DIVERGENT SELECTION FOR HEAT-LOSS IN MICE .1. SELECTION APPLIED AND DIRECT RESPONSE THROUGH 15 GENERATIONS

Divergent selection for heat production/loss (kcal.kg(-.75).d(-1)), me asured in 9- to 11-wk-old male mice, was conducted for 15 generations. Heat loss was measured for 15 h on individual animals placed overnigh t in direct, gradient-layer calorimeters. Selection for high (MH) and low (ML) heat loss and unselected control (MC) occurred in each of thr ee replicates for a total of nine unique lines. Repeatability of the h eat loss measurement was .45 and the CV was 10.5%. Cumulative realized selection differentials, averaged for the three replicates, were 145. 1 and -105.0 (kcal.kg(-.75).d(-1)) and ranged from 136.9 to 149.2 and -107.1 to -101.3 for MH and ML selection, respectively. Cumulative sta ndardized realized selection differentials, averaged for the three rep licates, were 10.06 and -9.51 for MH and ML selection, respectively. D irect responses (kcal.kg(-.75).d(-1)) in heat loss after 15 generation s were 44.2 for MH and -27.4 for ML as deviations from MC. Asymmetry o f response was evident (P =.03) by Generation 10. Realized heritabilit y was .28 +/- .01 based on divergence of MH and ML selection. For sele ction for higher and lower heat loss, realized heritabilities were .31 +/- .01 and .26 +/- .01, respectively.