Growth hormone transgenic livestock: A mouse model

Transgenic technology offers great potential to livestock production. Transgenic mice were used as a model to evaluate the effects of a MT/bGH transgene on growth, composition and reproduction. Male mice hemizygous for a metallothionein/bovine growth hormone fusion gene were mated to ICR, ND4, B6CBA/F1 and B6SJL/F1 females. Proportion classified transgenic (TG) or nontransgenic (N), weights at 21d (W21) and 42d (W42) and numbers born alive (NBA) and weaned (NW) were evaluated. Of 925 mice weaned, 35.9% were TG. This was unaffected by sire line, dam line or sex. Both W21 and W42 were greater for TG than N mice, and for males than for females. The transgene had its greatest effect on progeny of ND4 and B6CBA females for W21. Dam line affected reproductive traits. F2 and F3 mice were produced and evaluated similarly, except weight at 56d (W56) was also measured. Fewer F2 females from B6CBA and B6SJL backgrounds were TG than any other dam line-sex group. Among F3 animals, a lower proportion of females were TG than males among all backgrounds. Transgenic animals had higher W21, W42 and W56 for F2 and F3 generations. Interactions of transgenotype with sex occurred for W42 (F2) and W56 (F3); the difference between sexes being lower among TG animals. The TG minus N difference for W42 and W56 was greater in outbred than hybrid backgrounds. Dam line affected NBA and NW among F2 litters. Litters sired by TG males tended to be smaller at weaning than those sired by N males. Among F3 litters NBA and NW were unaffected by factors analyzed. A second study evaluated the transgenotype x sex interaction. Hemizygous males mated to ICR females produced experimental animals. At weaning, (21d), tissue samples were collected for PCR analysis, and one-half of males were surgically castrated. Weights at 25, 35 and 45d of age, and feed consumption were recorded. Males were heavier than castrates or females, and TG were heavier than N at 25 and 35d. Weight differences between males and females were smaller among TG mice at 45d. Transgenics consumed more feed than N mice, but no effects of gender or transgenotype were found for feed efficiency. Least-squares means for TOBEC estimated fat-free-mass at 45d ranked similarly to 45d weight. No effect of gender or transgenotype was observed for fat percentage.