Journal of Heredity Advance Access published online on February 28, 2008
Journal of Heredity, doi:10.1093/jhered/esn011
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Phenotypic Effects of the "Mini-Muscle" Allele in a Large HR x C57BL/6J Mouse Backcross
From the Department of Biology, University of California, Riverside, CA 92521 (Hannon, Kelly, Middleton, Kolb, and Garland); and the Departments of Nutrition and Cell and Molecular Physiology, University of North Carolina, Chapel Hill, NC 27599 (Pomp). Kevin M. Middleton is now at the Department of Biology, California State University, San Bernardino, CA 92407
Address correspondence to T. Garland Jr at the address above, or e-mail: tgarland{at}ucr.edu.
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 = 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.
Corresponding Editor: Robert Wayne
Received July 24, 2007
Accepted January 11, 2008
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