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

Variation in within-bone stiffness measured by nanoindentation in mice bred for high levels of voluntary wheel running

The hierarchical structure of bone, involving micro-scale organization and interaction of material components, is a critical determinant of macro-scale mechanics. Changes in whole-bone morphology in response to the actions of individual genes, physiological loading during life, or evolutionary processes, may be accompanied by alterations in underlying mineralization or architecture. Here, we used nanoindentation to precisely measure compressive stiffness in the femoral mid-diaphysis of mice that had experienced 37 generations of selective breeding for high levels of voluntary wheel running (HR). Mice (n = 48 total), half from HR lines and half from non-selected control (C) lines, were divided into two experimental groups, one with 13-14 weeks of access to a running wheel and one housed without wheels (n = 12 in each group). At the end of the experiment, gross and micro-computed tomography (microCT)-based morphometric traits were measured, and reduced elastic modulus (E(r)) was estimated separately for four anatomical quadrants of the femoral cortex: anterior, posterior, lateral, and medial. Two-way, mixed-model analysis of covariance (ancova) showed that body mass was a highly significant predictor of all morphometric traits and that structural change is more apparent at the microCT level than in conventional morphometrics of whole bones. Both line type (HR vs. C) and presence of the mini-muscle phenotype (caused by a Mendelian recessive allele and characterized by a approximately 50% reduction in mass of the gastrocnemius muscle complex) were significant predictors of femoral cortical cross-sectional anatomy. Measurement of reduced modulus obtained by nanoindentation was repeatable within a single quadrant and sensitive enough to detect inter-individual differences. Although we found no significant effects of line type (HR vs. C) or physical activity (wheel vs. no wheel) on mean stiffness, anterior and posterior quadrants were significantly stiffer (P < 0.0001) than medial and lateral quadrants (32.67 and 33.09 GPa vs. 29.78 and 30.46 GPa, respectively). Our findings of no significant difference in compressive stiffness in the anterior and posterior quadrants agree with previous results for mice, but differ from those for large mammals. Integrating these results with others from ongoing research on these mice, we hypothesize that the skeletons of female HR mice may be less sensitive to the effects of chronic exercise, due to decreased circulating leptin levels and potentially altered endocannabinoid signaling.

Voluntary exercise and its effects on body composition depend on genetic selection history

Little is known about how genetic variation affects the capacity for exercise to change body composition. We examined the extent to which voluntary exercise alters body composition in several lines of selectively bred mice compared to controls. Lines studied included high runner (HR) (selected for high wheel running), M16 (selected for rapid weight gain), Institute of Cancer Research (ICR) (randomly bred as control for M16), M16i (an inbred line derived from M16), HE (selected for high percentage of body fat while holding body weight constant), LF (selected for low percentage of body fat), C57BL/6J (common inbred line), and the F1 between HR and C57BL/6J. Body weight and body fat were recorded before and after 6 days of free access to running wheels in males and females that were individually caged. Total food intake was measured during this 6-day period. All pre- and postexercise measures showed significant strain effects. While HR mice predictably exercised at higher levels, all other selection lines had decreased levels of wheel running relative to ICR. The HR x B6 F1 ran at similar levels to HR demonstrating complete dominance for voluntary exercise. Also, all strains lost body fat after exercise, but the relationships between exercise and changes in percent body were not uniform across genotypes. These results indicate that there is significant genetic variation for voluntary exercise and its effects on body composition. It is important to carefully consider genetic background and/or selection history when using mice to model effects of exercise on body composition, and perhaps, other complex traits as well.

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.

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.

Long-term restricted index selection in mice designed to change fat content without changing body size

The objective of this study was to determine if low secondary selection differentials, caused by selecting within full-sib families, may have accounted for the failure of an intended restricted selection index to reduce epididymal fat pad weight (EF) without changing body weight (BW) in mice. Replicate lines that had been selected within full-sib families for high (HE) or low (LE) EF, while holding BW constant, were crossed. After two generations of random mating, two replicates were sampled and selection initiated for the same restricted index criteria except that mass selection was used to increase the selection differentials. In both phases of selection the HE restricted index selection, designed to increase EF without altering BW, was in agreement with expectation. In contrast, the LE index, designed to decrease EF without changing BW, did not agree with theory since BW increased while EF decreased only slightly. Therefore, reduced selection differentials could not explain the deviation from theory. A possible explanation may reside in the restricted selection index being more sensitive to changes in genetic parameters due to shifts in gene frequency as a consequence of the selection applied. However, linkage disequilibrium and genetic drift can not be ruled out as contributing factors to the asymmetry of response.

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

Correlated responses were studied in lines of mice selected for eight generations based on the criterion of a restricted selection index. Two replicate lines were selected in each treatment as follows: HE, high epididymal fat pad weight (EF) with zero change in body weight (BW) at 12 weeks of age; LE; low EF with zero change in BW; and RS, randomly. Correlated responses showed considerable variation between replicates, suggesting that genetic drift was important. Further, correlated responses for most traits were relatively small, probably because of low selection intensity. The HE line responded as expected in component traits of the restricted index. Associated compositional traits in HE responded as predicted since traits correlated with adiposity increased and hind carcass weight did not change significantly. Feed intake increased and feed efficiency (weight gain/feed intake) decreased in HE, as predicted. In contrast, the LE line did not respond in component traits as predicted since EF did not decrease and BW increased. Consequently, LE exhibited little change in traits associated with adiposity, but hind carcass weight, feed intake and feed efficiency increased. Of the correlated responses scored for fitness traits (littering rate, number of days from pairing of mate to littering, litter size and preweaning pup survival rate), significant effects were found for decreased littering rate in LE and increased prenatal survival rate in HE. In summary, correlated responses to restricted index selection generally agreed with expectation when responses in component traits of the index were considered.