A b s t r a c tConservation of biodiversity is one of the global challenges of the modern world. The preservation of animal genetic resources is considered essential for the food supply, since sustainable food production appears to be the greatest problem due to the human population growth, depletion of the Earth's natural resources, and many species becoming endangered. In situ and ex situ methods of preservation of the species (i.e. in/out of their natural habitats, respectively) are two major approaches to animal biodiversity conservation. Ex situ strategy involves the techniques for the genetic material cryopreservation. Cryopreservation of the wildlife biomaterials allows to use these genetic resources not only for the conservation and the renewal, but also for the introduction into the genotype of the farm animals. The bison (Bison bonasus) is identified as the rare and endangered species. At present, the free-living bison population in Russia comprises more than 1500 animals. A research concept of the Russian bison gene pool preservation includes creating cryo-preserved pool of bison spermatozoa. In this paper we report findings on biological adequacy of the cryopreserved epididymal bison semen after the long storage (for more than 20 years). The sperm samples were collected postmortem from the testicular appendages of four bison males sustained the injuries incompatible with life or culled and used for hunting. For the assessment of semen motility we used a computerassisted semen analysis (CASA) device; the DNA fragmentation index was assessed in AO-test with the acridine orange staining. The acrosomal integrity was studied by Diff-Quik staining method. It was shown that the semen quality parameters differed significantly due to the individual peculiarities of the bison. The spermatozoa of A + B grade which performed good motility and rectilinear motion reached more than 28 % in the semen of the males Mutfil and Morus, while in the Avel's and Misir's semen over 67 % spermatozoa were non-motile and 12.1 % and 10.4 % spermatozoa exhibited rotational and vibrational motions, respectively. The frequency of spermatozoa with pathomorphological changes significantly varied depending on the individual properties of the bison, with the greatest and the lowest values of 14.6 % and 6.8 %, respectively. The DNA fragmentation index reflecting sperm chromatin integrity can depend on the numerous biotic and abiotic factors and may vary in great ranges. In our surveys, it varied from 7 % to 86 %. For all the morphometric parameters, except the head width, the bison spermatozoa were inferior to the spermatozoa of the bulls though the differences between animal groups were not statistically significant. However, the area of the spermatozoa head in bulls was 3.14 m 2 larger than that of bison.
