The relative importance of genetics and phenotypic plasticity in dictating bone morphology and mechanics in aged mice: evidence from an artificial selection experiment

Large changes in detected selection signatures after a selection limit in mice bred for voluntary wheel-running behavior

In various organisms, sequencing of selectively bred lines at apparent selection limits has demonstrated that genetic variation can remain at many loci, implying that evolution at the genetic level may continue even if the population mean phenotype remains constant. We compared selection signatures at generations 22 and 61 of the "High Runner" mouse experiment, which includes 4 replicate lines bred for voluntary wheel-running behavior (HR) and 4 non-selected control (C) lines. Previously, we reported multiple regions of differentiation between the HR and C lines, based on whole-genome sequence data for 10 mice from each line at generation 61, which was >31 generations after selection limits had been reached in all HR lines. Here, we analyzed pooled sequencing data from ~20 mice for each of the 8 lines at generation 22, around when HR lines were reaching limits. Differentiation analyses of allele frequencies at ~4.4 million SNP loci used the regularized T-test and detected 258 differentiated regions with FDR = 0.01. Comparable analyses involving pooling generation 61 individual mouse genotypes into allele frequencies by line produced only 11 such regions, with almost no overlap among the largest and most statistically significant peaks between the two generations. These results implicate a sort of "genetic churn" that continues at loci relevant for running. Simulations indicate that loss of statistical power due to random genetic drift and sampling error are insufficient to explain the differences in selection signatures. The 13 differentiated regions at generation 22 with strict culling measures include 79 genes related to a wide variety of functions. Gene ontology identified pathways related to olfaction and vomeronasal pathways as being overrepresented, consistent with generation 61 analyses, despite those specific regions differing between generations. Genes Dspp and Rbm24 are also identified as potentially explaining known bone and skeletal muscle differences, respectively, between the linetypes.

Effects of long-term voluntary wheel running and selective breeding for wheel running on femoral nutrient canals

The nutrient artery provides ~50%-70% of the total blood volume to long bones in mammals. Studying the functional characteristics of this artery in vivo can be difficult and expensive, so most researchers have measured the nutrient foramen, an opening on the outer surface of the bone that served as the entry point for the nutrient artery during development and bone ossification. Others have measured the nutrient canal (i.e., the passage which the nutrient artery once occupied), given that the external dimensions of the foramen do not necessarily remain uniform from the periosteal surface to the medullary cavity. The nutrient canal, as an indicator of blood flow to long bones, has been proposed to provide a link to studying organismal activity (e.g., locomotor behavior) from skeletal morphology. However, although external loading from movement and activity causes skeletal remodeling, it is unclear whether it affects the size or configuration of nutrient canals. To investigate whether nutrient canals can exhibit phenotypic plasticity in response to physical activity, we studied a mouse model in which four replicate high runner (HR) lines have been selectively bred for high voluntary wheel-running behavior. The selection criterion is the average number of wheel revolutions on days 5 and 6 of a 6-day period of wheel access as young adults (~6-8 weeks old). An additional four lines are bred without selection to serve as controls (C). For this study, 100 female mice (half HR, half C) from generation 57 were split into an active group housed with wheels and a sedentary group housed without wheels for 12 weeks starting at ~24 days of age. Femurs were collected, soft tissues were removed, and femora were micro-computed tomography scanned at a resolution of 12 μm. We then imported these scans into AMIRA and created 3D models of femoral nutrient canals. We tested for evolved differences in various nutrient canal traits between HR and C mice, plastic changes resulting from chronic exercise, and the selection history-by-exercise interaction. We found few differences between the nutrient canals of HR versus C mice, or between the active and sedentary groups. We did find an interaction between selection history and voluntary exercise for the total number of nutrient canals per femur, in which wheel access increased the number of canals in C mice but decreased it in HR mice. Our results do not match those from an earlier study, conducted at generation 11, which was prior to the HR lines reaching selection limits for wheel running. The previous study found that mice from the HR lines had significantly larger total canal cross-sectional areas compared to those from C lines. However, this discrepancy is consistent with studies of other skeletal traits, which have found differences between HR and C mice to be somewhat inconsistent across generations, including the loss of some apparent adaptations with continued selective breeding after reaching a selection limit for wheel-running behavior.

Multiple solutions at the genomic level in response to selective breeding for high locomotor activity

Replicate lines under uniform selection often evolve in different ways. Previously, analyses using whole-genome sequence data for individual mice (Mus musculus) from 4 replicate High Runner lines and 4 nonselected control lines demonstrated genomic regions that have responded consistently to selection for voluntary wheel-running behavior. Here, we ask whether the High Runner lines have evolved differently from each other, even though they reached selection limits at similar levels. We focus on 1 High Runner line (HR3) that became fixed for a mutation at a gene of major effect (Myh4Minimsc) that, in the homozygous condition, causes a 50% reduction in hindlimb muscle mass and many pleiotropic effects. We excluded HR3 from SNP analyses and identified 19 regions not consistently identified in analyses with all 4 lines. Repeating analyses while dropping each of the other High Runner lines identified 12, 8, and 6 such regions. (Of these 45 regions, 37 were unique.) These results suggest that each High Runner line indeed responded to selection somewhat uniquely, but also that HR3 is the most distinct. We then applied 2 additional analytical approaches when dropping HR3 only (based on haplotypes and nonstatistical tests involving fixation patterns). All 3 approaches identified 7 new regions (as compared with analyses using all 4 High Runner lines) that include genes associated with activity levels, dopamine signaling, hippocampus morphology, heart size, and body size, all of which differ between High Runner and control lines. Our results illustrate how multiple solutions and "private" alleles can obscure general signatures of selection involving "public" alleles.

Effects of selective breeding for voluntary exercise, chronic exercise, and their interaction on muscle attachment site morphology in house mice

Skeletal muscles attach to bone at their origins and insertions, and the interface where tendon meets bone is termed the attachment site or enthesis. Mechanical stresses at the muscle/tendon-bone interface are proportional to the surface area of the bony attachment sites, such that a larger attachment site will distribute loads over a wider area. Muscles that are frequently active and/or are of larger size should cause attachment sites to hypertrophy (training effect); however, experimental studies of animals subjected to exercise have provided mixed results. To enhance our ability to detect training effects (a type of phenotypic plasticity), we studied a mouse model in which 4 replicate lines of High Runner (HR) mice have been selectively bred for 57 generations. Selection is based on the average number of wheel revolutions on days 5 & 6 of a 6-day period of wheel access as young adults (6-8 weeks old). Four additional lines are bred without regard to running and serve as non-selected controls (C). On average, mice from HR lines voluntarily run ~3 times more than C mice on a daily basis. For this study, we housed 50 females (half HR, half C) with wheels (Active group) and 50 (half HR, half C) without wheels (Sedentary group) for 12 weeks starting at weaning (~3 weeks old). We tested for evolved differences in muscle attachment site surface area between HR and C mice, plastic changes resulting from chronic exercise, and their interaction. We used a precise, highly repeatable method for quantifying the three-dimensional (3D) surface area of four muscle attachment sites: the humerus deltoid tuberosity (the insertion point for the spinodeltoideus, superficial pectoralis, and acromiodeltoideus), the femoral third trochanter (the insertion point for the quadratus femoris), the femoral lesser trochanter (the insertion point for the iliacus muscle), and the femoral greater trochanter (insertion point for the middle gluteal muscles). In univariate analyses, with body mass as a covariate, mice in the Active group had significantly larger humerus deltoid tuberosities than Sedentary mice, with no significant difference between HR and C mice and no interaction between exercise treatment and linetype. These differences between Active and Sedentary mice were also apparent in the multivariate analyses. Surface areas of the femoral third trochanter, femoral lesser trochanter, and femoral greater trochanter were unaffected by either chronic wheel access or selective breeding. Our results, which used robust measurement protocols and relatively large sample sizes, demonstrate that muscle attachment site morphology can be (but is not always) affected by chronic exercise experienced during ontogeny. However, contrary to previous results for other aspects of long bone morphology, we did not find evidence for evolutionary coadaptation of muscle attachments with voluntary exercise behavior in the HR mice.

Rapid and longer-term effects of selective breeding for voluntary exercise behavior on skeletal morphology in house mice

Selection experiments can elucidate the varying course of adaptive changes across generations. We examined the appendicular skeleton of house mice from four replicate High Runner (HR) lines bred for physical activity on wheels and four non-selected Control (C) lines. HR mice reached apparent selection limits between generations 17 and 27, running ~3-fold more than C. Studies at generations 11, 16, and 21 found that HR mice had evolved thicker hindlimb bones, heavier feet, and larger articular surface areas of the knee and hip joint. Based on biomechanical theory, any or all of these evolved differences may be beneficial for endurance running. Here, we studied mice from generation 68, plus a limited sample from generation 58, to test whether the skeleton continued to evolve after selection limits were reached. Contrary to our expectations, we found few differences between HR and C mice for these later generations, and some of the differences in bone dimensions identified in earlier generations were no longer statistically significant. We hypothesize that the loss of apparently coadapted lower-level traits reflects (1) deterioration related to a gradual increase in inbreeding and/or (2) additional adaptive changes that replace the functional benefits of some skeletal changes.

Effects of selective breeding for high voluntary wheel-running behavior on femoral nutrient canal size and abundance in house mice

Bone modeling and remodeling are aerobic processes that entail relatively high oxygen demands. Long bones receive oxygenated blood from nutrient arteries, epiphyseal-metaphyseal arteries, and periosteal arteries, with the nutrient artery supplying the bulk of total blood volume in mammals (~ 50-70%). Estimates of blood flow into these bones can be made from the dimensions of the nutrient canal, through which nutrient arteries pass. Unfortunately, measuring these canal dimensions non-invasively (i.e. without physical sectioning) is difficult, and thus researchers have relied on more readily visible skeletal proxies. Specifically, the size of the nutrient artery has been estimated from dimensions (e.g. minimum diameters) of the periosteal (external) opening of the nutrient canal. This approach has also been utilized by some comparative morphologists and paleontologists, as the opening of a nutrient canal is present long after the vascular soft tissue has degenerated. The literature on nutrient arteries and canals is sparse, with most studies consisting of anatomical descriptions from surgical proceedings, and only a few investigating the links between nutrient canal morphology and physiology or behavior. The primary objective of this study was to evaluate femur nutrient canal morphology in mice with known physiological and behavioral differences; specifically, mice from an artificial selection experiment for high voluntary wheel-running behavior. Mice from four replicate high runner (HR) lines are known to differ from four non-selected control (C) lines in both locomotor and metabolic activity, with HR mice having increased voluntary wheel-running behavior and maximal aerobic capacity (VO₂ max) during forced treadmill exercise. Femora from adult mice (average age 7.5 months) of the 11th generation of this selection experiment were μCT-scanned and three-dimensional virtual reconstructions of nutrient canals were measured for minimum cross-sectional area as a skeletal proxy of blood flow. Gross observations revealed that nutrient canals varied far more in number and shape than prior descriptions would indicate, regardless of sex or genetic background (i.e. HR vs. C lines). Canals adopted non-linear shapes and paths as they traversed from the periosteal to endosteal borders through the cortex, occasionally even branching within the cortical bone. Additionally, mice from both HR and C lines averaged more than four nutrient canals per femur, in contrast to the one to two nutrient canals described for femora from rats, pigs, and humans in prior literature. Mice from HR lines had significantly larger total nutrient canal area than C lines, which was the result not of an increase in the number of nutrient canals, but rather an increase in their average cross-section size. This study demonstrates that mice with an evolutionary history of increased locomotor activity and maximal aerobic metabolic rate have a concomitant increase in the size of their femoral nutrient canals. Although the primary determinant of nutrient canal size is currently not well understood, the present results bolster use of nutrient canal size as a skeletal indicator of aerobically supported levels of physical activity in comparative studies.

Predicting the bending properties of long bones: Insights from an experimental mouse model

OBJECTIVES

Analyses of bone cross-sectional geometry are frequently used by anthropologists and paleontologists to infer the loading histories of past populations. To address some underlying assumptions, we investigated the relative roles of genetics and exercise on bone cross-sectional geometry and bending mechanics in three mouse strains: high bone density (C3H/He), low bone density (C57BL/6), and a high-runner strain homozygous for the Myh4Minimsc allele (MM).

METHODS AND MATERIALS

Weanlings of each strain were divided into exercise (wheel) or control (sedentary) treatment groups for a 7-week experimental period. Morphometrics of the femoral mid-diaphysis and mechanical testing were used to assess both theoretical and ex vivo bending mechanics.

RESULTS

Across all measured morphological and bending traits, we found relatively small effects of exercise treatment compared to larger and more frequent interstrain differences. In the exercised group, total distance run over the experimental period was not a predictor of any morphological or bending traits. Cross-sectional geometry did not accurately predict bone response to loading.

DISCUSSION

Results from this experimental model do not support hypothesized associations among extreme exercise, cross-sectional geometry, and bending mechanics. Our results suggest that analysis of cross-sectional geometry alone is insufficient to predict loading response, and questions the common assumption that cross-sectional geometry differences are indicative of differential loading history.

Effects of activity, genetic selection, and their interaction on muscle metabolic capacities and organ masses in mice

Chronic voluntary exercise elevates total daily energy expenditure (DEE) and food consumption, potentially resulting in organ compensation supporting nutrient extraction/utilization. Additionally, species with naturally higher DEE often have larger processing organs, which may represent genetic differences and/or phenotypic plasticity. We tested for possible adaptive changes in organ masses of 4 replicate lines of house mice selected (37 generations) for high running (HR lines) compared with 4 non-selected control (C) lines. Females were housed with or without wheel access for 13-14 weeks beginning at 53-60 days of age. In addition to organ compensation, chronic activity may also require an elevated aerobic capacity. Therefore, we also measured hematocrit and both citrate synthase activity and myoglobin concentration in heart and gastrocnemius. Both selection (HR vs. C) and activity (wheels vs. no wheels) significantly affected morphological and biochemical traits. For example, with body mass as a covariate, mice from HR lines had significantly higher hematocrit and larger ventricles, with more myoglobin. Wheel access lengthened the small intestine, increased relative ventricle and kidney size, and increased skeletal muscle citrate synthase activity and myoglobin concentration. As compared with C lines, HR mice had greater training effects for ventricle mass, hematocrit, large intestine length, and gastrocnemius citrate synthase activity. For ventricle and gastrocnemius citrate synthase activity, the greater training was explainable quantitatively as a result of greater wheel running (i.e., “more pain, more gain”). For hematocrit and large intestine length, differences were not related to amount of wheel running and instead indicate inherently greater adaptive plasticity in HR lines.

Shape-shift: semicircular canal morphology responds to selective breeding for increased locomotor activity

Variation in semicircular canal morphology correlates with locomotor agility among species of mammals. An experimental evolutionary mouse model was used to test the hypotheses that semicircular canal morphology (1) evolves in response to selective breeding for increased locomotor activity, (2) exhibits phenotypic plasticity in response to early-onset chronic exercise, and (3) is unique in individuals possessing the minimuscle phenotype. We examined responses in canal morphology to prolonged wheel access and selection in laboratory mice from four replicate lines bred for high voluntary wheel-running (HR) and four nonselected control (C) lines. Linear measurements and a suite of 3D landmarks were obtained from 3D reconstructions of μCT-scanned mouse crania (μCT is microcomputed tomography). Body mass was smaller in HR than C mice and was a significant predictor of both radius of curvature and 3D canal shape. Controlling for body mass, radius of curvature did not differ statistically between HR and C mice, but semicircular canal shape did. Neither chronic wheel access nor minimuscle affected radius of curvature or canal shape These findings suggest that semicircular canal morphology is responsive to evolutionary changes in locomotor behavior, but the pattern of response is potentially different in small- versus large-bodied species.

Quantitative analysis of bone and soft tissue by micro-computed tomography: applications to ex vivo and in vivo studies

Micro-computed tomography (micro-CT) is a high-resolution imaging modality that is capable of analysing bone structure with a voxel size on the order of 10 μm. With the development of in vivo micro-CT, where disease progression and treatment can be monitored in a living animal over a period of time, this modality has become a standard tool for preclinical assessment of bone architecture during disease progression and treatment. For meaningful comparison between micro-CT studies, it is essential that the same parameters for data acquisition and analysis methods be used. This protocol outlines the common procedures that are currently used for sample preparation, scanning, reconstruction and analysis in micro-CT studies. Scan and analysis methods for trabecular and cortical bone are covered for the femur, tibia, vertebra and the full neonate body of small rodents. The analysis procedures using the software provided by ScancoMedical and Bruker are discussed, and the routinely used bone architectural parameters are outlined. This protocol also provides a section dedicated to in vivo scanning and analysis, which covers the topics of anaesthesia, radiation dose and image registration. Because of the expanding research using micro-CT to study other skeletal sites, as well as soft tissues, we also provide a review of current techniques to examine the skull and mandible, adipose tissue, vasculature, tumour severity and cartilage. Lists of recommended further reading and literature references are included to provide the reader with more detail on the methods described.

Identification of quantitative trait loci influencing skeletal architecture in mice: emergence of Cdh11 as a primary candidate gene regulating femoral morphology

Bone strength is influenced by many properties intrinsic to bone, including its mass, geometry, and mineralization. To further advance our understanding of the genetic basis of bone-strength-related traits, we used a large (n = 815), moderately (G(4) ) advanced intercross line (AIL) of mice derived from a high-runner selection line (HR) and the C57BL/6J inbred strain. In total, 16 quantitative trait loci (QTLs) were identified that affected areal bone mineral density (aBMD) and femoral length and width. Four significant (p < .05) and one suggestive (p < .10) QTLs were identified for three aBMD measurements: total body, vertebral, and femoral. A QTL on chromosome (Chr.) 3 influenced all three aBMD measures, whereas the other four QTLs were unique to a single measure. A total of 10 significant and one suggestive QTLs were identified for femoral length (FL) and two measures of femoral width, anteroposterior (AP) and mediolateral (ML). FL QTLs were distinct from loci affecting AP and ML width, and of the 7 AP QTLs, only three affected ML. A QTL on Chr. 8 that explained 7.1% and 4.0% of the variance in AP and ML, respectively, was mapped to a 6-Mb region harboring 12 protein-coding genes. The pattern of haplotype diversity across the QTL region and expression profiles of QTL genes suggested that of the 12, cadherin 11 (Cdh11) was most likely the causal gene. These findings, when combined with existing data from gene knockouts, identify Cdh11 as a strong candidate gene within which genetic variation may affect bone morphology.

Functional significance of genetic variation underlying limb bone diaphyseal structure

Limb bone diaphyseal structure is frequently used to infer hominin activity levels from skeletal remains, an approach based on the well-documented ability of bone to adjust to its loading environment during life. However, diaphyseal structure is also determined in part by genetic factors. This study investigates the possibility that genetic variation underlying diaphyseal structure is influenced by the activity levels of ancestral populations and might also have functional significance in an evolutionary context. We adopted an experimental evolution approach and tested for differences in femoral diaphyseal structure in 1-week-old mice from a line that had been artificially selected (45 generations) for high voluntary wheel running and non-selected controls. As adults, selected mice are significantly more active on wheels and in home cages, and have thicker diaphyses. Structural differences at 1 week can be assumed to primarily reflect the effects of selective breeding rather than direct mechanical stimuli, given that the onset of locomotion in mice is shortly after Day 7. We hypothesized that if genetically determined diaphyseal structure reflects the activity patterns of members of a lineage, then selected animals will have relatively larger diaphyseal dimensions at 1 week compared to controls. The results provide strong support for this hypothesis and suggest that limb bone cross sections may not always only reflect the activity levels of particular fossil individuals, but also convey an evolutionary signal providing information about hominin activity in the past.

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.

Glycogen storage and muscle glucose transporters (GLUT-4) of mice selectively bred for high voluntary wheel running

To examine the evolution of endurance-exercise behaviour, we have selectively bred four replicate lines of laboratory mice (Mus domesticus) for high voluntary wheel running (;high runner' or HR lines), while also maintaining four non-selected control (C) lines. By generation 16, HR mice ran approximately 2.7-fold more than C mice, mainly by running faster (especially in females), a differential maintained through subsequent generations, suggesting an evolutionary limit of unknown origin. We hypothesized that HR mice would have higher glycogen levels before nightly running, show greater depletion of those depots during their more intense wheel running, and have increased glycogen synthase activity and GLUT-4 protein in skeletal muscle. We sampled females from generation 35 at three times (photophase 07:00 h-19:00 h) during days 5-6 of wheel access, as in the routine selection protocol: Group 1, day 5, 16:00 h-17:30 h, wheels blocked from 13:00 h; Group 2, day 6, 02:00 h-03:30 h (immediately after peak running); and Group 3, day 6, 07:00 h-08:30 h. An additional Group 4, sampled 16:00 h-17:30 h, never had wheels. HR individuals with the mini-muscle phenotype (50% reduced hindlimb muscle mass) were distinguished for statistical analyses comparing C, HR normal, and HR mini. HR mini ran more than HR normal, and at higher speeds, which might explain why they have been favored by the selective-breeding protocol. Plasma glucose was higher in Group 1 than in Group 4, indicating a training effect (phenotypic plasticity). Without wheels, no differences in gastrocnemius GLUT-4 were observed. After 5 days with wheels, all mice showed elevated GLUT-4, but HR normal and mini were 2.5-fold higher than C. At all times and irrespective of wheel access, HR mini showed approximately three-fold higher [glycogen] in gastrocnemius and altered glycogen synthase activity. HR mini also showed elevated glycogen in soleus when sampled during peak running. All mice showed some glycogen depletion during nightly wheel running, in muscles and/or liver, but the magnitude of this depletion was not large and hence does not seem to be limiting to the evolution of even-higher wheel running.

Selective breeding as a tool to probe skeletal response to high voluntary locomotor activity in mice

We present a novel mouse-model for the study of skeletal structure and evolution, based on selective breeding for high levels of voluntary wheel running. Whereas traditional models (originally inbred strains, more recently knockouts and transgenics) rely on the study of mutant or laboratory-manipulated phenotypes, we have studied changes in skeletal morphometrics resulting from many generations of artificial selection for high activity in the form of wheel running, in which mice engage voluntarily. Mice from the four replicate High Runner (HR) lines run nearly three times as many revolutions during days 5 and 6 of a 6-day exposure to wheels (1.12 m circumference). We have found significant changes in skeletal dimensions of the hind limbs, including decreased directional asymmetry, larger femoral heads, and wider distal femora. The latter two have been hypothesized as evolutionary adaptations for long-distance locomotion in hominids. Exercise-training studies involving experimental groups with and without access to wheels have shown increased diameters of both femora and tibiafibulae, and suggest genetic effects on trainability (genotype-by-environment interactions). Reanalysis of previously published data on bone masses of hind limbs revealed novel patterns of change in bone mass associated with access to wheels for 2 months. Without access to wheels, HR mice have significantly heavier tibiafibulae and foot bones, whereas with chronic access to wheels, a significant increase in foot bone mass that was linearly related to increases in daily wheel running was observed. Mice exhibiting a recently discovered small-muscle phenotype ("mini-muscle," [MM] caused by a Mendelian recessive gene), in which the mass of the triceps surae muscle complex is ∼50% lower than in normal individuals, have significantly longer and thinner bones in the hind limb. We present new data for the ontogenetic development of muscle mass in Control, HR, and MM phenotypes in mice of 1-7 weeks postnatal age. Statistical comparisons reveal highly significant differences both in triceps surae mass and mass-corrected triceps surae mass between normal and MM mice at all but the postnatal age of 1 week. Based on previously observed differences in distributions of myosin isoforms in adult MM mice, we hypothesize that a reduction of myosin heavy-chain type-IIb isoforms with accounts for our observed ontogenetic changes in muscle mass.