The generation of transgenic pigs is an entirely new way of breeding. In contrast to classical breeding techniques the objects of manipulation in this case are individual genes rather than the entire genome of an organism. In pigs DNA-microinjection into the pronuclei of zygotes is the only available technique of transferring genetic material developed so far. The process involves collection, manipulation, microinjection, cultivation, and transfer of early embryos and also molecular-biological techniques allowing cloning of gene constructs, preparation of suitable injection solutions, and techniques allowing detection of integrated and expressed transgenes in transgenic animals. Gene transfer in pigs usually yields less than 1% transgenic piglets per injected zygote. Our own experiments have shown that simultaneous transfer of untreated control embryos increases yields from 0.5% to 1%.
Gene transfer in pigs can be employed in particular to increase growth performance and carcass composition by using genes encoding hormones of the growth hormone cascade (GHRH, GH, IGF-I). So far, the effects already known from experiments in mice have not been reproduced in pigs.
We are currently investigating whether the transfer of the influenza resistance gene Mx+ of mice will yield disease-resistant pigs.
Breeding with transgenic animals must take into account that approximately 30% of the primary transgenic animals will be mosaics which will not pass on the transgene to their offspring. Unwanted side effects may also occur during gene transfer. Most important examples are instability of integrated transgenes and variability of gene expression over many generations.
In about 5% of all primary transgenic animals integration of the transgene can be assumed to lead to the generation of insertion mutations. Animals carrying these mutations should not be used for breeding. Furthermore severe health problems may be caused by uncontrolled over-expression of the transgene.
Much more work will be necessary in future before we will be able to employ gene transfer techniques in practical breeding programmes.