Correlated responses to selection for large body size in oMt1a-oGH mice: growth, feed efficiency and body composition

Correlated responses were determined for growth, feed consumption, feed eff iciency and body composition following short-term selection for large 8-wee k body weight in transgenic and nontransgenic mice. Replicate lines which e ither carried or did not carry the sheep metallothionein 1a-sheep growth ho rmone transgene (oMt1a-oGH) were established. The lines carrying the transg ene at an initial frequency of 0.5 came from a high-growth (TM) and a rando mly selected (TC) background. The respective nontransaenic lines were ident ified as NM and NC. Control replicates (CC) came from the randomly selected background. During the selection process the transgene was activated by ad ding 25 mM ZnSO4 to the drinking water of all mice. Correlated responses we re measured with (Z) and without (C) the addition of zinc. After seven and eight generations of selection, the frequency of transgenic mice in line TM had fallen sharply, whereas transgene frequency had risen moderately in TC . The reduced frequency of oMt1a-oGH in the high-growth genetic background may have been caused by a lower additive effect compared with the randomly selected background combined with a fitness disadvantage of the transgene. Therefore, the utility of a transgene In Improving a quantitative trait may depend in part on genetic background. Correlated responses for most traits in NC were similar for Z and C. In contrast, correlated responses in TC sh owed marked differences in C compared with Z. For example, daily gain and f eed efficiency showed no significantly correlated responses under C and pos itive responses (p < 0.001) under Z, and the reverse was found for indicato rs of body fat percentage. These line by environment (Z versus C) interacti ons may represent a genetic correlation of less than one between a trait ex pressed in two distinct environments. Thus, in developing lines with a tran sgene that can be regulated, a critical question is whether selection for q uantitative trait(s) should be conducted when the transgene Is activated or not activated.