Genomic potential in mammals.

Embryos of amphibians, fish, sheep, cattle, swine and rabbits have been multiplied by nuclear transfer. Successful nuclear transfer in these species has been accomplished by transfer of a blastomere from a late stage embryo into an enucleated oocyte or egg with large scale multiplication achieved by serial repetition of the procedure using blastomeres from nuclear transfer embryos. This allows the production of clonal lines, which when appropriately selected for performance in a given trait, can be reproduced to capture in the offspring expression of both additive and nonadditive inheritance. The efficiency of producing offspring from nuclear transfer is low in mammals in both frequency of morula or blastocyst produced and maintenance of pregnancy after embryo transfer. In domestic animals the largest number of offspring from one embryo has been eight calves. Embryos as late as the 64-cell stage in cattle and 120-cell blastocyst in sheep have been used successfully as donors of blastomeres. Recloning has also been done in cattle. Potentially, nuclear transfer provides a mechanism for multiplication and production testing of clonal lines, a method for rapid genetic improvement and a means for rapid propagation of a selected genotype.