Genetic analysis of nurse dams selected for six-week body weight or postweaning gain in mice.

A modified crossfostering technique was developed to compare the performance of nurse dams in selected and control populations of mice. The H6 and M16 populations were selected for increased 6-week body weight and 3- to 6-week postweaning gain, respectively, while the C2 and ICR populations were the respective controls. Crossfostering was performed using H6, M16 and their reciprocal F1 crosses as nurse dams in the selected crossfostering group and C2 ICR and their reciprocals in the control group. Measurements recorded for nurse dams included mean body weight of 8 young within a nursed litter at birth (MWB) and 12 days of age (MW12). The latter was used as a measure of postnatal maternal performance. Other traits recorded for nurse dams were number born (NB), body weight at parturition (DWP) and 12 days postpartum (DW12), and weight gain (DWG), feed intake (FED) and efficiency (EFF = DWG/FED) for the first 12 days of lactation. The correlated response in MW12 was negative (P less than .01) for M16 and essentially zero for H6. Both lines exhibited positive (P less than .01) correlated responses in DWP and DW12 and no change in EFF. Only the H6 line increases significantly in DWG and FED as a result of selection. NB increased in M16 and H6, but was significant for the latter population only. Population differences in selection response [(M16-ICR)-(H6-C2)] were significant for FED only, primarily due to average direct genetic effects. Direct comparisons of M16 and H6 indicated that M16 was larger in DWP and DW12 but smaller in DWG and EFF. Average direct genetic effects favored M16 for NB, DWP, and DW12, whereas average maternal genetic effects favored H6 for NB, DWP, DW12 and FED. Percent direct heterosis, in F1 crosses of selected populations was significant for MW12 (13.7%) ,FED (10.8%) and NB (11.4%). Direct heterosis in F1 crosses of the controls was significant for MW12 (9.4%), NB (6.6%), DWP (3.5%), DW12 (3.3%) and FED (4.4%). The effects of MW12, DWG and metabolic body size (MBS) accounted for 47% of the variation in FED, pooled within populations. Of these variables, MW12 accounted for the highest proportion (32%) of variation in total feed intake.