Description
Few studies have investigated heterogeneity of selection response in replicate lines subjected to equivalent selection. We developed 4 replicate lines of mice based on high levels of voluntary wheel running (high runner or HR lines) while also maintaining 4 non-selected control lines. This led to the unexpected discovery of the HR mini-muscle (HRmini) phenotype, recognized by a 50% reduction in hindlimb muscle mass, which became fixed in 1 of the 4 HR selected lines.