Cryobanking of somatic cells in conservation of animal genetic resources: prospects and successes (review)

Extinction of many species is irreversible and is a part of the natural evolution, but human activities have influenced this process, making it much faster comparing to speciation. According to FAO, approximately 20 % of the breeds of cattle, goats, pigs, horses and poultry in the world are currently at risk of disappearance, many have died in the past few years, as a result their genetic characteristics lost forever. The role of banks in the management of genetic resources and the conservation of endangered species is particularly noticeable in the last decade. Most cryobanks focus on the cryopreservation of gametes (primarily sperm) and embryos. Their main goal is to produce offspring using reproductive technologies, which include artificial insemination, in vitro fertilization and embryo transfer. The discovery of the phenomenon of reprogramming somatic cell nuclear allowed expanding the range of forms of biological material in programs for cryopreservation. Creating cryobanks of somatic cells as donors of nuclei for cloning considered an auxiliary instrument for the preservation and improvement of the gene pool of farm animals and poultry. To obtain viable cryopreserved cell lines very small amount of biopsy material, including that of dead animals, is sufficient, but such lines contain the complete genome and proteome. In contrast to germ cells, embryos and generative tissues, the cryopreserved somatic cells after repeated thawing are capable to regenerate, i.e. almost infinitely may serve as a source of biomaterial for use in assisted reproductive technologies and biological research, including retrospective reconstruction. Furthermore, due to the small size the somatic cells are more resistant to cryopreservation. This review also provides a brief description of the principles and history of cloning. The advantages of the use of different cell types as karyoplasts are discussed. In particular, almost all types of cells (e.g. embryonic cells, mammary cells, cumulus, granulosa, oviduct, liver, fibroblasts, white blood cells and embryonic stem cells) can be used for the production of cloned animals, but the cloning efficiency depends significantly on the type of cells. Aiming embryo development and birth of live offspring, the fetal fibroblasts as donors of nuclear material for cloning are most effective. Alternatively, the stem cells may be a source of the nuclei. Stem or progenitor cells (i.e., stem, determined to differentiate in specific type cells) are easier reprogrammed than terminally differentiated cells. Also when stem cells nuclei are used as karyoplasts the number of cloned embryos significantly increased. The advances in interspecific cloning as a strategy for restoration of rare and endangered species are discussed. Numerous examples show that somatic cells can be considered the most promising material for the recovery of animal genetic resources of different types. Particularly from 1997 to 2012 using the differentiated somatic cells the domestic and wild animals of different species such as sheep, mice, cows, goats, pigs, guar, mouflon, domestic cat, rabbits, mule, horse, rat, wildcat, dog, banteng, ferret, wolves, buffalo, deer, mountain goat, camel, coyote were obtained. Cattle are still the leader in the production of cloned offspring with the efficacy 10 % on average, and in some cases up to 25 %, while for most other animals it does not exceed 1 %. Under controlled conditions in farms with good management, the productivity of clones should vary only within the remaining natural variability and mitochondrial genetic variability due to cloning technology.