Effects of concurrent pregnancy and lactation on reproduction in four strains of mice

The influence of concurrent pregnancy until the third lactation on reproduction was studied in four strains of mice (SHN, SLN, C3H/He and GRS/A) as a possible step to evaluate the efficiency of this system on offspring production. In all strains, little difference was observed between sequential and concurrent pregnancy and lactation groups in any parameter as the index of reproductivity--delivery interval, litter size, average weight and growth rate of pups, mother weight, rearing rate, still-birth rate and rate of still-born pups. Also, no difference was seen between groups of any strain, in the number of females which had three litters. The estimated number of pups weaned per female during 50 days after first being placed with males was higher in the concurrent pregnancy groups than in the sequential pregnancy groups of all strains. Meanwhile, the incidence of spontaneous mammary tumors, the major characteristic of these strains, was consistently lower in the concurrent pregnancy group in all strains. These results have demonstrated that the concurrent pregnancy and lactation system is an efficient means of mouse production.     

Mammary tumour incidence in relation to the pattern of oestrous cycles in mice

The patterns of oestrous cycles were examined in new strains of mice exhibiting a high (SHN) and a low (SLN) mammary tumour incidence. These mice originated from the same stock of Swiss albino mice. At the age of 3-4 months, SHN mice showed much longer cycles with continual dioestrous periods than SLN mice. The average lengths of cycles and dioestrous periods were, respectively, 8.3 +/- 0.3 and 6.0 +/- 0.4 days in SHN mice and 4.6 +/- 0.1 and 2.5 +/- 0.1 days in SLN mice. Furthermore, the distinct alteration in the patterns of cycles from SHN to SLN type was associated with complete reduction in mammary tumour incidence in highly inbred C3H/He female mice maintained by brother X sister mating with no selection for mammary tumourigenesis. The relation between the pattern of oestrous cycles and mammary tumourigenesis is discussed from the viewpoint of the endogenous hormonal milieu and the responsiveness of target organs.     

Comparison of mammary tumorigenesis between litters in relation to mammary tumor virus antigenic expression in the milk of C3H/He mice

As a possible step to estimate the relationship between mammary tumor virus (MTV) and mammary tumorigenesis in mice, MTV antigenic expression in mother's milk and spontaneous mammary tumorigenesis in their daughters were compared between the 1st, the 2nd and the 3rd litters of the highly inbred strains of C3H/He mice with low mammary tumor incidence. While MTV antigenic expression was detected in all undiluted samples at each litter by immunodiffusion test, the amount of antigen as measured by the single radial immunodiffusion method was increased with increasing litter numbers. On the other hand, the development of preneoplastic mammary hyperplastic alveolar nodules was different little between litters and mammary tumor incidence by 13 months of age was very low with no difference in all litters. The pattern of estrous cycles and plasma prolactin level were also similar in each litter. The results suggest that spontaneous mammary tumorigenesis in mice is not always affected quantitatively by the amount of MTV when endocrine and genetical conditions are similar.     

Two inbred strains of mice with high and low mammary tumor potentials established from the same basal stock of Swiss albino (SHN and SLN): history of mammary tumorigenesis and reproduction after the 30th generation of full-sib mating.

We established SHN and SLN strains of mice with a high and a low mammary tumor potentials from the same basal stock of Swiss albino. The selection history of females (mothers) for subsequent generations, mammary tumorigenesis and reproduction up to recent generations are presented and compared with the results up to F30 published in 1979. The changes with generation of each parameter varied in both strains except the conditions of female (mother) selection for subsequent generations, which were generally constant. However, the fluctuations in parameters up to F30 became generally smaller with advancing generations. Based on the fluctuations from generation to generation of mammary tumorigenic and reproductive parameters, an interdependency between mammary tumor potential and reproductivity is suggested in SLN, but less so in SHN. The pup growth rate was increased and associated with selection for mammary tumorigenesis in SHN; this parameter is usually used as an indicator of mother lactational performance in mice. The significance of selection for mammary tumor potential in association with reproductivity was further discussed.     

A device for milk collection from mice

Approximately 14 days after SHN, SLN and C3H/He strains of mice gave birth, their pups were removed. After 14--16 hours away from their pups, the dam was anesthetized, injected with oxytocin and milked. The milking device consisted of a rubber-stoppered centrifuge tube containing a stainless steel catheter and polyvinyl chloride tube. It was operated by vacuum.     

Relationship between milk composition and pup's growth in mice

The effect of lactation numbers (first, second, and third) on milk composition and the relationship between milk composition and pup's growth were examined in four strains of mice (SHN, SLN, C3H/He, and GR/A). Although the percentage of milk fat and moisture increased and decreased, respectively, through the first to third lactations in C3H/He mice, no differences in milk composition among lactation numbers were observed in the other mouse strains. In pooled data derived from the four strains of mice, there was no significant correlation between milk composition and body weight or growth rate of pups on Day 12 or 20 of the first lactation. In contrast, the amount of milk fat and moisture was the positively and negatively, respectively, correlated with the pup's growth parameters during the second and third lactations. This was reflected by a lower fat content and a higher moisture content in the milk of GR/A mice, especially evident during the second and third lactations and reduced pup's growth, when compared with the other mouse strains. Milk lactose and ash content was not correlated with pup's growth parameters. The results of our study stress the importance of milk fat for the growth of mouse pups.     

Effects of age and parity on litter size and offspring sex ratio in golden hamsters (Mesocricetus auratus)

Golden hamsters that were mated repeatedly from 55 days of age produced 6-12 litters. Litter size at birth rose between the 1st and 2nd litters, peaked on the 3rd, and declined steadily after the 5th litter. Offspring sex ratio (% male) at birth followed a similar pattern: increasing between the 1st and 2nd litters, remaining high through the 3rd, and becoming increasingly female-biased thereafter. Weaning success decreased sharply after the 6th litter and most dams failed to raise any young to weaning after the 9th litter. These sequential effects on litter size, offspring sex ratio and weaning success were also observed in females mated once at different ages, but they occurred considerably later in life, i.e. increasing parity hastened the effects of advanced age. These age- and parity-related changes in litter composition are consistent with the Trivers-Willard hypothesis that physiologically-stressed females would skew offspring sex ratios to favour daughters. However, since the observed changes in sex ratio were probably due to differential prenatal mortality, their adaptive significance is unclear.     

Sex ratio and litter size in relation to parity and mode of conception in three inbred strains of mice

Breeding records from three inbred strains of mice (BALB/c ABom, C57BL/10ScSn, C3H/He/Kon) were examined with respect to the effects of parity and mode of conception upon litter size and sex ratio at birth. Litters from 3 modes of conception were considered: litters of primipares, litters of multipares conceived during postpartum oestrus and litters conceived after lactational anoestrus. Litters of multipares were assigned to one of these latter groups according to the inter-litter intervals. Parity had no significant effect upon the sex ratio but had a significant one upon the litter size, which did not vary between the strains when first litters were excluded from analysis. The expected variations in response to the mode of conception were found in BALB/c ABom mice but both the effects on the litter size as well as on the sex ratio varied significantly between the strains. Litter size reduction per se could be ruled out to be the cause of the sex ratio variations found. Rather, it is suggested that sex-specificity of embryonic loss depends upon the mode of conception.     

Relationship of cryptorchidism with sex ratios and litter sizes in 12 dog breeds

The aim of this study was to identify the influence of genetic carriership for cryptorchidism on litter sizes and sex ratios in the offspring. Weaning data of 11,230 litters in 12 purebred dog breeds were evaluated. Parents were classified as cryptorchidism 'carriers' (C) when at least one of their offspring was found cryptorchid. Subsequently the effects of 'carrier' and 'non-carrier' (NC) parents on their litters were studied. In litters from C x C parents we found an increased number of males per litter in all breeds, a reduced number of females per litter in 8 breeds and an increased litter size in 11 breeds in comparison with litters from NC x NC parents. Over all breeds the effects on litter size, on number of males per litter and on sex ratio were highly significant. Mixed litters from C x NC and NC x C did not show these effects and were not significantly different from the NC x NC offspring. Our results suggest a general mechanism in the dog species which causes cryptorchidism as well as increased male/female ratios and increased litter sizes. A consequence of such a mechanism is that selection in favor of increasing reproduction output frustrates selective efforts to eliminate cryptorchidism.     

Explaining the seasonal decline in litter size in European ground squirrels

In European ground squirrels Spermophilus citellus as in many ground squirrel species. late born litters are composed of fewer young than early born litters. Two alternative though not mutually exclusive hypotheses may explain this seasonal pattern of change in litter size. On the one hand. the production of few large young late in the season may be an adaptation to time limitations on the offspring. that have to complete growth and fattening prior to hibernation. Then one would expect a trade-off between offspring number and size as the breeding season progresses. At its extreme. this hypothesis would predict that total maternal effort should be equal independent of litter size. Alternatively. litter size may be determined by physiological limitations on the mother. in that highly constrained mothers breed later and produce smaller litters. Then one would expect reduced overall maternal effort in highly constrained mothers of smaller litters. In this case. a trade-off between litter size and offspring size would not be expected. We found that total maternal effort in terms of gestation length and the duration of lactation increased with increasing litter size. thus supporting the second hypothesis. Lactation was not terminated at natal emergence. It extended a relatively long period of time beyond the time of first litter emergence depending on litter size. During prolonged lactation. individual young of large litters made up body mass to young of small litters. As a consequence. juvenile weaning body mass was unaffected by litter size although offspring body mass at natal emergence was inversely related to litter size. This additional weight gain in young of large litters compensated for initial survival disadvantages and presumably affected fecundity at yearling age.     

Genetic Factors Controlling the Proliferative Activity of Mouse Epidermal Melanocytes during the Healing of Skin Wounds

A cut was made on the middorsal skin of newborn mice of strains C57BL/10J, C57BL/10J-A/A, and C3H/He using fine iridectomy scissors. In the epidermis within 1 mm of the wound edge in C57BL/10J and C57BL/10J-A/A, the melanocyte population positive to the dopa reaction as well as the melanoblast-melanocyte population positive to the combined dopa-premelanin reaction increased dramatically until the 3rd day, then gradually decreased. In contrast, the melanocyte population of C3H/He did not increase after wounding, despite that the melanoblast-melanocyte population increased. Pigment-producing melanocytes in mitosis were frequently found in C57BL/10J and C57BL/10J-A/A, but not in C3H/He. The F1, F2, and backcross matings were performed to get some information about the genetic basis of the difference between C57BL/10J and C3H/He. In the F1 generation the offspring from reciprocal crosses exhibited intermediate values in both populations on the 3rd day after wounding. The F2 generation included the C3H/He type, F1 type, and C57BL/10J type in a ratio of 1:2:1 in both populations. Moreover, both reciprocal backcrosses gave 1:1 ratios of parent type to F1 type in both populations. These results indicate that the proliferative activity of mouse epidermal melanocytes during the healing of skin wounds are controlled by semidominant genes.     

Incomplete proviral genome of mouse mammary tumour virus is present on chromosome Y of NIH Swiss and some genetically related mouse strains.

An incomplete proviral genome of endogenous mammary tumour virus (MMTV) was found in DNA of several strains of mice. This MMTV-related sequence was assigned to the Y chromosome since it was clearly observed in male mice only. This MMTV provirus contained a sequence related to LTR (long terminal repeat), but not to gag-pol and env genes. NFS, NIH Swiss/S, STS/A, and DD/Tbr mice have this sequence but BALB/cHeA, SHN, SLN, C57BL/6NJcl, C3H/HeNJcl and CBA/JJcl mice are negative. In the strains containing this sequence, a DNA test for the sequence makes it possible to easily distinguish the DNAs of male or female mice.     

Differential Growth of Own and Alien Pups in Mixed Litters of Mice: A Role for Genomic Imprinting?

Female mice are said to be unable to distinguish own from alien offspring and will indiscriminately nurse each other's young in communal nests. Here, we present results of a split‐litter experiment that tested whether offspring growth was affected depending on whether they were nursed by their own or unrelated foster mothers. Pups of reciprocal crosses between C57/B6 and CBA/Ca strains were fostered in mixed litters that consisted half of their natural siblings and half of unrelated littermates of the reciprocal genotype. Analysing the relative growth of the two pup types showed that offspring gained proportionally more weight when nursed by their own mothers than their cross‐fostered litter mates during the period from day 15 until weaning, during which maternal provisioning effort contributes to pup weight gain. Before day 15 of the pups’ life, however, we found no advantage of being nursed by biological mothers, and we suggest that this may be due to the effects of paternally expressed genes in young that mask their maternal identity, thus favouring indiscriminate nursing of all young in a communal nest.     

Hormonal manipulation of offspring number: maternal effort and reproductive costs

We used exogenous gonadotropin hormones to physiologically enlarge litter size in the bank vole (Clethrionomys glareolus). This method allowed the study design to include possible production costs of reproduction and a trade-off between offspring number and body size at birth. Furthermore, progeny rearing and survival and postpartum survival of the females took place in outdoor enclosures to capture salient naturalistic effects that might be present during the fall and early winter. The aim of the study was to assess the effects of the manipulation on the growth and survival of the offspring and on the reproductive effort, survival, and future fecundity of the mothers. Mean offspring body size was smaller in enlarged litters compared to control litters at weaning, but the differences disappeared by the winter. Differences in litter sizes disappeared before weaning age due to higher mortality in enlarged litters. In addition to the effects of the litter size, offspring performance was probably also influenced by the ability of the mother to support the litter. Experimental females had higher reproductive effort at birth, and they also tended to have higher mortality during nursing. Combined effects of high reproductive effort at birth and high investment in nursing the litter entailed costs for the experimental females in terms of decreased probability of producing a second litter and a decreased body mass gain. Thus, enlarged litter size had both survival and fecundity costs for the mothers. Our results suggest that the evolution of litter size and reproductive effort is determined by reproductive costs for the mothers as well as by a trade-off between offspring number and quality.     

Litter size variation in hypothalamic gene expression determines adult metabolic phenotype in Brandt's voles (Lasiopodomys brandtii)

Background

Early postnatal environments may have long-term and potentially irreversible consequences on hypothalamic neurons involved in energy homeostasis. Litter size is an important life history trait and negatively correlated with milk intake in small mammals, and thus has been regarded as a naturally varying feature of the early developmental environment. Here we investigated the long-term effects of litter size on metabolic phenotype and hypothalamic neuropeptide mRNA expression involved in the regulation of energy homeostasis, using the offspring reared from large (10–12) and small (3–4) litter sizes, of Brandt''s voles (Lasiopodomys brandtii), a rodent species from Inner Mongolia grassland in China.

Methodology/Principal Findings

Hypothalamic leptin signaling and neuropeptides were measured by Real-Time PCR. We showed that offspring reared from small litters were heavier at weaning and also in adulthood than offspring from large litters, accompanied by increased food intake during development. There were no significant differences in serum leptin levels or leptin receptor (OB-Rb) mRNA in the hypothalamus at weaning or in adulthood, however, hypothalamic suppressor of cytokine signaling 3 (SOCS3) mRNA in adulthood increased in small litters compared to that in large litters. As a result, the agouti-related peptide (AgRP) mRNA increased in the offspring from small litters.

Conclusions/Significance

These findings support our hypothesis that natural litter size has a permanent effect on offspring metabolic phenotype and hypothalamic neuropeptide expression, and suggest central leptin resistance and the resultant increase in AgRP expression may be a fundamental mechanism underlying hyperphagia and the increased risk of overweight in pups of small litters. Thus, we conclude that litter size may be an important and central determinant of metabolic fitness in adulthood.     

烟蚜茧蜂寄生对烟蚜生长发育和繁殖的影响

【目的】为探究烟蚜茧蜂Aphidius gifuensis对烟蚜Myzus persicae寄生后烟蚜生长发育和繁殖影响及其在烟蚜防控中的作用。【方法】采用室内叶片培养皿法,测定了不同龄期烟蚜(1-4龄若蚜及成蚜)被烟蚜茧蜂寄生后烟蚜当代(F₀)及后代(F₁)的发育历期、寿命、产仔量。【结果】烟蚜1龄若蚜被烟蚜茧蜂寄生后,1龄若蚜历期受到显著影响,较未被寄生对照缩短了0.65 d (33.00%),而2-4龄若蚜期和成蚜历期未受显著影响;2龄若蚜被寄生后,4龄若蚜和成蚜历期均受到显著影响,较未被寄生的对照分别缩短了1.50 d (100.00%)和0.83 d (100.00%),而2和3龄若蚜期未受显著影响;3龄若蚜被寄生后,4龄若蚜和成蚜期均受到显著影响,较未被寄生对照分别缩短了1.50 d (100.00%)和0.83 d (100.00%),而3龄若蚜期未受显著影响。2龄若蚜被寄生后烟蚜的寿命和产仔量均受到显著影响,寿命较未被寄生对照缩短了8.39 d(54.00%),每雌产仔量较未被寄生对照减少了48.67头(100.00%)。此外,2-3龄若蚜被寄生后其后代(F₁)1龄若蚜历期受影响最大,较未被寄生烟蚜后代1龄若蚜期缩短了0.80 d(40.00%),其F₁代平均寿命较未被寄生烟蚜后代平均寿命延长了5.72 d(41.20%),但其产仔量与对照差异不显著。【结论】不同龄期烟蚜被烟蚜茧蜂寄生后,其发育历期和寿命比未被寄生的缩短,产仔量比被未寄生的少,但其F₁代平均寿命比未被寄生后代烟蚜的长且其产仔量未受显著影响。当代2龄若蚜期是控制烟蚜增长的关键时期。这些结果对确定烟蚜茧蜂防治烟蚜适期及认识其机理具有重要参考意义。     

Optimal offspring size in a small mammal: an exception to the tradeoff invariant life-history rule

Offspring size and number were examined in a captive population of wild guinea pigs ( Cavia aperea ), and findings were compared with models of optimal offspring size for small litters. Median and modal litter size was two, regardless of maternal size or parity. Females producing their second litter tended to have litters that were larger than average. In contrast, young females that were still growing never had litters that were larger than average. Mean offspring size decreased and variation in offspring size tended to decrease with increasing litter size. Optimal offspring size models, in which offspring survival depended on the amount of resources invested, as well as litter size, predict such a trend. Little support was found for Charnov and Downhower's (1995) tradeoff invariant life-history rule that the range in offspring sizes between litters is inversely proportional to the size of the litter. Cavia aperea may be an exception to this rule because pup mass at birth did not reflect total reproductive investment, because conversion of resources into litter mass may not be linearly related to litter size and because resources were not equally partitioned among offspring within large litters. Experimental data are needed to determine the relevance of these results among mammals in general.     

Trade-offs in Energy Allocation During Lactation

SYNOPSIS. During lactation, mothers require energy to meet bothmaternal and offspring requirements. If a mother exports toomuch energy to dependent offspring (in milk), her weight lossmay be excessive and maternal risk may increase. Conversely,too little energy allocation to offspring may reduce the growthrate or induce mortality of dependent offspring. This paradigmwas evaluated in cotton rats (Sigmodon hispidus) supportingsmall (3 pup) and large (6 pup) litters from early to late lactation.Several types of evidence indicate that physiological constraintslimit the ability of mothers with large litters to provide resourcesto offspring. Mothers with large litters produced a dilute,energy-poor milk and their rates of food intake, weight lossand energy export per litter appeared to approach physiologicalmaxima. Whereas the energy exported to pups in small littersincreased from early to late lactation, the energy flow perpup in large litters was consistently low; consequently, offspringin large litters had low growth rates. An increase in eithermaternal food intake or weight loss (catabolism of maternaltissue) could have provided additional energy to offset thelow growth rate of pups in large litters. However, mothers withlarge litters did not substantially increase their food intakeor weight loss compared with mothers supporting small litters.These results indicate that the maternal support of offspringin large litters is limited. The pattern of energy allocationshown by cotton rats with large litters likely reflects a compromisebetween meeting maternal and offspring energy requirements (cf.,Parker and Macnair, 1979). The energy flow is greater than optimalfor the parent but less than optimal for the offspring. Lessmaternal-offspring conflict occurs in small than large littersbecause offspring in small litters maintain a high growth rateat a relatively low maternal cost. Yet, under favorable environmentalconditions, the reduction in maternal-offspring conflict hasno apparent fitness benefit.     

Offspring growth, survival and reproductive success in the bank vole: a litter size manipulation experiment

To estimate the optimality of brood size, it is essential to study the effects of brood size manipulation on offspring survival and reproductive success. Moreover, testing the generality of the hypothesis of reproductive costs requires experimental data from a diversity of organisms. Here I present data on the growth, survival and reproductive success of bank vole Clethrionomys glareolus individuals from manipulated litters. Furthermore, the survival of mothers whose litter size was manipulated was studied. At weaning, the mean weight of pups from enlarged litters was lower and from reduced litters higher compared to control litters. After winter, at the start of the breeding season, individuals from enlarged litters, especially males, were still lighter than individuals from the other two treatments. Litter enlargements did not increase the number of reproducing female offspring per mother, nor did the litter sizes of female offspring differ between treatments. There were no differences between treatments in winter survival of offspring after weaning, but among female offspring, weaning weight explained the survival probabilities over winter. A higher weight of females at winter determined the probability of starting to reproduce in spring. The survival of mothers did not seem to be influenced by litter manipulation performed the previous year. According to the results, mothers nursing enlarged or reduced litters do not gain any fitness benefits in terms of number of offspring surviving to breeding. The results are consistent with the majority of experiments conducted in birds, which have found costs of enlarged brood appearing as offspring trade-offs rather than parent trade-offs. Received: 14 December 1997 / Accepted: 1 March 1998