How to run far: multiple solutions and sex-specific responses to selective breeding for high voluntary activity levels

Abstract

The response to uniform selection may occur in alternate ways that result in similar performance. We tested for multiple adaptive solutions during artificial selection for high voluntary wheel running in laboratory mice. At generation 43, the four replicate high runner (HR) lines averaged 2.85-fold more revolutions per day as compared with four non-selected control (C) lines, and females ran 1.11-fold more than males, with no sex-by-linetype interaction. Analysis of variance indicated significant differences among C lines but not among HR for revolutions per day. By contrast, average speed varied significantly among HR lines, but not among C, and showed a sex-by-linetype interaction, with the HR/C ratio being 2.02 for males and 2.45 for females. Time spent running varied among both HR and C lines, and showed a sex-by-linetype interaction, with the HR/C ratio being 1.52 for males but only 1.17 for females. Thus, females (speed) and males (speed, but also time) evolved differently, as did the replicate selected lines. Speed and time showed a trade-off among HR but not among C lines. These results demonstrate that uniform selection on a complex trait can cause consistent responses in the trait under direct selection while promoting divergence in the lower-level components of that trait.

Figure 1.

Least-squares means and associated standard errors (corresponding to analyses presented in the electronic supplementary material, table S1 without body mass as a covariate). Numbers indicate ratios of those means for HR/C lines (interaction: (a), p = 0.0355; (b), p = 0.4732; (c), p = 0.0142; (d), p = 0.0195).

Figure 2.

Sex-specific least-squares means and standard errors (from SAS procedure mixed analysis of all eight lines, excluding linetype as a factor) for each of the eight lines for mean daily wheel running and body mass prior to the start of the 6-day wheel test. This graph illustrates the 2.85-fold higher daily running by HR mice (lines numbered 3, 6, 7 and 8), as well as the smaller body sizes of HR mice (filled circle, high runner female; filled square, high runner male; open circle, control female; open square, control male).

Figure 3.

Sex-specific least-squares means and standard errors (from SAS procedure mixed analysis of all eight lines, excluding linetype as a factor) for each of the eight lines for the components of mean daily wheel running, number of 1 min intervals with at least one revolution and mean speed (revolutions per day divided by number of active intervals). This graph illustrates that the among-line variation in speed (y-axis) is significantly greater for high runner lines (numbered 3, 6, 7 and 8) than for control lines (numbered 1, 2, 4 and 5), whereas the among-line variation in time (x-axis) is greater for control lines (see electronic supplementary material, tables S1 and S2). The HR lines show a significant negative relation (‘trade-off’), but the C lines do not (see Results; filled circle, high runner female; filled square, high runner male; open circle, control female; open square, control male).