LONG-TERM RESTRICTED INDEX SELECTION IN MICE DESIGNED TO CHANGE FAT-CONTENT WITHOUT CHANGING BODY-SIZE

The objective of this study was to determine if low secondary selectio n differentials, caused by selecting within full-sib families, may hav e accounted for the failure of an intended restricted selection index to reduce epididymal fat pad weight (EF) without changing body weight (BW) in mice. Replicate lines that had been selected within full-sib f amilies for high (HE) or low (LE) EF, while holding BW constant, were crossed. After two generations of random mating, two replicates were s ampled and selection initiated for the same restricted index criteria except that mass selection was used to increase the selection differen tials. In both phases of selection the HE restricted index selection, designed to increase EF without altering BW, was in agreement with exp ectation. In contrast, the LE index, designed to decrease EF without c hanging BW, did not agree with theory since BW increased while EF decr eased only slightly. Therefore, reduced selection differentials could not explain the deviation from theory. A possible explanation may resi de in the restricted selection index being more sensitive to changes i n genetic parameters due to shifts in gene frequency as a consequence of the selection applied. However, linkage disequilibrium and genetic drift can not be ruled out as contributing factors to the asymmetry of response.