Embryo production and cloning: Their impact on livestock improvement

For farmers and animal scientists, there has always been a certain fascination with the possibility of producing copies or 'clones' of superior animals in some numbers. Attempts to increase the number of embryos available for transfer, by splitting or even quatering in vitro produced (IVP) embryos from genetically superior animals have been reported. Such procedures were regarded as an interim measure before the advent of commercially acceptable large scale cloning technology. Nuclear transfer (NT) has been used as a method for the cloning of 'elite' embryos. Despite much work in NT in amphibians, dated back to the early 1950' s, it is only in recent decades that similar cloning approaches have been successfully applied in farm animals. Willadsen in 1986 showed that it was possible to produce multiples (clones) by fusing a whole nucleated blastomeres from a donor sheep embryo into an enucleated recipient oocyte which opened up an important new exploration in cloning. The ultimate goal for a commercial embryo cloning project is likely to be the production of an unlimited number of identical offspring from and original high-genetic-quality embryos. The establishment of embryonic stem (ES) cell lines would be of great value for large scale cloning. It would be possible to use such ES cells as donors of nuclei for transfer to enucleated oocytes. The number of clones would then become practically limitless. However, only Campbell et al. in 1996 first reported of live lambs born following NT of an established cell line which derived from sheep embryos. The same group of investigators also reported the first success of cloning from sheep. The lamb born after NT from a mammary gland cell is the first mammal to develop from a cell derived from an adult tissue. Cloning could be useful in dairy cattle by allowing genotypes of the highest genetic merit to be introduced directly into commercial herds, thus circumventing the tradition breeding process of upgrading. In experimental animals cloning could be employed to produced genetically identical offspring for biomedical research. Cloning is also useful to multiply the transgenic animals for the production of human proteins of biomedical importance.