Application of zona-free cloning technologies to livestock breeding programmes

Advanced reproductive technologies are powerful tools for accelerating genetic gain because they enable both increased selection intensity and the generation of large numbers of offspring from elite animals. Among these, somatic cell nuclear transfer (SCNT) cloning offers a means to rapidly multiply elite genetics from nucleus herds into commercial populations. Genomic estimated breeding values allow for the accurate assessment of genetic merit in embryos and newborns, creating opportunities to identify elite young stock for cloning. Cloning will also be an important complement to gene-editing as a way to generate animals from cell lines carrying targeted genetic modifications. Nevertheless, the widespread application of cloning remains constrained by low efficiency and high costs, underscoring the need for continued optimisation. Zona-free cloning, also known as handmade cloning, has potential as a simpler and more scalable alternative to conventional micromanipulator-based methods to prepare cloned embryos, and has demonstrated promising improvements in pregnancy and live birth rates across several livestock species. This review summarises advances in zona-free cloning, tracing its development from the microblade to the two-pipette and micropipette methods, and evaluates their relative advantages and limitations. Approaches to improve the health and welfare of clones are examined, and potential applications of cloning in livestock breeding programmes, along with associated regulatory considerations, are discussed. Cloning may become increasingly important as a crucial bridge between lines of genetically elite and possibly gene-edited embryonic cells, and their manifestation as breeding animals in livestock improvement programs.