QUANTITATIVE GENETICS OF SPRINT RUNNING SPEED AND SWIMMING ENDURANCE IN LABORATORY HOUSE MICE (MUS-DOMESTICUS)

We tested the hypothesis that locomotor speed and endurance show a neg ative genetic correlation using a genetically variable laboratory stra in of house mice (Hsd:ICR: Mus domesticus). A negative genetic correla tion would qualify as an evolutionary ''constraint,'' because both asp ects of locomotor performance are generally expected to be under posit ive directional selection in wild populations. We also tested whether speed or endurance showed any genetic correlation with body mass. For all traits, residuals from multiple regression equations were computed to remove effects of possible confounding variables such as age at te sting, measurement block, observer, and sex. Estimates of quantitative genetic parameters were then obtained using Shaw's (1987) restricted maximum-likelihood programs, modified to account for our breeding desi gn, which incorporated cross-fostering. Both speed and endurance were measured on two consecutive trial days, and both were repeatable. We i nitially analyzed performances on each trial day and the maximal value . For endurance, the three estimates of narrow-sense heritabilities ra nged from 0.17 to 0.33 (full ADCE model), and some were statistically significantly different from zero using likelihood ratio tests. The he ritability estimate for sprint speed measured on trial day I was 0.17, but negative for all other measures. Moreover, the additive genetic c ovariance between speeds measured on the two days was near zero, indic ating that the two measures are to some extent different traits. The a dditive genetic covariance between speed on trial day 1 and any of the four measures of endurance was negative, large, and always statistica lly significant. None of the measures of speed or endurance was signif icantly genetically correlated with body mass. Thus, we predict that a rtificial selection for increased locomotor speed in these mice would result in a decrease in endurance, but no change in body mass. Such ex periments could lead to a better understanding of the physiological me chanisms leading to trade-offs in aspects of locomotor abilities.