ALTERING DEVELOPMENTAL TRAJECTORIES IN MICE BY RESTRICTED INDEX SELECTION

A restricted index selection experiment on mice was carried out for 14 generations on rate of early postnatal development (growth rate from birth to 10 days of age) vs. rate of development much later in ontogen y (growth rate from 28 to 56 days of age). Early rate of development ( E) approximates hyperplasia (changes in cell number) and later rate (L ) reflects hypertropy (changes in cell size). The selection criteria w ere as follows: E+L0 was selected to increase early body weight gain w hile holding late body weight gain constant; E-L0 was selected to decr ease early body gain while holding late gain constant; E0L+ was select ed to increase late gain holding early gain constant; and E0L- was sel ected to decrease late gain holding early gain constant. After 14 gene rations of selection, significant dive among lines has occurred and th e changes in the growth trajectories are very close to expectation. Th e genetic and developmental bases of complex traits are discussed as w ell as the concept of developmental homoplasy.