GENETIC DRIFT AND INBREEDING DEPRESSION MEASURED FROM CONTROL POPULATIONS OF MICE

Abstract

A pedigree control line of mice (ICR stock) with a mean effective population size [Formula: see text] of 110.52 was evaluated over 26 generations. Control lines, sampled from the ICR stock, of 16, 8, 4, 2 and 1 pairs, replicated 2, 2, 4, 8 and 16 times, respectively, were maintained for 15 generations. Mean inbreeding coefficients (F) were 0.10 in the ICR stock and 0.12, 0.20, 0.42, 0.76 and 0.95 in the respective control lines. Traits evaluated were per cent females littering (PFL), number born (NB), 12-day litter weight (W12), body weights at 3 (WK3), 6 (WK6) and 8 (WK8) weeks, gain from 3 to 6 weeks (PWG) and 6-week tail length (TAIL). The significant (P < 0.05) regressions of means on generation number in the ICR stock for PFL (0.22 ± 0.11), W12 (0.18 ± 0.09) and NB (−.04 ± 0.02) were probably due to unknown environmental effects. The remaining traits showed no trends. Analysis of generation trends in the WK6 variance components of the ICR stock showed no significant changes in heritability, total maternal effects or environmental effects. Mean trends in the replicate control lines indicated that smaller Ne tended to show greater average divergence among replicates. For about 70% of the Ne-trait combinations the observed variance among replicates exceeded the predicted drift variance. Linear regressions of control line means for each trait on F, within generations, were significantly (P < 0.01) negative. Male means for WK6, WK8 and PWG showed a significantly greater inbreeding depression than females, even after adjustment for differences in phenotypic standard deviations. When the 1 and 2 pair lines were excluded, the regressions on F were reduced for all traits except NB and female PWG.