Effect of selection for development rate on reproductive onset in female mice

This research reports analyses of correlated response in reproductive onset in ICR mice after 23 generations of restricted index selection for diverge nt body weight gain, early (birth-10 days) or later (28-56 days) in life. L ong-term selection altered growth trajectories and 56 day body weight of in dividuals under different selection regimes in this study. Mice in lines un der early selection have the same percentage mature weight at vaginal openi ng as controls (63%). Vaginal opening is delayed in mice selected for slow early growth, which take longer to reach what appears to be a weight thresh old. In contrast, individuals in lines selected for later slow growth under go vaginal opening at the same age as controls, but at a lower weight and i ncreased percentage mature weight. Pre-compensation or 'counter-balance gro wth' is observed in these lines, with mice selected for late enhanced growt h reaching 52% of mature weight at vaginal opening while mice with late slo w growth attain 71% of mature weight prior to vaginal opening. Only 42% of mice with late slow growth attain first oestrus by 56 days. We speculate th is is a function of growth rate and fat/lean ratio. Mice with early slow gr owth show compensatory growth, reaching first oestrus at a similar time to controls. We conclude that selection for growth rate has asymmetrically aff ected reproductive onset, with lines selected for suppressed gains experien cing delays in the reproductive onset traits measured.