Correlated changes in fertility and fitness traits in lines of oMt1a-oGH transgenic mice selected for increased 8-week body weight

Correlated responses in fitness and fertility traits were compared in trans genic and nontransgenic lines of mice selected for increased 8-week body we ight. Two replicates of lines which either earned or did not carry the shee p metallothionein-la sheep growth hormone transgene (oMt1a-oGH) were establ ished. Host lines had been previously selected for rapid growth or selected randomly. Within-litter selection was carried out for 13 generations, and a randomly selected control line was kept for each set of replicate lines. Mice were genotyped every generation for the presence of the transgene, but this information was not used in selection decisions. The oMt1a-oGH constr uct was activated by adding 25 mM ZnSO4 to the drinking water from 3 weeks (weaning) until 8 weeks of age. Zinc stimulation of the transgene was not c arried out during mating, gestation and lactation. Correlated responses in fitness traits were measured by regression of least-squares means las devia tions from the control lines) on generation number. Two fitness indexes wer e defined to combine the information on individual fitness traits. The prop ortion of infertile matings was higher in generations 7 to 13 than in gener ations 0 to 6. Correlated responses to selection showed an Increase in the cohabitation to littering interval in nontransgenic lines and an increase i n litter sizes in lines from the selected background. Preweaning pup surviv al did not change over generations. Overall fitness increased in the transg enic line from the selection background whereas no changes were observed in the transgenic line from the control background. The initial frequency of 0.5 of the transgene was reduced to less than 0.10 in the selected backgrou nd, but increased to an average of 0.62 in the control lines. The compariso n of specific mating groups involving transgenic and nontransgenic mates re vealed that the only consistent disadvantage in a transgenic parent was the increase in the length of the cohabitation to littering interval. Major fi tness problems were not associated with the oMt1a-oGH transgene, which make s this construct a potential choice for use in livestock breeding programme s.