Reproduction Supplement 61, 131–150 Cloning in livestock agriculture

A tremendous amount is required of the oocyte cytoplasm to reprogramme a differentiated donor nucleus after somatic cell nuclear transfer so that it re-acquires a state of totipotency and can form a cloned individual. These reprogramming events must occur in a relatively short period after embryo reconstruction, quite unlike the situation during gametogenesis. It is remarkable that nuclear transfer can produce physiologically normal animals, but the process is highly prone to epigenetic errors. Aberrant patterns of gene expression are believed to contribute to the cumulat-ive losses and abnormal phenotypes observed throughout development, from embryonic to adult stages. As a consequence, cloning efficiencies with current nuclear transfer technology are low and limit practical ap-plications in farming. These applications depend upon the reliability with which nuclear transfer can reproduce specific genotypes and phenotypes in resulting offspring. Present animal welfare issues are a significant bar-rier for acceptance of nuclear transfer in agriculture. Any long lastin