Phenotypic Effects of the ‘‘Mini-Muscle’’ Allele in a Large HR 3 C57BL/6J Mouse Backcross

From outbred Hsd:ICR mice, we selectively bred 4 replicate lines for high running (High-Runner [HR] lines) on wheels while maintaining 4 nonselected lines as controls (C lines). An apparent Mendelian recessive, the ‘‘mini-muscle’’ (MM) allele, whose main phenotypic effect is to reduce hindlimb muscle mass by 50%, was discovered in 2 HR lines and 1 C line. This gene of major effect has gone to fixation in one selected line, remains polymorphic in another, and is now undetectable in the one C line. Homozygotes exhibit various pleiotropic effects, including a doubling of mass-specific muscle aerobic capacity, and larger hearts, livers, and spleens. To create a population suitable for mapping the genomic location of the MM allele and to better characterize its pleiotropic effects, we crossed females fixed for the MM allele with male C57BL/6J. F1 males were then backcrossed to the MM parent females. Backcross (BC) mice (N 5 404) were dissected, and a 50:50 ratio of normal to MM phenotype was observed with no overlap in relative muscle mass. In the BC, analysis of covariance revealed that MM individuals ran significantly more on days 5 and 6 of a 6-day exposure to running wheels (as in the routine selective-breeding protocol), were smaller in body mass, and had larger ventricles and spleens. A basic assumption of quantitative genetics is that most aspects of the phenotype are affected by alleles segregating at multiple loci, each having a small effect on the trait (Fisher 1930). However, recent studies suggest that genes of major effect (GOMEs) are important during the adaptive evolution of many traits and that these genes are almost always associated with a large number of pleiotropic effects (Orr and Coyne 1992; Bradshaw et al. 1998; Orr 1998; Agrawal et al. 2001). GOMEs have been identified in both wild and captive populations. Some important examples in domestic livestock are the muscle-doubling gene in cattle (Rollins et al. 1972), dwarfing gene in poultry (Merat and Ricard 1974), DGAT1 gene in dairy cattle (Grisart et al. 2002), and IGF2 gene in pigs (Van Laere et al. 2003). Notable examples in natural populations are the PITX1 gene, which alters the pelvis in stickleback fishes (Shapiro et al. 2004), and the Mc1r gene, which affects coat color in beach mice (Hoekstra et al. 2006). GOMEs also play a critical role in genetic research surrounding several major human health problems. For example, gene expression studies of GOMEs have provided valuable insight concerning treatments for both leukemia (Mullighan et al. 2007) and autism (Muhle et al. 2004). We identified a GOME, termed the ‘‘mini-muscle’’ (MM) allele (Garland et al. 2002), within some of the replicate lines of a long-term selection experiment for high locomotor activity in house mice. Selection began in 1993 from a base population of outbred Hsd:ICR mice (Swallow et al. 1998; Garland 2003). After initial generations of random mating, the base population was divided randomly into 8 lines, 4 of which serve as controls (C lines), whereas the other 4 are selectively bred for high voluntary wheel running (HighRunner [HR] lines). After 16 generations, mice from the HR lines ran an average 170% more revolutions/day than C-line mice. These mice have been the subjects of many behavioral and physiological studies, which are summarized elsewhere (e.g., Garland 2003; Rhodes et al. 2005; Swallow et al. 2005; Rezende, Gomes et al. 2006; Rezende, Kelly et al. 2006; Belke and Garland 2007; Girard et al. 2007; Malisch et al. 2007).