EFFECT OF GENETIC BACKGROUND ON GROWTH OF MICE HEMIZYGOTIC FOR WILD-TYPE OR DWARF MUTATED BOVINE GROWTH-HORMONE TRANSGENES

The effects of a high-growth genetic background on the growth of mice hemizygous for one of two growth hormone transgenes were examined. Mal e mice hemizygous for wild-type (W) and dwarf mutant (M) bovine growth hormone (bGH) transgenes were crossed with females of a high-growth s elected (S) and control (C) line as follows: W x S, W x C, M x S and M x C. Body weights of progeny were recorded weekly from 2 to 10 weeks of age. F1 progeny were classified as carriers (P) or non-carriers (N) of the transgene by assaying tail DNA for bGH using the polymerase ch ain reaction and agarose gel electrophoresis. A deficiency in the numb er of F1 progeny carrying the W (P < 0.05) and M (P < 0.01) bGH transg ene was most likely due to differential prenatal and early postnatal m ortality. Body-weight means of wild-type transgenic mice were larger ( P < 0.05) than those of non-transgenic littermates by 3 weeks of age i n a C background in contrast to 5 weeks in S. The wild-type bGH transg ene increased adult body weights more in the C (155%) than in the S (1 36%) background, indicating transgene expression by selection backgrou nd interaction (P < 0.05). However, the growth response to the wild-ty pe transgene in the S background was still large. The dwarf mutant tra nsgene had a greater effect on growth reduction in the S (70%) than in the C (84%) background, thus causing transgene expression by selectio n background interaction (P < 0.05). Gender by wild-type transgene eff ect interactions (P < 0.001) for adult body weight were caused by the transgene reducing the gender difference for body weight in C and elim inating it in S. The dwarf mutant caused a larger negative effect on g rowth in males than in females, resulting in a gender by dwarf mutant transgene interaction (P < 0.001) for adult body weights. Results indi cate that the effect of a GH transgene on growth can be affected both by a high-growth genetic background and the gender of progeny.