Muscle-bone relationships in mice selected for different body conformations.
Some muscle-bone relationships were studied in terms of gastrocnemius muscle weight, femur and tibia length and femur and tibia weight in four lines of mice (CBi-, CBi+, CBi/L and CBi/C) artificially selected for different body conformations and in the unselected control line (CBi). CBi- (low body weight--short tail) and CBi+ (high body weight--long tail) lines were divergently selected following the positive genetic correlation between body weight and tail (skeleton) length (agonistic selection). In contrast, CBi/L (low body weight--long tail) and CBi/C (high body weight--short tail) were also divergently selected but against the aforementioned correlation (antagonistic selection). The relationship between bone length and muscle weight was interpreted based on the assumption that the increased tension generated by the longitudinal growth of a bone, brings about an increase in the mass of the muscles attached to it. All CBi+, CBi/C and CBi/L mice showed enlarged femurs and tibias, but only those genotypes simultaneously selected for high body weight (CBi+ and CBi/C) showed heavier muscles than controls. The CBi+ and CBi- genotypes with agonistic selection differ in bone length and muscle weight, as it would be expected of the allometric modification of their body conformation, showing the associated longitudinal bone growth-muscle growth. CBi/C and CBi/L mice, with a non-allometric modification of body conformation, exhibited the same bone length but different muscle weight. Consequently, the antagonistic criterion allowed to confirm that the genetic influence on of the proposed muscle-bone relationships could be modified, thus making it possible to lengthen the bone through selection of a long skeleton and to avoid the correlated effect on muscle mass, by selecting for a low body weight, bringing forth presumptive evidence that both processes were genetically independent.