Transgenic perspectives in livestock science: a review

The limitations of existing transgenic technology, the potential of cloning technology to overcome these, as well as technologies which may be available in the future for inserting new genetic material are discussed. Currently, transgenic livestock are produced by injecting hundreds to thousands of copies of a particular transgene into the pronucleus of a fertilised egg. This method suffers from a number of inherent limitations that prevent the full potential of this technology from being explored. Most of these limitations stem from the fact that it is impossible to control the site at which the transgene becomes inserted. Transgenic technology holds considerable promise for the livestock industries as well as having important biomedical applications. However, before any of these possibilities can be realised, technology is required whereby a single copy of a particular transgene can be inserted or ‘knocked in’ at a site that does not interfere with expression, as well as having the capacity to ‘knockout’ existing genes. This is possible in mice using a combination of homologous recombination and embryonic stem cell technologies. However, despite considerable effort worldwide, embryonic stem cells are yet to be isolated from any of the livestock species. The ability to clone these now means that somatic cells most notably fetal fibroblasts, can used for gene targeting purposes instead of embryonic stem cells. However, this method is not without its limitations and it is possible that more efficient methods will be developed in the future. In particular, the use of mammalian artificial chromosomes will extend this technology to allow combinations of transgenes as well as chromosomal segments to be incorporated, allowing us to explore the full potential of transgenic technology for agricultural as well as biomedical applications.