Comparison of lifetime performance in mice under crisscross, repeat hybrid male cross and random mating systems

Mating systems that capitalize on heterosis in dairy cattle are the criss-cross (CC), the repeat hybrid male cross (RHMC) and random mating within a synthetic population (SYN). When performance is determined solely by direct additive genetic and dominance genetic effects, expected performance under CC (averaged over four generations after F1 generation), relative to that under RHMC (or SYN) is (59 G1+69 G2 +82 H)/64(G1+G2+H), where Gi is direct additive genetic effect of breed i and H is direct heterosis. Five CC, five RHMC and one SYN population of mice were prepared to test 533, 534 and 410 females, respectively for performance during lifetime (155 days after mating). Each female was pair-mated at day 42 with a male from the SYN population and the number of lactations during the lifetime (NL), total number (TN) and weight (TW in g) of young born alive during lifetime, total number (AN) and weight (AW in g) of young raised to weaning (18 days), and actual length of reproductive life (RL in days) were recorded. Observed performance averaged over four generations was, under CC, RHMC and SYN, 4.74, 4.62 and 4.56 for NL, 49.9, 48.2 and 48.8 for TN, 86.0, 83.6 and 85.1 g for TW, 47.5, 45.5 and 46.3 for AN, 512.1, 517.9 and 521.1 g for AW, and 120.0, 117.6 and 116.7 for RL, respectively. Heterosis due to the female (H) was 10, 30, 33, 34, 43 and 9% for NL, TN, TW, AN, AW and RL, respectively. Direct additive genetic values were estimated for each pair of lines involved with CC or RHMC. These values were used in the formula to calculate expected performance in each mating system. The ratio of CC to RHMC for the expected and observed performance was 1.01 and 1.01 for NL, 1.04 and 1.04 for TN, 1.04 and 1.03 for TW, 1.04 and 1.04 for AN, 1.05 and 0.99 for AW, and 1.01 and 1.02 for RL, respectively. The ratio of CC to SYN for the observed performance was 1.04 for NL, 1.02 for TN, 1.01 for TW, 1.03 for AN, 0.98 for AW, and 1.03 for RL. As expected, the observed mean performance under CC was slightly larger than that under RHMC or SYN.