Effects of selection for independent changes in two highly correlated body weight traits of mice

The effects of selection for high and low body weight on the proportion and distribution of fat in mice

The growth of adipose tissue was investigated in lines of mice selected for high and low body weight at 5 and 10 weeks of age, by dissecting and weighing individual fat depots from mice aged 5, 7·5, 10 and 15 weeks. At fixed ages most depots were heavier in the High lines and lighter in the Low lines. Depots grew at different rates and the rate of total fat deposition relative to gain in body weight was greater in High than in Low lines. At about 20g live weight High and Low lines had similar proportions of total fat; at lower weights the High lines were relatively leaner. These correlated effects of selection were more pronounced in the faster growing depots of the kidneys and gonads. In consequence, the High an d Low lines had different fat distributions at fixed ages, at fixed body weights and at fixed weights of fat.

A CENTENNIAL CELEBRATION FOR QUANTITATIVE GENETICS

Quantitative genetics is at or is fast approaching its centennial. In this perspective I consider five current issues pertinent to the application of quantitative genetics to evolutionary theory. First, I discuss the utility of a quantitative genetic perspective in describing genetic variation at two very different levels of resolution, (1) in natural, free-ranging populations and (2) to describe variation at the level of DNA transcription. Whereas quantitative genetics can serve as a very useful descriptor of genetic variation, its greater usefulness is in predicting evolutionary change, particularly when used in the first instance (wild populations). Second, I review the contributions of Quantitative trait loci (QLT) analysis in determining the number of loci and distribution of their genetic effects, the possible importance of identifying specific genes, and the ability of the multivariate breeder's equation to predict the results of bivariate selection experiments. QLT analyses appear to indicate that genetic effects are skewed, that at least 20 loci are generally involved, with an unknown number of alleles, and that a few loci have major effects. However, epistatic effects are common, which means that such loci might not have population-wide major effects: this question waits upon (QTL) analyses conducted on more than a few inbred lines. Third, I examine the importance of research into the action of specific genes on traits. Although great progress has been made in identifying specific genes contributing to trait variation, the high level of gene interactions underlying quantitative traits makes it unlikely that in the near future we will have mechanistic models for such traits, or that these would have greater predictive power than quantitative genetic models. In the fourth section I present evidence that the results of bivariate selection experiments when selection is antagonistic to the genetic covariance are frequently not well predicted by the multivariate breeder's equation. Bivariate experiments that combine both selection and functional analyses are urgently needed. Finally, I discuss the importance of gaining more insight, both theoretical and empirical, on the evolution of the G and P matrices.

Correlated responses in body composition to divergent selection for exponential growth rate to 14 or 42 days of age in chickens

Chicks divergently selected for 14-d (14H and 14L) or 42-d (42H and 42L) exponential growth rate (EGR) over five generations were used to determine correlated responses between growth at different ages and body composition. Regression coefficient estimates across five generations of selection were not significant for any line at either age for percentage total body water or protein. Genetic correlations between EGR from hatching to 14 d of age (EGR14) and 42-d percentage carcass fat were -0.18, -0.57, 0.63, and -0.79 among the 14H, 14L, 42H, and 42L lines, respectively. Genetic correlations between EGR from hatching to 42 d of age (EGR42) and 42-d percentage carcass fat were 0.09, -0.67,0.50, and -0.75 among the 14H, 14L, 42H, and 42L lines, respectively. During the short-term selection experiment, selection for fast EGR14 or EGR42 increased fat at the age of selection. However, selection for fast EGR42 increased body weight and percentage fat at 42 d of age (DOA), whereas selection for fast EGR14 increased body weight but not fat at 42 DOA. Therefore, it is possible to simultaneously select for high body weight at, or near, the inflection point of the growth curve without increasing fat deposition or obesity by taking advantage of the lack of a genetic correlation between EGR14 and body fat percentage at later ages.

Body weight and tail length divergence in mice selected for rate of development

A series of mouse lines has been produced by 19 generations of restricted index selection for rate of development during early and late ontogeny. The selection program was based on an index with the following four replicated selection treatments: E(+) and E(-) were selected to alter birth to 10-day body weight gain while holding late gain for both selection lines constant; correspondingly, L(+) and L(-) were selected to alter 28- to 56-day body weight gain holding early gain for both lines constant. Herein, we characterize response to selection for growth rate by analyzing age-specific mouse body weight and tail lengths and for growth curves using a logistics model. Selection on developmental rate has resulted in divergence in both age-specific and growth curve traits. E(+) and L(+) lines reached identical weights during the late selection interval, then diverged to unique mature weights. E(-) and L(-) lines similarly achieved identical weights during late selection and diverged to unique mature weights. However, the shapes of early and late growth curves were significantly divergent, and at least two distinct growth patterns are shown to result from selection. Response in body weight gain was accompanied by similar, though less pronounced, change in tail length traits. Significant response during intervals of restricted growth was also found, especially in lines selected for late gain. The evolution of the growth trajectory under restricted index selection is discussed in terms of drift and available additive genetic variation and covariation.

Asymptotic weight and maturing rate in mice selected for body conformation

Growth patterns of four lines of mice selected for body conformation were analyzed with the logistic function, in order to provide baseline information about the relationship between asymptotic weight and maturing rate of body weight. Two lines were divergently selected favoring the phenotypic correlation between body weight and tail length (agonistic selection: CBi+, high body weight and long tail; CBi-, low body weight and short tail), whereas the other two lines were generated by a disruptive selection performed against the correlation between the aforementioned traits (antagonistic selection: CBi/C, high body weight and short tail; CBi/L, low body weight and long tail). The logistic parameters A (asymptotic weight) and k (maturing rate) behaved in CBi/C and CBi- mice and in CBi+ females as expected in terms of the negative genetic relationship between mature size and earliness of maturing. An altered growth pattern was found in CBi/L mice and in CBi+ males, because in the former genotype, selected for low body weight, the time taken to mature increased, whereas in the latter, selected for high body weight, there was a non-significant increase in the same trait. In accordance with the selective criterion, different sources of genetic variation for body weight could be exploited: one inversely associated with earliness of maturing (agonistic selection), and the other independent of maturing rate (antagonistic selection), showing that genetic variation of A is partly independent of k.

Estimation of changes in genetic parameters in selected lines of mice using REML with an animal model. 2. Body weight, body composition and litter size

Restricted Maximum Likelihood (REML) with an animal model was used to estimate genetic parameters of body weight, body consumption and litter size of lines of mice selected for 20 generations on an index of lean mass at 10 weeks in males, highly correlated with body weight, and for a further 18 generations on body weight at 10 weeks in males and females. Univariate and multivariate estimates of heritability were about 0.5 and those of common environment correlations were about 0.25 for both body weight and composition. Body weight and fat pad weight had genetic and phenotypic correlations of about 0.5. The heritability estimate of litter size was about 0.15 from univariate analysis, rather lower from multivariate, and the estimate of its genetic correlation with body weight was about 0.25. There were reductions in heritability of both body weight and litter size in later generations, even though full pedigrees were fitted and inferences made to the base population, but a plateau in response to selection for increased body weight could not be explained by a complete attenuation of genetic variance.

Asymmetrical correlated responses to selection under an infinitesimal genetic model

Asymmetry in correlated responses to selection is expected when more than one cycle of selection is practised due to changes in genetic parameters produced by selection. In large populations, under the infinitesimal model these changes are due to linkage disequilibrium generated by selection and not to gene frequency changes. This study examines the conditions under which asymmetrical correlated responses are to be expected when an infinitesimal model is considered. Asymmetrical correlated responses in two traits in respect to which trait is selected are expected if the two traits have different heritabilities. Predicted asymmetry increases with the absolute value of the genetic correlation between the two traits, the difference between the two heritabilities, the intensity of selection and the number of generations of selection. Linkage disequilibrium generated by selection should be taken into account in explaining asymmetrical correlated responses observed in selection experiments.

Restricted index selection in mice designed to change body fat without changing body weight: direct responses

Replicated within full-sib family restricted index selection was conducted for eight generations in mice for high or low epididymal fat pad weight (EF) holding body weight (BW) constant. Pooled realized heritability estimates of index units based on high, low and divergent selection were 0.42±0.20, 0.44±0.19 and 0.42± 0.05, respectively, which were not different from the base population estimate of 0.33±0.10. Realized responses per generation pooled across replicates in the high-fat restricted index lines were in the expected directions for EF (17.5±7.2 mg; P<0.05) and BW (0.03±0.58 g; P>0.05), but responses in the low-fat restricted index lines were discrepant for EF (0.3±5.1 mg; P>0.05) and BW (0.38±0.01 g; P<0.01). Consequently, the realized responses in component traits were decidedly asymmetric (P<0.05). A technique for estimating realized genetic parameters from index selection lines gave realized heritabilities for BW and EF of 0.68±0.04 and 0.45±0.05, respectively, and a realized genetic correlation between BW and EF of 0.93±0.01 compared with base population estimates of 0.43±0.08, 0.49±0.10 and 0.78±0.05, respectively. Possible explanations for the disparity between observed and expected responses in the low-fat restricted index lines include genetic drift, poor estimates of base population parameters, changes in genetic parameters with selection, linkage disequilibrium resulting from selection and asymmetric realized relative index weights.

Phenotypic and genetic variability of estimated growth curve parameters in mice

Data from 1,919 outbred ICR mice were used to examine the potential usefulness of growth curve parameters as selection criteria for altering the relationship between body weight and age. A logistic growth function was used to model growth through 12 weeks of age. Estimates of asymptotic weight (A), maximum growth rate (r) and age at point of inflection (t(*)) were obtained by nonlinear least-squares. A log transformation was also used to stabilize residual variance. Phenotypic and genetic parameters were estimated for the estimated growth curve parameters and for body weights at 2, 3, 4.5, 6, 8 and 12 weeks of age. Heritabilities of estimated growth curve parameters (obtained with and without a log transformation, respectively) were: A (0.28±0.07, 0.28±0.07), r (0.35±0.07, 0.53±0.09) and t(*) (0.41±0.08, 0.44±0.08). Estimated genetic correlations suggest that t(*) may be useful in selecting for rapid early growth without increasing mature weight.

Changes in genetic parameters under restricted index selection

The ability of restricted selection indices to prevent genetic change in a restricted trait over several generations of selection was studied using deterministic computer models. Four loci, two affecting each trait independently, and two pleiotropic loci, one affecting each trait in the same direction, and one with opposite effects, were modelled. In general, continued effectiveness of the restriction was achieved only when the restricted trait was affected by only one locus. In some conditions (equal gene frequencies), an independent locus and one pleiotropic locus affecting the restricted trait allowed maintenance of the restriction. The results suggest that long-term restriction may be very difficult without re-estimation of parameters.

A genetic analysis of targeted growth in mice

Effects of normal growth regulation on components of phenotypic variance and covariance of body weight were examined in a cross-fostering study of growth between 2 and 10 wk of age in ICR randombred mice. Different early growth rates caused genetic, postnatal maternal and residual environmental variances to increase, but these variances were subsequently reduced by negative autocorrelation between early and later growth. Postnatal maternal variance continued to increase for about 1 wk after weaning but then decreased substantially. Genetic variance caused by preweaning growth followed a pattern of increase and decrease very similar to that of postnatal maternal variance, but this pattern was masked by new genetic variance. Normal growth regulation affects the magnitudes of genetic variances and serial autocorrelations . The timing of these changes suggests that regulation of cell numbers reduces variance near the end of exponential growth, but this may be obscured by subsequent increase in cell size. In contrast with earlier studies, we find that targeted growth reduces both genetically and environmentally determined differences among early growth trajectories. Final size may be determined by an antagonistic balance between early growth rate and age at initiation of puberty.

Effects of selection on growth, body composition and food intake in mice. I. Responses in selected traits

Mice were selected for one of three criteria: appetite (A), measured as 4- to 6-week food intake, adjusted by phenotypic regression to minimize change in 4-week body weight, fat percentage (F), using the ratio of gonadal fat pad weight to body weight in 10-week-old males, and total lean mass (protein, P), using the index, body weight in 10-week-old males − (8 × gonadal fat pad weight). For each selection criterion, there were 3 high, 3 low and 3 unselected control lines. At generation 11, the high and low A lines diverged by 17% of the control mean and the realized heritability from within family selection of adjusted food intake was 15%. Selection for this character produced changes in body weight, gross efficiency from 4 to 6 weeks, and percentage of fat, the high lines being heavier, more efficient and less fat than the lows. The high and low F lines diverged by 80% of the control mean and the realized heritability of the ratio of gonadal fat pad weight to body weight was 44%. Selection for this character produced changes in total fat per cent, but little change in percentage protein, body weight, food intake or gross efficiency. The high and low P lines diverged by 40% of the control mean and realized heritability of the lean mass index (10-week weight − [8 × gonadal fat pad weight]) was 51%. Selection for an increase in the index increased body weight at all ages, food intake and 4- to 6-week gross efficiency. There was no change in percentage fat. Responses in the selected traits were not highly correlated, and the different lines provide an opportunity for investigating responses in physiology, metabolism and gene products.

Correlated changes in feeding behavior on selection for large and small body size in mice

An automated method was used to record the temporal pattern of feeding of lines of mice selected over 15 generations for high and low body weight (L-mice and S-mice, respectively). Both L-mice and S-mice eat in meals concentrated during the night, and meal frequency is similar in the two lines, but L-mice consume much larger meals, each made up of many more separate feeding bouts. The outbred strain from which the selected lines were derived has a similar basic pattern of feeding in meals, which becomes like that of L-mice when the animal's thermogenic metabolic rate is high, and like that of S-mice when it is low, suggesting that the differences between the feeding patterns of the two selected lines are a secondary consequence of alterations in whole body metabolic rate.

The effects of selection for different combinations of weights at two ages on the growth curve of mice

The weights of mice in lines selected for different combinations of high and low body weights at 5 and at 10 weeks of age were recorded from 3 to 21 weeks of age. The average growth curve for each line was computed using the Gompertz function. The growth curves of lines selected for high or low weight at a single age (ST lines) showed large differences in estimates of mature size and small differences in estimates of maturing rate, i.e. of the relative rate of growth to maturity. The growth curves of lines selected by independent culling for divergent combinations of deviations of opposite sign in 5- and 10-week weights (ICL lines) showed little difference in estimates of mature size and a large difference in estimates of maturing rate. The growth curves of lines selected by index for divergence in 5-week weight with no change in 10-week-weight or for divergence in 10-week-weight with no change in 5-week weight showed large differences in estimates of mature size and large differences in estimates of the maturing rate. The relationship between mature size and maturing rate was affected in different ways by the three types of selection.

A replicated single generation test of a restricted selections index in poultry

Predicted and realized responses in a single generation of mass selection for an index and for its component traits were compared. The index included the log transformed traits determining egg mass in chickens to 40 weeks of age (days tested from sexual maturity, egg weight, rate of lay). The index was restricted to allow no increase in log days tested. Other traits measured were egg mass, age at first egg, egg weight, rate of lay, number of eggs and body weight. When averaged over replicates, realized and predicted responses were in close agreement for index values and for the component traits. Significant corresponding correlated responses were obtained for egg mass and weight. The restricted trait, log days tested, and the correlated trait age at first egg did not change. Egg mass was increased solely through change in egg weight.