Restricted index selection in mice designed to change body fat without changing body weight: direct responses

Replicated within full-sib family restricted index selection was conducted for eight generations in mice for high or low epididymal fat pad weight (EF) holding body weight (BW) constant. Pooled realized heritability estimates of index units based on high, low and divergent selection were 0.42±0.20, 0.44±0.19 and 0.42± 0.05, respectively, which were not different from the base population estimate of 0.33±0.10. Realized responses per generation pooled across replicates in the high-fat restricted index lines were in the expected directions for EF (17.5±7.2 mg; P<0.05) and BW (0.03±0.58 g; P>0.05), but responses in the low-fat restricted index lines were discrepant for EF (0.3±5.1 mg; P>0.05) and BW (0.38±0.01 g; P<0.01). Consequently, the realized responses in component traits were decidedly asymmetric (P<0.05). A technique for estimating realized genetic parameters from index selection lines gave realized heritabilities for BW and EF of 0.68±0.04 and 0.45±0.05, respectively, and a realized genetic correlation between BW and EF of 0.93±0.01 compared with base population estimates of 0.43±0.08, 0.49±0.10 and 0.78±0.05, respectively. Possible explanations for the disparity between observed and expected responses in the low-fat restricted index lines include genetic drift, poor estimates of base population parameters, changes in genetic parameters with selection, linkage disequilibrium resulting from selection and asymmetric realized relative index weights.