KELLY, S.A.*; GARLAND, T., Jr.; Univ. of Calif., Riverside: Experimental Evolution and Phenotypic Plasticity of Hindlimb Bones in High-Activity House Mice
A primary factor in the acquisition and maintenance of vertebrate skeletal mass and strength is mechanical loading. Studies of rodents have generally found that both forced and voluntary chronic exercise causes increased hindlimb bone diameter, mass, and strength. Among species of mammals, “cursoriality” is generally associated with lengthening of distal limb segments and an increased metatarsal/femur (MT/F) ratio. Therefore, we tested for possible adaptive changes (genetic and/or phenotypic plasticity) in hindlimb bones of 4 replicate lines of house mice that had been selectively bred for high voluntary wheel running (S lines) for 22 generations and in 4 non-selected control (C) lines. In the present study, we examined hindlimb bones (femur, tibia-fibula, and metatarsal) of males housed either with or without wheel access for 8 weeks beginning at 21 days of age (from generation 22). Here, we report on lengths and diameters; future studies will examine mass, mineral content, and histomorphometry. As expected from our previous studies, S mice were smaller than C, and wheel access reduced body mass of both groups (no interaction). ANCOVA showed that body mass was a significant predictor of all bone measures except MT/F ratio; therefore, all results reported are from ANCOVAs. Several traits were significantly affected by selection history and/or exercise history (P < 0.05), but interactions between these two main effects were never significant (all P > 0.2). With body mass as a covariate, mice from S lines had significantly thicker femurs and tibia-fibulas. Wheel access also significantly increased diameters of femurs and tibia-fibulas. Additionally, S-line individuals with the mini-muscle phenotype (homozygous for a Mendelian recessive allele that halves hindlimb muscle mass [Evolution, 2002, 56:1267]) exhibited significantly longer and thinner femurs and tibia-fibulas. MT/F ratio did not significantly vary among groups. Supported by NSF IBN-0212567 to T.G.
