Correlated responses in development and distribution of fat depots in mice selected for body composition traits

CORRELATED RESPONSE IN THE DEVELOPMENTAL CHOREOGRAPHIES OF THE MOUSE MANDIBLE TO SELECTION FOR BODY COMPOSITION

The correlated response to 13 generations of selection for percent fatness and leanness is investigated in 11 mandible traits in mice. Five selection lines are examined including high fat (HF), low fat (LF), high lean (HL), low lean (LL) and a randomly selected control strain (RC. The ontogenetic patterns of growth in the RC strain serve as a model to evaluate the developmental consequences of directional selection. Selection has systematically altered the patterns of mandible growth in selection lines relative to the control strain. Further, selection has significantly altered the age-specific phenotypic covariance among these traits. In the HF strain, growth in the mandible is completed by 12 weeks of age for most traits. In other selected strains, notably LF and LL, there is a significant growth spurt that occurs between 12 and 15 weeks of age. Changes in the patterns of mandibular growth produce significant differences among strains in the final form of the mandible. Because of the changes in the patterns of growth, the differences among strains are themselves shown to vary at different postnatal ages. The phenotypically similar strains, i.e., HF and LL or LF and HL, show different but correlated patterns of divergence. Multivariate statistical analyses suggest that the temporal strain differences in these traits are multidimensional.

Lack of Socs2 expression reduces lifespan in high-growth mice

The high-growth (HG) phenotype in mice is characterized by a 30-50% postweaning overgrowth with a substantial increase in plasma insulin-like growth factor I (IGF1) levels, which is directly related to a deletion (hg) on chromosome 10 that includes the suppressor of cytokine signaling 2 (Socs2) gene. Reduced plasma IGF1 levels have been associated with extended lifespan in mice, although the aging-related effects of abnormally high IGF1 levels without elevated growth hormone levels have never been assessed in mammals. Within this context, the hg deletion was introgressed into C57BL/6J (B6) and FVB backgrounds, and a survival analysis was performed on the longevity records of 200 B6 (91 wild-type and 109 homozygous hg mutants) and 69 FVB (32 wild-type and 37 hg mutants) mice. Longevity was examined using a piecewise Weibull proportional hazards model solved through a Bayesian perspective and Markov chain Monte Carlo sampling. Lifespan was significantly reduced in both strains in homozygous hg mice, with a death risk between 3.689 (B6) and 4.347 (FVB) times higher than in wild-type mice (non-overlapped highest posterior density regions at 95%). These results highlight the effects of the Socs2 gene on aging regulation, likely related with variations described in plasma IGF1 levels. This result is consistent with previous research in dwarf mutant mice and other species, and characterizes the HG mutant mice as a unique and interesting animal model for accelerated aging research.

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

Laboratory strains of house mice (Mus domesticus) are increasingly used as model organisms in evolutionary physiology, so information on levels of genetic variation is important. For example, are levels of genetic variation comparable to those found in populations of wild house mice? We studied allozymes to estimate genetic variation in outbred Hsd:ICR mice, which have been used in several studies with evolutionary emphasis. The physiological significance of allozyme variation remains obscure. Several workers have reported relationships between multi-locus heterozygosity and metabolic traits, but endotherms have not been studied. Therefore, we also measured mice for basal metabolic rate (BMR), maximal oxygen consumption during forced treadmill exercise (VO2max), and 12 other traits related to locomotor physiology, before genotyping them for 10 allozyme loci. Four of these loci were polymorphic, all were in Hardy-Weinberg equilibrium, and inbreeding coefficients were not significantly different from zero. Average heterozygosities were 11%, similar to values reported for wild populations of house mice. Fourteen percent of the associations between single-locus genotype and physiological traits were statistically significant. Multi-locus heterozygosity was not significantly related to VO2max, but was positively correlated with BMR, a result opposite to the negative correlation between standard metabolic rate and heterozygosity reported in many ectotherms. Therefore, the proposed mechanisms for the effect of multi-locus heterozygosity on metabolic rate in ectotherms may not apply to endotherms.

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.

Analysis of lines of mice selected on fat content. 4. Correlated responses in growth and reproduction

Lines of mice have been selected for 32 generations for either high or low fat content, resulting in a threefold divergence in the selection criterion (estimated fat content of males at 14 weeks of age). Male mice from both lines were dissected at a series of ages between 4 and 26 weeks and the following traits measured or estimated: body weight, fat content, lean weight, and the weights of several fatpads and internal organs. The lines appeared to have a similar underlying lean weight upon which the Fat line accumulated fat at a faster rate. This accumulation continued unabated in the Fat lines for at least 26 weeks but had effectively ceased by 8 weeks of age in the Lean. The liver and kidneys were slightly larger in the Fat line but there were no differences in the weights of heart, lung or spleen. This detailed phenotypic description of the lines complemented previous reports describing correlated changes in their physiology. The threefold divergence in estimated fat content was less than that in one of its component traits, growth of gonadal fatpad, but was greater than the divergence in other physiological indicators, i.e. the activity of lipogenic enzymes in vitro and direct measurement of lipogenic flux. Testis size in the Fat line was consistently lower than in the Lean although the Fat line was slightly more fecund, apparently due to a higher prenatal survival rate.