Progress and emerging problems in livestock transgenesis: a summary perspective

The creation of transgenic livestock is a complex multistep procedure the successful execution of which demands a high level of skill and application. Useful animals have been generated by transfer of genes to zygotes by microinjection, but further extension to livestock breeding is severely limited by the present low efficiency and lack of precision in gene transfer procedures. There are major developments in alternative approaches to gene transfer and those based on embryonic stem (ES) cell lines show particular promise as a broadly adaptable means of allowing precise manipulation of specific genes within the animal genome. Rapid progress is being made in adapting ES cell technology to livestock species but as yet no one has demonstrated the totipotency of the putative cell lines so far generated. The demonstration of the feasibility of the chimaeric route for reinstating an ES cell genome into the germ line of the pig is a major advance. For other livestock breeds, particularly those with long generation times and bearing single young where the chimaeric route is much less useful, there are encouraging developments in nucleus transfer (cloning) technology which could provide practical solutions. Overall, there are now good reasons to be optimistic that transgenesis will eventually be available to all livestock breeders with the proviso that there are no further unanticipated phenomena such as the effect of tissue culture on imprinting, to be discovered to threaten the predictability of outcome of ES cell-derived pregnancies and further limit the potential usefulness of this futuristic technology to the livestock industry.