The impact of a transgene for ovine growth hormone on the performance of two breeds of sheep.

The effect of a transgene encoding ovine growth hormone and regulated by a metallothionein promoter was examined in progeny of 69 Merino ewes and 49 Poll Dorset ewes that were inseminated by rams heterozygous for the gene construct. The presence of the transgene had no effect on the progeny from one of the three rams used, as evinced by a normal concentration and secretion pattern of growth hormone and normal growth rate and fatness. In progeny from the other two rams that bore an actively transcribed and translated copy of the transgene, the mean concentration of growth hormone in the plasma was twice that of controls, but the pulsatility of secretion was lost. These animals grew faster (P < 0.001) and were leaner (P < 0.001), but had a greater parasite fecal egg count (P < 0.001). The impact of the transgene differed between breeds with greater wool growth rate (P < 0.01) and live weight increase (P = 0.06) in Merino progeny compared with Poll Dorset cross. At 18 mo of age, the depth of the eyemuscle was decreased (P < 0.001), particularly in female sheep (P < 0.01). The results indicate that the production effects of genetic manipulation may depend on the age, the breed, and the sex of the animal. Furthermore, the transgene may fail to be expressed in some progeny so that its activity cannot be detected, even though the sheep bear the DNA construct.