Phenotypic effects of the "mini-muscle" allele in a large HR x 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. F(1) 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.
Hannon
Robert M
RM
Department of Biology, University of California, Riverside, CA 92521, USA.
Kelly
Scott A
SA
Middleton
Kevin M
KM
Kolb
Erik M
EM
Pomp
Daniel
D
Garland
Theodore
T
D000483:Alleles; D000818:Animals; D049628:Body Size; D003433:Crosses, Genetic; D005260:Female; D008297:Male; D051379:Mice; D008810:Mice, Inbred C57BL; D018482:Muscle, Skeletal / Q000033:anatomy & histology* / Q000502:physiology; D009929:Organ Size / Q000235:genetics; D010641:Phenotype; D012420:Running; D012641:Selection, Genetic