1 mg base/kg and 1 mg base/kg. The substance was administered to male and female rats of each generation daily from the weaning onward till the conclusion of the study. The pups of two consecutive generations F0 and F1 were surveilled always in two litters. There were assessed the numbers and body weights of the pups from birth till the weaning, the postnatal developmental milestones, and the results of neuromuscular and locomotor activity tests. In the parents the fertility parameters were followed. --Isofloxythepin at higher dosage, 1 mg/kg, influenced the estrous cycles in the females, prolonging the diestrus stage, but exerted no significant effects upon their fertility and pregnancy. The growth of the offspring of parents treated with isofloxythepin in both generations was slightly retarded. In the F0 generation the pup's mortality increased in the perinatal period. The sedative effect of the neuroleptic manifested itself by a lowering of the locomotor activity. The proved effects of isofloxythepin in the higher dose are mostly connected with the drug's neuroleptic properties. Analogous effects of other substances of this type have been reported in the literature, too. In the course of the study the effects of isofloxythepin did not intensify from the first to the second generation.     

Specific inhibition of hybrid resistance in F1 hybrid mice pretreated with parent strain spleen cells. I. Induction of a nylon-adherent, Thy-1+Lyt-1+2- suppressor cell

Hybrid resistance, which is observed in certain strain combinations when parent-strain bone marrow cells are grafted into lethally irradiated F1 hybrids, can be specifically overcome by the i.v. injection, 1 wk before the graft, of spleen cells syngeneic with the bone marrow graft. This phenomenon is due to a suppressor mechanism, induced in the spleen of the F1 hybrid by the injection of parent-strain spleen cells and mediated by a nylon-adherent Thy-1+Lyt-1+2- cell population of hybrid origin, because hybrid resistance can be inhibited by the transfer into a normal B6D2F1 of nylon-adherent Thy-1+Lyt-1+2- spleen cells from B6D2F1 mice pretreated with B6 spleen cells 1 wk earlier (B6-pretreated B6D2F1); spleen cells from B6-pretreated B6D2F1 mice not depleted of their nylon-adherent subpopulation cannot restore hybrid resistance when they are injected into a B6D2F1 rendered nonresistant by split-dose irradiation; and spleen cells from normal B6D2F1 mice cannot restore hybrid resistance when they are injected into B6-pretreated B6D2F1 hybrids. The suppressor cells specifically inhibit resistance against bone marrow cells syngeneic with the spleen cells used for pretreatment, because transfer of nylon-adherent B6-pretreated B6D2F1 spleen cells into a normal B6D2F1 does not enhance syngeneic B6D2F1 or parent-strain D2 bone marrow growth, and when injected into normal B6D2F1 hybrids, nylon-adherent spleen cells from B6D2F1 mice pretreated with D2 spleen cells 1 wk earlier (D2-pretreated B6D2F1) are not able to transfer the inhibition of hybrid resistance against B6 bone marrow cells. Moreover, the activity of the suppressor cells depends on the genetic environment of the hybrid host mice, because nylon-adherent B6-pretreated B6D2F1 spleen cells injected into normal B6C3F1 hybrids do not transfer an inhibition of hybrid resistance, and when injected into B6C3F1 hosts previously rendered nonresistant by split-dose irradiation, spleen cells from B6-pretreated B6D2F1 mice can, in contrast, transfer hybrid resistance.     

Maternal genotype affects adult offspring lipid, obesity, and diabetes phenotypes in LGXSM recombinant inbred strains

Maternal effects on offspring phenotypes occur because mothers in many species provide an environment for their developing young. Although these factors are correctly "environmental" with respect to the offspring genome, their variance may have both a genetic and an environmental basis in the maternal generation. Here, reciprocal crosses between C57BL/6J and 10 LGXSM recombinant inbred (RI) strains were performed, and litters were divided at weaning into high-fat and low-fat dietary treatments. Differences between reciprocal litters were used to measure genetic maternal effects on offspring phenotypes. Nearly all traits, including weekly body weights and adult blood serum traits, show effects indicative of genetic variation in maternal effects across RI strains, allowing the quantitative trait loci involved to be mapped. Although much of the literature on maternal effects relates to early life traits, we detect strong and significant maternal effects on traits measured at adulthood (as much as 10% of the trait variance at 17 or more weeks after weaning). We also found an interaction affecting adult phenotype between the effects of maternal care between RI strain mothers and C57BL/6J mothers and a later environmental factor (dietary fat intake) for some age-specific weights.     

Onset of puberty in CD-1 mouse pups exposed prenatally through weaning to endophyte-infected tall fescue seed

This study was undertaken to investigate the effects of feeding endophyte - infected (Acremonium coenophialum ) tall fescue seed to CD-1 mouse dams (P(1)) during gestation and lactation, and on the subsequent growth and sexual maturity (onset of puberty) of their male and female offspring (F(1)). Forty-eight 21 d old pups (24 male and 24 female F(1) mice) were weaned from dams fed one of two diets containing 50% rodent chow (w/w) and 50% KY-31 tall fescue (Festuca arundinacea ) seed. The seed in Diet 1 was noninfected, while the seed in Diet 2 was 80% endophyte-infected. At weaning (21 d), the F(1) pups were fed rodent chow, ad libitum throughout the remaining experimental period. At 24 d, they were paired with sexually mature non-treated virgin CD-1 mice (fed 100% rodent chow) for one parturition cycle. Male F(1) mice were sacrificed at 84 d to determine testicular development. The age at the birth of the first litter for Diet 2 F(1) male (76.8 +/- 2.2 d) and female (58.4 +/- 2.1 d) was significantly greater (P<0.05) than the age at parturition for Diet 1 male and female F(1) test mice (64.1 +/- 1.8 and 51.9 +/- 1.2 d, respectively). At parturition, the female F(1) mice showed no significant differences (P>0.05) in either mean parturition weight or number of F(2) pups born per litter. However, total F(2) litter wight (11.38 +/- 1.14 g) and mean weight per F(2) pup (1.40 +/- 0.04 g) for Diet 2 female F(1) mice litters were lower (P<0.05) when compared with Diet 1 females (14.53 +/- 0.57 g and 1.66 +/- 0.02 g, respectively). No significant differences were observed between the two male F(1) treatment groups, for total F(2) litter weight or the number of pups born per F(2) litter. Although Diet 2 F(1) males weighed significantly less (P<0.05) at weaning and at pairing, final body weights at sacrifice (84 d) were not different (P>0.05) from the Diet 1 males.     

Influence of thyroxine and propylthiouracil administration on the intake of maternal milk in sucklings of the laboratory rat

Young laboratory rats received a daily subcutaneous injection of thyroxine (T4) and propylthiouracil (PTU) in saline, from birth until the period of spontaneous weaning. The rate of growth and the time of eye opening as well as the spontaneous weaning was assessed. The intake of maternal milk was followed directly, i.e. by the transport of 85Sr in maternal milk from the maternal body to the sucklings. In the young treated with T4 the eyes opened earlier than in the young treated with PTU. The body mass differed in both groups between the 4th and 20th day of life (the T4 pups were heavier). The 85Sr transfer in maternal milk was higher in the PTU young from the 22nd day of life. The PTU sucklings weaned spontaneously one week later than those receiving T4. A hypothesis has been proposed on the primary significance of maturation of oral mechanisms of sucking for spontaneous weaning.