GARLAND, T.*; MORGAN, M.T.; SWALLOW, J.G.; RHODES, J.S.; GIRARD, I.; CARTER, P.A.: Evolution of a Small-muscle Polymorphism in Lines of House Mice Selected for High Activity Levels

We conducted artificial selection to study the correlated evolution of locomotor behavior and exercise physiology. From the outbred Hsd:ICR strain, we began 8 closed lines, each of 10 breeding pairs. In 4 lines, we used within-family selection to increase voluntary wheel running. The other 4 lines were random-bred (within lines) as controls. Various traits are monitored for correlated responses. Here, we report on organ masses, with emphasis on the triceps surae muscle complex, an important extensor of the ankle. Mice from the selected lines exhibit reduced total body mass, increased relative (mass-corrected) kidney mass, and reduced relative triceps surae mass. A discrete muscle-mass polymorphism was observed: some individuals had triceps surae that were almost 50% lighter than normal for their body mass. This small-muscle phenotype was observed in 3 of the 8 lines: in one control line, it has fluctuated in frequency between 0 and 10%, but in two of the selected lines it has increased in frequency to approximately 50% by generation 22. Data from parents and offspring indicate inheritance as a single autosomal recessive allele. This “major gene” is estimated to have been present at low frequency (7%) in the base population, and analysis indicates that strong selection favors it in the selected but not control lines. We hypothesize that the small muscles possess functional characteristics and/or that the underlying allele has pleiotropic effects (e.g., reduced total body mass; increased relative heart, liver, and kidney mass) that facilitate high wheel running. Studies of muscle function are currently underway. NSF IBN-9728434 to TG, DEB-9974017 to MM, DEB-0083638 to PAC.