Genetic variation and correlations between genotype and locomotor physiology in outbred laboratory house mice (Mus domesticus)

Does Behavior Evolve First? Correlated Responses to Selection for Voluntary Wheel-Running Behavior in House Mice

AbstractHow traits at multiple levels of biological organization evolve in a correlated fashion in response to directional selection is poorly understood, but two popular models are the very general "behavior evolves first" (BEF) hypothesis and the more specific "morphology-performance-behavior-fitness" (MPBF) paradigm. Both acknowledge that selection often acts relatively directly on behavior and that when behavior evolves, other traits will as well but most with some lag. However, this proposition is exceedingly difficult to test in nature. Therefore, we studied correlated responses in the high-runner (HR) mouse selection experiment, in which four replicate lines have been bred for voluntary wheel-running behavior and compared with four nonselected control (C) lines. We analyzed a wide range of traits measured at generations 20-24 (with a focus on new data from generation 22), coinciding with the point at which all HR lines were reaching selection limits (plateaus). Significance levels (226 P values) were compared across trait types by ANOVA, and we used the positive false discovery rate to control for multiple comparisons. This meta-analysis showed that, surprisingly, the measures of performance (including maximal oxygen consumption during forced exercise) showed no evidence of having diverged between the HR and C lines, nor did any of the life history traits (e.g., litter size), whereas body mass had responded (decreased) at least as strongly as wheel running. Overall, results suggest that the HR lines of mice had evolved primarily by changes in motivation rather than performance ability at the time they were reaching selection limits. In addition, neither the BEF model nor the MPBF model of hierarchical evolution provides a particularly good fit to the HR mouse selection experiment.

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.

Mitochondrial haplotypes are not associated with mice selectively bred for high voluntary wheel running

Mitochondrial haplotypes have been associated with human and rodent phenotypes, including nonshivering thermogenesis capacity, learning capability, and disease risk. Although the mammalian mitochondrial D-loop is highly polymorphic, D-loops in laboratory mice are identical, and variation occurs elsewhere mainly between nucleotides 9820 and 9830. Part of this region codes for the tRNA^Arg gene and is associated with mitochondrial densities and number of mtDNA copies. We hypothesized that the capacity for high levels of voluntary wheel-running behavior would be associated with mitochondrial haplotype. Here, we analyzed the mtDNA polymorphic region in mice from each of four replicate lines selectively bred for 54 generations for high voluntary wheel running (HR) and from four control lines (Control) randomly bred for 54 generations. Sequencing the polymorphic region revealed a variable number of adenine repeats. Single nucleotide polymorphisms (SNPs) varied from 2 to 3 adenine insertions, resulting in three haplotypes. We found significant genetic differentiations between the HR and Control groups (F_st = 0.779, p ≤ 0.0001), as well as among the replicate lines of mice within groups (F_sc = 0.757, p ≤ 0.0001). Haplotypes, however, were not strongly associated with voluntary wheel running (revolutions run per day), nor with either body mass or litter size. This system provides a useful experimental model to dissect the physiological processes linking mitochondrial, genomic SNPs, epigenetics, or nuclear-mitochondrial cross-talk to exercise activity.

A Mixed Model Approach to Genome-Wide Association Studies for Selection Signatures, with Application to Mice Bred for Voluntary Exercise Behavior

Selection experiments and experimental evolution provide unique opportunities to study the genetics of adaptation because the target and intensity of selection are known relatively precisely. In contrast to natural selection, where populations are never strictly "replicated," experimental evolution routinely includes replicate lines so that selection signatures-genomic regions showing excessive differentiation between treatments-can be separated from possible founder effects, genetic drift, and multiple adaptive solutions. We developed a mouse model with four lines within a high running (HR) selection treatment and four nonselected controls (C). At generation 61, we sampled 10 mice of each line and used the Mega Mouse Universal Genotyping Array to obtain single nucleotide polymorphism (SNP) data for 25,318 SNPs for each individual. Using an advanced mixed model procedure developed in this study, we identified 152 markers that were significantly different in frequency between the two selection treatments. They occurred on all chromosomes except 1, 2, 8, 13, and 19, and showed a variety of patterns in terms of fixation (or the lack thereof) in the four HR and four C lines. Importantly, none were fixed for alternative alleles between the two selection treatments. The current state-of-the-art regularized F test applied after pooling DNA samples for each line failed to detect any markers. We conclude that when SNP or sequence data are available from individuals, the mixed model methodology is recommended for selection signature detection. As sequencing at the individual level becomes increasingly feasible, the new methodology may be routinely applied for detection of selection.

Sex-specific heterosis in line crosses of mice selectively bred for high locomotor activity

When populations with similar histories of directional selection are crossed, their offspring may differ in mean phenotype as compared with the average for the parental populations, often exhibiting enhancement of the mean phenotype (termed heterosis or hybrid vigor). We tested for heterosis in a cross of two replicate lines of mice selectively bred for high voluntary wheel running for 53 generations. Mice were paired to produce four sets of F1 offspring: two purebred High Runner (HR) lines and the hybrid reciprocal crosses. The purebred HR showed statistically significant, sex-dependent differences in body mass, wheel revolutions, running duration, mean running speed, and (controlling for body mass) organ masses (heart ventricles, liver, spleen, triceps surae muscle). Hybrid males ran significantly more revolutions than the purebred males, mainly via increased running speeds, but hybrid females ran intermediate distances, durations, and speeds, as compared with the purebred females. In both sexes, ventricles were relatively smaller in hybrids as compared with purebred HR. Overall, our results demonstrate differential and sex-specific responses to selection in the two HR lines tested, implying divergent genetic architectures underlying high voluntary exercise.

Erythropoietin elevates VO2,max but not voluntary wheel running in mice

Voluntary activity is a complex trait, comprising both behavioral (motivation, reward) and anatomical/physiological (ability) elements. In the present study, oxygen transport was investigated as a possible limitation to further increases in running by four replicate lines of mice that have been selectively bred for high voluntary wheel running and have reached an apparent selection limit. To increase oxygen transport capacity, erythrocyte density was elevated by the administration of an erythropoietin (EPO) analogue. Mice were given two EPO injections, two days apart, at one of two dose levels (100 or 300 microg kg(-1)). Hemoglobin concentration ([Hb]), maximal aerobic capacity during forced treadmill exercise (VO2,max) and voluntary wheel running were measured. [Hb] did not differ between high runner (HR) and non-selected control (C) lines without EPO treatment. Both doses of EPO significantly (P<0.0001) increased [Hb] as compared with sham-injected animals, with no difference in [Hb] between the 100 microg kg(-1) and 300 microg kg(-1) dose levels (overall mean of 4.5 g dl(-1) increase). EPO treatment significantly increased VO2,max by approximately 5% in both the HR and C lines, with no dosexline type interaction. However, wheel running (revolutions per day) did not increase with EPO treatment in either the HR or C lines, and in fact significantly decreased at the higher dose in both line types. These results suggest that neither [Hb] per se nor VO2,max is limiting voluntary wheel running in the HR lines. Moreover, we hypothesize that the decrease in wheel running at the higher dose of EPO may reflect direct action on the reward pathway of the brain.

Relationships between quantitative and reproductive fitness traits in animals

The relationships between quantitative and reproductive fitness traits in animals are of general biological importance for the development of population genetic models and our understanding of evolution, and of great direct economical importance in the breeding of farm animals. Two well investigated quantitative traits--body weight (BW) and litter size (LS)--were chosen as the focus of our review. The genetic relationships between them are reviewed in fishes and several mammalian species. We have focused especially on mice where data are most abundant. In mice, many individual genes influencing these traits have been identified, and numerous quantitative trait loci (QTL) located. The extensive data on both unselected and selected mouse populations, with some characterized for more than 100 generations, allow a thorough investigation of the dynamics of this relationship during the process of selection. Although there is a substantial positive genetic correlation between both traits in unselected populations, caused mainly by the high correlation between BW and ovulation rate, that correlation apparently declines during selection and therefore does not restrict a relatively independent development of both traits. The importance of these findings for overall reproductive fitness and its change during selection is discussed.

Molecular and quantitative genetic divergence among populations of house mice with known evolutionary histories

Evolutionary biologists have long been interested in the processes influencing population differentiation, but separating the effects of neutral and adaptive evolution has been an obstacle for studies of population subdivision. A recently developed method allows tests of whether disruptive (ie, spatially variable) or stabilizing (ie, spatially uniform) selection is influencing phenotypic differentiation among subpopulations. This method, referred to as the F(ST) vs Q(ST) comparison, separates the total additive genetic variance into within- and among-population components and evaluates this level of differentiation against a neutral hypothesis. Thus, levels of neutral molecular (F(ST)) and quantitative genetic (Q(ST)) divergence are compared to evaluate the effects of selection and genetic drift on phenotypic differentiation. Although the utility of such comparisons appears great, its accuracy has not yet been evaluated in populations with known evolutionary histories. In this study, F(ST) vs Q(ST) comparisons were evaluated using laboratory populations of house mice with known evolutionary histories. In this model system, the F(ST) vs Q(ST) comparisons between the selection groups should reveal quantitative trait differentiation consistent with disruptive selection, while the F(ST) vs Q(ST) comparisons among lines within the selection groups should suggest quantitative trait differentiation in agreement with drift. We find that F(ST) vs Q(ST) comparisons generally produce the correct evolutionary inference at each level in the population hierarchy. Additionally, we demonstrate that when strong selection is applied between populations Q(ST) increases relative to Q(ST) among populations diverging by drift. Finally, we show that the statistical properties of Q(ST), a variance component ratio, need further investigation.

Different effects of intensity and duration of locomotor activity on circadian period

An outstanding unresolved issue in chronobiology is how the level of locomotor activity influences length of the free-running, endogenous circadian period (tau). To address this issue, the authors studied a novel model, 4 replicate lines of laboratory house mice (Mus domesticus) that had been selectively bred for high wheel-running activity (S) and their 4 unselected control (C) lines. Previous work indicates that S mice run approximately twice as many revolutions/day and exhibit an altered dopaminergic function as compared with C mice. The authors report that S mice have a tau shorter by about 0.5 h as compared with C mice. The difference in tau was significant both under constant light (control lines: tau = 25.5 h; selected: tau = 24.9 h) and under constant dark (control lines: 23.7 h; selected: 23.4 h). Moreover, the difference remained statistically significant even when the effects of running speed and time spent running were controlled in ANCOVA. Thus, something more fundamental than just intensity or duration of wheel-running activity per se must underlie the difference in tau between the S and C lines. However, despite significant difference in total wheel-running activity between females and males, tau did not differ between the sexes. Similarly, among individuals within lines, tau was not correlated with wheel-running activity measured as total revolutions per day. Instead, tau tended to decrease with average running speed but increase with time spent running. Finally, within individuals, an increase in time spent running resulted in decreased tau in the next few days, but changes in running speed had no statistically significant effect. The distinctions between effects of duration versus intensity of an activity, as well as between the among- versus within-individual correlations, are critical to understanding the relation between locomotor activity and pace of the circadian clock.

Food wasting by house mice: variation among individuals, families, and genetic lines

Under ad libitum conditions, laboratory house mice (Mus domesticus) fragment considerable amounts of pelleted food and leave it scattered in their cages. The proportion of food thus wasted (in relation to food eaten) varies remarkably among individuals, from 2% to 40%, but is highly consistent in consecutive trials, even when the mice were moved from 22 to -10 degrees C and food consumption doubled. Food wasting did not differ either between the sexes or between genetic lines that had been selected (10 generations) for high voluntary wheel-running behavior (n=4) and their unselected control lines (n=4). However, it varied significantly among replicate lines within the selection groups and among families within the lines (coefficient of intraclass correlation for full sibs, rhof=0.41 in room temperature trials and rhof=0.34 in cold trials). Moreover, the percent of food wasted was negatively correlated with food consumption in the cold trials (males: r=-.36, females: r=-.20) and with total litter mass at weaning (the litters into which they were born; r=-.24), two traits that may affect Darwinian fitness. We conclude that food wastage should not be ignored without justification in calculations of food consumption. In addition, "table manners" can convey reliable information about family origin of an individual and its quality, and therefore could potentially play a role in establishment of social status.

Dominance, plasma testosterone levels, and testis size in house mice artificially selected for high activity levels

Male house mice (Mus domesticus) from four replicate lines selectively bred for high voluntary wheel-running behavior were compared with four random-bred control lines with respect to dominance, testis size, and plasma testosterone level. Behavior was measured with a tube apparatus in which focal mice encountered a standard opponent from an inbred strain, and positions of mice were scored over a 10-min period; the test was replicated the following day. Blood samples were taken from undisturbed mice 1 week prior to testing (baseline condition) and immediately after the first tube test; plasma testosterone was measured by enzyme immunoassay with chromatography. As compared with control lines, mice from selected lines tended to be smaller in body mass, to have larger testes, and were significantly less likely to advance towards their opponent during the second tube-test encounter. However, no significant differences in either baseline or post-encounter testosterone levels were detected. Significant differences in body mass, relative testis size, position during the first tube-test encounter, and baseline testosterone were found among the replicate lines within linetype, which indicates founder effects, random genetic drift, unique mutations, and/or multiple responses to selection. At the level of individual variation (residuals from nested analysis of covariance models), an inverse relationship between baseline testosterone and advancing in the tube test was observed, and the relationship was stronger during the second test day. This unexpected result may reflect an alternate coping strategy.

Evolution of a small-muscle polymorphism in lines of house mice selected for high activity levels

To study the correlated evolution of locomotor behavior and exercise physiology, we conducted an artificial selection experiment. From the outbred Hsd:ICR strain of Mus domesticus, we began eight separate lines, each consisting of 10 breeding pairs. In four of the lines, we used within-family selection to increase voluntary wheel running. The remaining four lines were random-bred (within lines) to serve as controls. Various traits have been monitored to test for correlated responses. Here, we report on organ masses, with emphasis on the triceps surae muscle complex, an important extensor of the ankle. Mice from the selected lines exhibit reduced total body mass, increased relative (mass-corrected) kidney mass, and reduced relative triceps surae mass. In addition, a discrete muscle-mass polymorphism was observed: some individuals had triceps surae that were almost 50% lighter than normal for their body mass. This small-muscle phenotype was observed in only three of the eight lines: in one control line, it has fluctuated in frequency between zero and 10%, whereas in two of the selected lines it has increased in frequency to approximately 50% by generation 22. Data from a set of parents and offspring (generations 23 and 24) are consistent with inheritance as a single autosomal recessive allele. Evidence for the adaptive significance of the small-muscle allele was obtained by fitting multiple-generation data to hierarchical models that include effects of genetic drift and/or selection. The small-muscle allele is estimated to have been present at low frequency (approximately 7%) in the base population, and analysis indicates that strong selection favors the allele in the selected but not control lines. We hypothesize that the small muscles possess functional characteristics and/or that the underlying allele causes pleiotropic effects (e.g., reduced total body mass; increased relative heart, liver, and kidney mass) that facilitate high levels of wheel running. Nevertheless, at generation 22, wheel running of affected individuals did not differ significantly from those with normal-sized muscles, and the magnitude of response to selection has been similar in all four selected lines, indicating that multiple genetic "solutions" are possible in response to selection for high activity levels.

Response of Sod-2 enzyme activity to selection for high voluntary wheel running

The objective of this study was to examine the correlated response of anti-oxidant enzyme activity to selective breeding for increased voluntary wheel running in house mice. Activity of liver superoxide dismutase-2 (Sod-2), a free radical scavenger, was measured in four groups of mice. 'Active' individuals were housed in cages with attached wheels for 8 weeks beginning at weaning; 'sedentary' individuals were housed in cages with attached wheels that were prevented from rotating. Both of these treatments were applied to male and female mice from generation 14 of a replicated artificial selection experiment, which is composed of four lines selected for high wheel running and four randomly bred lines that serve as controls. In females, Sod-2 activity was significantly lower in selected vs control animals, regardless of presence/absence of a free-turning wheel. This difference suggests a trade-off between early-age voluntary wheel-running activity and Sod-2 activity. In males, Sod-2 activity was significantly affected by an interaction between selection group and activity group, with males from selected lines having lower Sod-2 activity relative to control males only in the sedentary treatment. These negative correlated responses of Sod-2 activity to selection on wheel running are discussed in the context of antagonistic pleiotropy models of aging and with respect to potential effects on lifespan.

The quantitative genetics of maximal and basal rates of oxygen consumption in mice

A positive genetic correlation between basal metabolic rate (BMR) and maximal (VO(2)max) rate of oxygen consumption is a key assumption of the aerobic capacity model for the evolution of endothermy. We estimated the genetic (V(A), additive, and V(D), dominance), prenatal (V(N)), and postnatal common environmental (V(C)) contributions to individual differences in metabolic rates and body mass for a genetically heterogeneous laboratory strain of house mice (Mus domesticus). Our breeding design did not allow the simultaneous estimation of V(D) and V(N). Regardless of whether V(D) or V(N) was assumed, estimates of V(A) were negative under the full models. Hence, we fitted reduced models (e.g., V(A) + V(N) + V(E) or V(A) + V(E)) and obtained new variance estimates. For reduced models, narrow-sense heritability (h(2)(N)) for BMR was <0.1, but estimates of h(2)(N) for VO(2)max were higher. When estimated with the V(A) + V(E) model, the additive genetic covariance between VO(2)max and BMR was positive and statistically different from zero. This result offers tentative support for the aerobic capacity model for the evolution of vertebrate energetics. However, constraints imposed on the genetic model may cause our estimates of additive variance and covariance to be biased, so our results should be interpreted with caution and tested via selection experiments.

Body temperatures of house mice artificially selected for high voluntary wheel-running behavior: repeatability and effect of genetic selection

We studied rectal body temperatures of house mice (Mus domesticus) that had been artificially selected for high voluntary wheel running.1. At generation 17, mice from the four replicate selected lines ran, on average, 2.5-times as many revolutions/day as did mice from the four random-bred control lines.2. During the day, repeatability of individual differences in body temperature measured 4 days apart was low; at night, repeatability was statistically significant across three time scales (1 day, 1 week, 2 weeks).3. During the day, body temperatures of selected and control animals did not differ; at night, mice from selected lines had higher body temperatures. However, when amount of wheel running immediately prior to measurement was included as a covariate, the difference was no longer statistically significant.Higher body temperatures, associated with increased activity, might enhance locomotor abilities through Q10 effects, increase metabolic rate and food requirements, affect sleep patterns, and alter expression of heat-shock proteins.

Effects of voluntary activity and genetic selection on muscle metabolic capacities in house mice Mus domesticus

Selective breeding is an important tool in behavioral genetics and evolutionary physiology, but it has rarely been applied to the study of exercise physiology. We are using artificial selection for increased wheel-running behavior to study the correlated evolution of locomotor activity and physiological determinants of exercise capacity in house mice. We studied enzyme activities and their response to voluntary wheel running in mixed hindlimb muscles of mice from generation 14, at which time individuals from selected lines ran more than twice as many revolutions per day as those from control (unselected) lines. Beginning at weaning and for 8 wk, we housed mice from each of four replicate selected lines and four replicate control lines with access to wheels that were free to rotate (wheel-access group) or locked (sedentary group). Among sedentary animals, mice from selected lines did not exhibit a general increase in aerobic capacities: no mitochondrial [except pyruvate dehydrogenase (PDH)] or glycolytic enzyme activity was significantly (P < 0.05) higher than in control mice. Sedentary mice from the selected lines exhibited a trend for higher muscle aerobic capacities, as indicated by higher levels of mitochondrial (cytochrome-c oxidase, carnitine palmitoyltransferase, citrate synthase, and PDH) and glycolytic (hexokinase and phosphofructokinase) enzymes, with concomitant lower anaerobic capacities, as indicated by lactate dehydrogenase (especially in male mice). Consistent with previous studies of endurance training in rats via voluntary wheel running or forced treadmill exercise, cytochrome-c oxidase, citrate synthase, and carnitine palmitoyltransferase activity increased in the wheel-access groups for both genders; hexokinase also increased in both genders. Some enzymes showed gender-specific responses: PDH and lactate dehydrogenase increased in wheel-access male but not female mice, and glycogen phosphorylase decreased in female but not in male mice. Two-way analysis of covariance revealed significant interactions between line type and activity group; for several enzymes, activities showed greater changes in mice from selected lines, presumably because such mice ran more revolutions per day and at greater velocities. Thus genetic selection for increased voluntary wheel running did not reduce the capability of muscle aerobic capacity to respond to training.

Behaviour of house mice artificially selected for high levels of voluntary wheel running

We have developed a novel model to study the correlated evolution of behavioural and morphophysiological traits in response to selection for increased locomotor activity. We used selective breeding to increase levels of voluntary wheel running in four replicate lines of laboratory house mice, Mus domesticus, with four random-bred lines maintained as controls. The experiment presented here tested for correlated behavioural responses in the wheel-cage complex, with wheels either free to rotate or locked (environmental factor). After 13 generations, mice from selected lines ran 2.2 times as many revolutions/day as controls on days 5 and 6 of initial exposure to wheels (10 826 versus 4890 revolutions/day, corresponding to 12.1 and 5.5 km/day, respectively). This increase was caused primarily by mice from selected lines running faster, not more minutes per day. Focal-animal observations confirmed that the increase in revolutions/day involved more actual running (or climbing in locked wheels), not an increase in coasting (or hanging). Not surprisingly, access to free versus locked wheels had several effects on behaviour, including total time spent in wheels, sniffing and biting. However, few behaviours showed statistically significant differences between the selected and control lines. Selection did not increase the total time spent in wheels (either free or locked), the frequency of nonlocomotor activities performed in the wheels, nor the amount of locomotor activity in cages attached to the wheels; as well, selection did not decrease the amount of time spent sleeping. Thus, wheel running is, at the genetic level, a largely independent axis of behaviour. Moreover, the genetic architecture of overall wheel running and its components seem conducive to increasing total distance moved without unduly increasing energy or time-related costs. The selection experiment also offers a new approach to study the proximate mechanisms of wheel-running behaviour itself. For example, frequencies of sniffing and wire biting were reduced in selected females but not males. This result suggests that motivation or function of wheel running may differ between the sexes. Copyright 1999 The Association for the Study of Animal Behaviour.