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 efficiency and body composition following short-term selection for large 8-week body weight in transgenic and nontransgenic mice. Replicate lines which either carried or did not carry the sheep metallothionein 1a-sheep growth hormone transgene (oMt1a-oGH) were established. The lines carrying the transgene at an initial frequency of 0.5 came from a high-growth (TM) and a randomly selected (TC) background. The respective nontransgenic lines were identified as NM and NC. Control replicates (CC) came from the randomly selected background. During the selection process the transgene was activated by adding 25 mm ZnSO4 to the drinking water of all mice. Correlated responses were 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 showed marked differences in C compared with Z. For example, daily gain and feed efficiency showed no significantly correlated responses under C and positive responses (p < 0.001) under Z, and the reverse was found for indicators of body fat percentage. These line by environment (Z versus C) interactions may represent a genetic correlation of less than one between a trait expressed in two distinct environments. Thus, in developing lines with a transgene that can be regulated, a critical question is whether selection for quantitative trait(s) should be conducted when the transgene is activated or not activated.
Document Type: Research Article
Publication date: December 1, 2000