New advances in cloning and their potential impact on genetic variation inlivestock

Cloning has advanced through the recent demonstrations that it is feasible to produce, in principle and with significant effort, an unlimited number o f individuals of identical genotype from differentiated cell lines that hav e been frozen and thawed. These advances have been based upon understanding the importance of interactions between the stage of the cell cycle of both the oocyte and donor cell for the success of the nuclear transfer. Whilst the impact of the biological advance is immense for biomedical applications , the significance is less clear for livestock breeding. In our view the sc ientific issues for breeding programmes lie in whether clones can increase genetic progress without a cost to biodiversity. Biodiversity within a spec ies may be categorized as: (i) between-breed variation; (ii) genetic variat ion among parents within breeds; (iiii) genetic variation among individuals within a farm; and (iv) allelic variation within an individual. In the fac e of a vapid global decline in breed diversity, cloning, in particular clon ing of adults, may be an important route to protect biodiversity since it m ay allow far more genetic variation to be made available for new breed deve lopment in the future than is practicable at present. For variation among p arents, the judicious use of clones may give significantly faster rates of progress without increasing the rate of loss of genetic variation and furth ermore can help improve traits associated with health and welfare which are at present less tractable than, say, milk yield. Local diversity within a farm may be greatly affected if cloning is utilized to disseminate genetic progress widely and move answers are required on the importance of genetic variation within any one locality either in buffering diseases or ameliorat ing other management problems. Experience from clonal forestry can provide some indications but now there are models capable of answering this questio n directly in livestock. Allelic variation within individuals per se is not generally advantageous but at loci where it is identified to be beneficial , the use of cloning may exploit it more widely.