The fertility of mice selected for large or small body size

Reproductive performance over repeated parities of lines of mice selected for appetite, lean growth and fatness

Female reproductive performance over four parities was studied for lines of mice selected for one three criteria: appetite (A), total lean mass (P), or proportion of fat (F). Female mice were first bred 8 weeks of age, and thereafter at intervals of about 7 weeks until fourth parity when they were dissected in late pregnancy to measure components of litter size.

The high A lines had higher litter sizes at the first three parities and higher ovulation rates, numbers of implantation sites and live foetuses at the fourth parity than the low A lines. The high P lines were also higher than the low P lines for each of these traits. In contrast, litter sizes and ovulation rates differed little between the high and low F lines. Fitting body weight as a covariate removed the high-low differences in ovulation rate and litter size between the P lines, but not all the differences between the A lines.

Pre-implantation survival at the fourth parity was slightly lower in the high than in low A and in high than in low P lines. Differences in post-implantation survival were very small. No component of prenatal survival differed substantially at fourth parity between the high and low F lines.

Differences in fertility at each parity and the percentages of females surviving to the end of the study were small. At each litter, dams of the high A lines produced heavier total weights of litters at 12 and 21 days than dams of the low A lines and the corresponding high-low differences in the P lines were larger. There were only small differences in these traits between the F lines. When litter size, fertility and survival were summed over parities, the average total number of young produced per female was substantially higher in the high than in the low A lines (5·0 young born alive or dead) and the P lines (7·2), but the high-low difference was small in the F lines (2-0). For all lines, litter size at first parity gave a reliable indication of reproductive rate during a major portion of the reproductive lifespan.

The efficiency of food utilization, digestibility of foodstuffs and energy expenditure of mice selected for large or small body size

The efficiency of food utilization, the digestibility of foodstuffs, energy metabolism, and body activity have been studied in three lines of mice, one selected for large, another for small body size, and a third, control, line.

The gross efficiency of food utilization was highest in the large line, intermediate in the control line and lowest in the small line between 21 and approximately 35 days of age. During this period, gross efficiency declined in the large and control lines with increasing size and decreasing growth-rate, presumably due to an increase in maintenance costs in comparison with the weight gained. In the small line, the efficiency of food utilization increased up to 35 days of age but declined thereafter. The energetic efficiency (measured in Calories) was higher in the large than in the small line up to 4 weeks of age, i.e. when the growth-rate was high, and after 6 weeks of age, when fat was being deposited at an increased rate.

The increased efficiency of large mice was not entirely associated with a greater proportion of the ingested food being absorbed from the gut. Large mice absorbed a greater proportion of protein, though the difference was not sufficient to account for the large weight difference between the large and small lines.

The energy expenditure of mice of the large line was greater than that of the small line at all ages and similar for the same body weights. The reduced growth-rate of small mice was not due to abnormally high or low energy costs. There was no evidence that body activity determined or restricted the rate of growth in either line.

Mice selected for small size were phenotypically unlike pituitary dwarf mice, although the low nitrogen retention during the growing-period indicated a deficiency of some growth stimulus.

‘Midget’, a new dwarfing gene in the house mouse dependant on a genetic background of small body size for its expression

Midget, a new dwarfing gene in the house mouse, was found in a line (NS) selected for small body size. Backcrosses to, and heterozygous matings of NS mice gave midget offspring in the expected 1:1 and 1:3 ratios. Backcrosses to two other lines of mice selected for small body size resulted in fewer midget offspring than expected and in a reduced phenotypic expression of the gene, though matings between these midget offspring gave all midget progeny.

A characteristic check in growth occurred between 15 and 21–24 days of age, and midgets rarely exceeded 10 g. at 6 weeks of age. They were 1·2 g. lighter than NS litter-mates at 3 weeks, and 3·6–4·4 g. lighter at 6 weeks. Fostering or transferring 3½-day-old midget embryos to the uteri of large mothers slightly increased the growth-rate, though the characteristic growth check still persisted. Transfer as embryos did not influence the midget phenotype.

Midget males were of near-normal fertility. The females were semi-sterile due to irregularity or absence of oestrous cycles, failure to ovulate, or the death of embryos before implantation. Each of these factors, which were also found in NS mice, could be remedied by the administration of exogenous hormones.

The genetic factors underlying the expression of midget are discussed, and the physiological traits associated with this and with other dwarfing genes are compared.

The size and endocrine activity of the pituitary in mice selected for large or small body-size

Mice of two strains, N and C, were used in studies on body-size, pituitary size, and endocrine potency of the pituitary. Strain N had been selected for large (NL) and small (NS) body-size; strain C had also been selected for large body-size (CL) but had been crossed to an outbred strain segregating pituitary dwarfism.

Pituitary weights and body-weights were highly correlated, the regression lines being common in NL and NS mice. Female pituitaries were considerably heavier than male pituitaries in CL mice. In relation to body-weight, CL pituitaries were consistently heavier than those of NL or NS mice.

No differences were detected in the unit potency of gonadotrophins in the pituitaries of NL and NS mice as estimated by the uterine response of immature outbred mice to subcutaneous injections of pituitary tissue. The uptake of 131I into the thyroid was comparable in NL and NS mice per unit of body-weight, and the thyroid secretion rate was also similar using animals of the same body-weight. Immature mice of both lines responded by increased growth to injections of growth hormone or fresh mouse pituitary, though the response was greater in NS than in NL mice.

The primary response to selection has probably been in the size of the pituitary rather than in its unit potency. The interrelationships between body-size, body components, organ size and endocrine levels are discussed.

Ovarian structure in mice lines selected for weight

Selection for body weight at 49 day of age (s and h, downward selected lines; s' and h', upward selected lines) affected reproductive traits in CF1 mice lines. The objective of this study was to compare ovarian structures in females of these lines, as well as in unselected controls (Line t). The number of ovarian follicles (N), follicle diameter (FD), number of corpora lutea (CL), litter size (LS), and body weight (W), were recorded. There were significant differences among lines for N, FD, CL, LS and W; means values for the lines with the greatest difference for post-pubertal females were: N(s) = 19.3 and N(s') = 32.7; FD(h') = 161.7 and FD(s') = 178.2; CL(h) = 10.3 and CL(s') = 21.9; LS(s) = 6.0 and LS(h') = 11.1; W(h) = 18.9 and W(s') = 32.4. There were also differences between positive lines; Line s' had a higher proportion of large follicles in pre-pubertal females, a greater capacity to convert these follicles into CL, but a lower capacity to maintain embryos until term than Line h'. For negative lines, Line h apparently had a reduced incidence of embryonic loss when compared with Line s. In conclusion, selection for body weight modified ovarian structure, as well as reproductive efficiency.

Semen production in two rabbit lines divergently selected for 63-d body weight

Thirty-one bucks from two lines divergently selected for 63-d body weight (low, L and high, H) were solicited every week (twice at a 15min interval) during 18 weeks resulting in 482 ejaculates. While differing markedly on adult body weight (L: 4650g versus H: 5925g), both lines had the same testis weight. Libido did not differ between the lines. The proportion of ejaculates suitable for insemination was markedly higher in the L line (66.5% versus 44.2%). Mass motility and the volume of the ejaculates were higher in the L line while the sperm concentration was higher in the H line. Overall, the total number of spermatozoa per ejaculate was similar in both lines but the efficient number of spermatozoa per ejaculate, a synthetic criterion taking into account the ability of the ejaculate for insemination was higher in the L line (229 versus 170x10(6)). The L line had higher values of average path velocity, linearity and curvilinear velocity but a lower value of beat cross frequency. In the L line, both ejaculates had the same concentration, while in the H line, the first ejaculate was more concentrated than the second one. Some male reproductive traits are therefore genetically related to body weight.

Life history and bioeconomy of the house mouse

1. More is known about the western European house mouse, Mus (musculus) domesticus than any other non-human mammal. If laboratory and field information is combined, an extremely valuable understanding of the species' bioeconomy could be obtained. 2. The seven stages of mouse life-history are surveyed (up to birth, nest life, sex life, social structure, population statics and stability, senescence, and death), and the interactions between the changing phenotype and the environment are described. 3. These interactions can be used to build up a model of the opportunities and compromises which result in the fitness of individual mice. It is not yet possible to quantify such a model, but this should in principle be achievable.

Reversal of stress-induced pregnancy blocks in mice by progesterone and metyrapone

It is established that diverse psychological stressors administered in the first trimester of pregnancy can cause the pregnancy to fail. The physiological mediators of this phenomenon are unknown. In Experiment 1, two strains of female mice were inseminated and randomly assigned to either first trimester physical restraint, restraint plus progesterone, or control conditions. Restrained females produced fewer pregnancies than did controls, and a reversal of this block through the daily administration of 500 micrograms progesterone was seen in HS but not C57 mice. Experiment 2 demonstrated that exposure to a predator will also block pregnancy in the first trimester in C57 mice, and that this can also be counteracted with progesterone administration. Experiment 3 showed that metyrapone, a drug which blocks the conversion of progesterone to corticosterone, also partially reverses a restraint-induced pregnancy block. These results suggest that lowered progesterone levels may be involved in pregnancy blocks induced by psychological stress.

Effects of selection on growth, body composition and food intake in mice. II. Correlated responses in reproduction

Female reproductive performance is reported in mice selected for ten generations for one of three criteria: either appetite (A), fat percentage (F) or total lean mass (P). For each criterion lines were selected for high (H) or low (L) performance, with contemporary unselected controls (C). In theAandPlines, litter size changed in the direction of the selected criterion, the changes being larger and more rapidly established in theAthan in thePlines. At generation 10, the differences in litter size between high and low lines were 2·6 live young born in theAlines, and 1·0 live young born in thePlines. The differences in 6-week weight between the high and low lines were 3·5 g in theAlines, 6·5 g in thePlines. Changes in ovulation rate were the primary reason for changes in litter size, the differences between the high and low lines being 3·8 corpora lutea for theAlines, and 3·1 corpora lutea for thePlines. Fitting body weight at mating as a covariate within lines in the analysis of ovulation rate and live foetus number removed the differences between the high and low selectedPlines, but not those in theAlines. The high and low selectedAandPlines did not differ in prenatal survival. There were no consistent differences in litter size, ovulation rate or pre-natal survival in theFlines.

Effects of selection for postweaning gain on testicular function in mice

Changes in testicular and epididymal function and selected endocrine organ size were examined in a line of mice selected for rapid postweaning gain (M16) and in reciprocal crosses with an unselected pedigree control line (ICR). The larger body weight of the M16 line was accompanied by larger testes, epididymides, seminal vesicle, pituitary, thyroid and adrenal weights (P smaller than .01) although the testes, epididymides, seminal vesicle and adrenal weights of M16 mice, expressed per gram body weight, actually decreased (P smaller than .01) relative to the ICR line. Testicular and epididymal sperm reserves were higher in M16 mice but the difference was not significant. However, when adjusted for gland size, testicular and epididymal sperm reserves were lower in the M16 line (P smaller than .01). Absolute and relative weights of testes, epididymides and pituitary were larger (P smaller than .01) in M16 male times ICR female crosses than in ICR male times M16 female crosses. Although testicular and epididymal sperm reserves were higher in M16 male times ICR female males the reciprocal difference was significant only for testicular sperm (P smaller than .05). Heterotic effects were significant for both absolute and relative weights of testes, epididymides (P smaller than.01), pituitary (P smaller than .05) and relative weight of siminal vesicles (P smaller than .05). Although percent heterosis was 8.5 for testicular and epididymal sperm reserves, significant heterotic effects were found only for epididymal sperm (P smaller than .05).

Preweaning growth of inbred, F 1 hybrid, and random-bred mice as a measure of mother's lactation

A cross-fostering experiment was performed in which inbred and random-bred mothers suckled three groups of young: inbred (the SWR strain), F1hybrid (SWR × C3H), and random-bred (originally synthesized from SWR, C3H, CBA and C57BL). Two inbred mothers and one random-bred mother were placed in each of 43 cross-fostering sets, and each mother suckled two of her own young and two from each of the other two mothers. Body weights of young were recorded individually at birth and at 12 days.In 12-day weight and gain from birth to 12 days, differences among the three groups of young and differences between inbred and random-bred mothers were significant (P < 0.01). Although the ratios of the between-dam to within-dam mean squares in log scale varied among nine mother-young subgroups, the differences between any pair of these ratios were statistically insignificant. It was therefore concluded that in detecting lactation differences among mothers the value of a given type of young was not different between inbred and random-bred mothers and that as a means of detecting lactation differences among random-bred mothers, inbred and F1hybrid young did not appear to offer any substantial advantage over random-bred young.

Correlated response in male and female sterility to selection for pupa weight in Tribolium castaneum

The correlated responses in male and female sterility to 50 generations of individual selection for pupa weight in Tribolium were analyzed. Two replicate lines (S-lines) were selected for heavier pupa weight and stabilizing selection for pupa weight was practiced in two replicate control lines (C-lines). There was close agreement between replicates in both sets of lines for direct and correlated responses. The rate of inbreeding has been constant for all lines (approximately 0.5% per generation).Regression of generation means for pupa weight on generation of selection indicated a significant linear regression in the direct response for both lines. The linear increases of 46 and 55 μg. per generation in the S-lines accounted for 98% of the variation among generations and the linear decreases of 5 and 10 μg. per generation in the C-lines accounted for 70-90% of the variation in the generation means.Maximum likelihood estimators were used to calculate the frequency of male and female sterility for each generation and line. Average sterility in the base population ranged from about 4 to 12% for both sexes. Polynomial regressions of percent sterility on generation of selection showed that quadratic and higher order regressions were occasionally significant but accounted for a relatively small fraction of the total variation. In the two S-line replicates, linear regression coefficients of percent sterility on generation number were 0.16±.09 and 0.20±.07 for males and 0.72±.08 and 0.54±.08 for females, suggesting a larger correlated response in female than in male sterility. In the C-lines, linear regression coefficients were 0.02±.08 and -.12±.05 for males in the two replicates and -.05±.05 and -.05±.05 for females. Estimates of realized genetic correlations between pupa weight and sterility in the S-lines ranged from 0.04 to 0.14 for males and from 0.14 to 0.37 for females when the heritability of sterility was allowed to take on values from 0.05 to 0.25.

Genetic and phenotypic relationships between ovulation rate and body weight in the mouse

The genetic and phenotypic regressions and correlations between ovulation rate and body weight were examined in a random bred strain (Q) of laboratory mice during the course of three experiments. These experiments were (1) a sib analysis; (2) selection for natural and induced primiparous ovulation rate; and (3) replicated selection for 6-week weight. The following results were obtained:

(a) The genetic correlations between body weight and natural and induced ovulation rate were positive, and approximately equal to 0·4 and 0·6 respectively.

(b) The genetic regressions of natural and of induced ovulation rate on body weight were approximately 0·4 and 0·9 eggs per gram respectively.

(c) The genetic regressions of body weight on natural and on induced ovulation rate were approximately 0·5 and 0·25 g per egg respectively.

(d) The phenotypic correlation between natural ovulation rate and body weight was approximately 0·4 and the corresponding regression of ovulation rate on body weight approximately 0·4 eggs per gram.

(e) The phenotypic correlation between induced ovulation rate and body weight declined from 0·4 at 6 weeks of age to zero at the time of scoring, the corresponding regressions of ovulation rate on body weight declining from 0·1 eggs per gram to zero.

It was concluded that natural ovulation rate itself, and both its components (FSH activity and ovarian sensitivity) are positively genetically correlated with body weight. Furthermore, the observation that large mice shed at least as many eggs as small ones in response to the same dose of PMS showed that the response was more closely related to the absolute dose than to the resultant concentration.

Genetic studies of ovulation rate in the mouse

Genetic and physiological studies of ovulation rate, both natural and induced, in a random bred population of laboratory mice led to the following results.

(1) The heritability of natural ovulation rate in nulliparous females was 22±19%, estimated from the correlation between paternal half-sibs.

(2) Selection applied to natural ovulation in primiparous females led to a response in both directions, with a realized heritability of 31% (h2within litters = 0·18 ± 0·013). The number of eggs shed by the High, Control and Low lines after 12 generations were respectively 21, 16 and 14.

(3) Selection applied to ovulation induced by 4 i.u. of PMS led also to a response in both directions, with a realized heritability of 22% (h2within litters = 0·11 ± 0·038). The induced ovulation rates of the High, Control and Low lines after 12 generations were respectively 29, 19 and 14 eggs.

(4) PMS-equivalents were calculated from dose-response curves, and were found to be nearly the same in all lines except the line selected for high natural ovulation rate, which had a PMS-equivalent nearly double that of the Control. From this it was inferred that the increase of ovulation rate resulting from selection for high natural ovulation was due to an increased FSH activity, whereas the changes of ovulation rate resulting from selection for low natural ovulation and for both high and low induced ovulation were due to changes of ovarian sensitivity.

(5) Genetic correlations, estimated from correlated responses to selection, were: (i) between natural and induced ovulation in primiparous females, 0·33; (ii) between primiparous and nulliparous females in natural ovulation, 0·46; (iii) between primiparous and nulliparous females in induced ovulation, 0·82.

(6) The selection for induced ovulation produced changes in litter size following natural ovulation that were nearly equal to the changes in natural ovulation rate. The selection for natural ovulation, however, produced no clear changes of litter size.

The limits to artificial selection for body weight in the mouse. II. The genetic nature of the limits

1. The effects of long-continued selection for body weight in two lines of mice, one large and one small, are described.

2. The large line showed a sharp increase in weight after remaining at an apparent limit for twenty generations. A rare combinational event is suggested as the most likely explanation.

3. Reversed and relaxed selection from the large line at the limit failed to yield any response. This indicates that effectively, the additive genetic variance in this line had been exhausted.

4. In contrast, the small line at the limit regressed slightly towards the base population when selection was relaxed. Reversed selection yielded a ready response until a new limit was apparently reached. Loci affecting body weight in this line had therefore not been fixed by selection.

5. Natural selection, operating on viability between conception and the time when the selection was made, appears to explain best the lack of fixation in the small line.

6. Attention is drawn to the necessity of more experimental work to elucidate the genetic nature of the limits to artificial selection.