Fundamental Principles of Cryobiology and Application to Ex situ Conservation of Avian Species

Abstract

Cryopreservation of animal germplasm is an important ex situ conservation strategy. Germplasm can be preserved at subzero temperatures and recovered in a way that allows germline development to be resumed at a later time. This strategy is routinely used for genetic improvement of dairy cattle, maintenance of lab strains of rodents and in human reproductive medication, and it contributes to genetic conservation of a growing number of wild mammalian species. Although there is an urgent need, cryoconservation of germplasm for birds is limited, partly because of skepticism with respect to the effectiveness of cryopreservation techniques. An understanding of fundamental principles of cryobiology and comparative cryobiological and physiological properties of germplasm is essential to the development and optimisation of cryopreservation protocols. In mammals, slow freezing has been used to cryopreserve gametes and embryos but the effectiveness of slow freezing is specific to the cell type and it does not protect extracellular structures. Vitrification, which is a process of solidification without crystallisation, can be used to effectively preserve multicellular structures such as gonadal tissue. In some domestic and wild avian species, the fertility produced by cryopreserved semen is sufficient for conservation but the w chromosome is lost. Avian eggs and embryos cannot be cryopreserved, but gonadal tissue can be cryopreserved and recovered by transplantation. Cryopreservation of germplasm allows ex situ conservation of avian species because it allows repopulation without the limitations of geographic isolation or reproductive longevity.