Body weight and tail length divergence in mice selected for rate of development

A series of mouse lines has been produced by 19 generations of restricted i ndex selection for fate of development during early and late ontogeny. The selection program was based on an index with the following four replicated selection treatments: E+ and E- were selected to alter birth to 10-day body weight gain while holding late gain for both selection lines constant; cor respondingly, L+ and L- were selected to alter 28- to 56-day body weight ga in holding early gain for both lines constant. Herein, we characterize resp onse to selection for growth rate by analyzing age-specific mouse body weig ht and tail lengths and for growth curves using a logistics model. Selectio n on developmental rate has resulted in divergence in both age-specific and growth curve traits. E+ and L+ lines reached identical weights during the late selection interval, then diverged to unique mature weights. E- and L- lines similarly achieved identical weights during late selection and diverg ed to unique mature weights. However, the shapes of early and late growth c urves were significantly divergent, and at least two distinct growth patter ns are shown to result from selection. Response in body weight gain was acc ompanied by similar, though less pronounced, change in tail length traits. Significant response during intervals of restricted growth was also found, especially in lines selected for late gain. The evolution of the growth tra jectory under restricted index selection is discussed in terms of drift and available additive genetic variation and covariation. (C) 2000 Wiley-Liss, Inc.