Ph.D. in Zoology, August 1998
Artificial selection for high voluntary activity in house mice:
a study in evolutionary physiology
Replicated within-family selection for increased
voluntary wheel running was applied to a base population of outbred house
mice (Mus domesticus; Hsd:ICR strain). Four high-selected and four
randombred control lines were maintained. Prior to selection, heritabilities
of mean revolutions run per day (rev/day), average running velocity (rpm),
and number of minutes during which any activity occurred (min/day) were
estimated by midparent-offspring regression. Heritabilities were statistically
significant for rev/day (0.18+0.064 [S.E.]) and rpm (0.28+0.074
[S.E.]), but not for min/day (0.14+0.088 [S.E.]). After 10 generations,
selected mice ran 75% more rev/day than controls. Rev/day increased mainly
through changes in rpm rather than min/day.
Progeny of these selected and control lines were then used in a series of experiments to test simultaneously effects of genotype (selected vs. control) and environment (wheel access or none) on traits with potential functional relationships to activity levels. In male mice from generation 10 given access to wheels for 7-8 weeks since weaning, both selection and wheel access had significant positive effects on maximal oxygen consumption during forced treadmill exercise (VO2max). Wheel-access selected mice had the highest mass-corrected VO2max, sedentary control mice the lowest. These results demonstrate a genetic correlation between voluntary activity and aerobic capacity.
After 14 generations of selection, body masses of mice given access to wheels at weaning were compared. At 79 days of age, selected mice weighed 15.3% less than controls, whereas wheel-access mice weighed 4.4% less than "sedentary" individuals. This result supports the hypothesis of a negative genetic correlation between body size and activity in house mice.
Finally, body composition was measured to test whether the correlated response in body mass was the result of changes in lean body mass or total fat mass. Mice from generation 13 were given access to running wheels beginning at seven weeks of age. After eight weeks, selected mice were smaller than controls, but the magnitude of the difference was modest compared to that observed in generation-14 mice given wheel-access at weaning. Lean mass, determined by hydrogen isotope dilution, did not differ significantly between selected and control lines, but mice from selected lines had less total body fat.