European bison is the only species of large mammal that has a very high inbreeding coefficient; which reaches 50% in the L line and 28% in the LC line . The high inbreeding coefficient is due to the fact that the rescue of the species was carried out by crossing a very small number of founder animals. Uniformity should prevail in the population of the European bison and variability should be minimal. However, this assumption is not valid at all if we examine the lifespan of individual animals. The lifespan of individual animals is highly variable. Although the median lifespan is only 3.54 years , some animals live to 28 years [15, 16]. Very interesting is the fact, that the median lifespan of females (6.01 years) exceeded more than twice the median lifespan of males (2.71 years). The genus Bison has the biggest sex-related difference in longevity among mammals .
Statistical evaluation of the lifespan of individual animals does not correspond with the normal distribution that is common for most animal species .
The European bison thus represents a very interesting model for the study of longevity-associated genes. Identifying genes associated with longevity in European bison could be useful in long-term conservation of this species and could improve current and future reintroduction programs thanks to selective breeding and deeper knowledge about its genetic background.
We used GWAS followed by confirmation of the results by sequencing to study these genes. The association of the BCKDHB gene with longevity has not been described so far. The BCKDHB gene encodes the E1 beta subunit of the branched-chain keto acid dehydrogenase, which is a multienzyme complex associated with the inner membrane of mitochondria. This enzyme complex is active in the catabolism of branched-chain amino acids. Mutations of this gene have been associated with the maple syrup disease type 1B, a disease characterized by a maple syrup odor of the urine, mental and physical retardation, feeding problems and dihydrolipoamide dehydrogenase deficiency .
The association of the FER1L6 gene with longevity has not been described so far. The FER1L6 gene (FER-1 like family member 6) is associated with diseases including cerebellar ataxia type 43  and Miyoshi muscular dystrophy .
Determining the causal relationship between a particular nucleotide substitution and longevity can be crucial in identifying the predisposition for longevity at the molecular genetic level. SNPs located in gene exons are of greatest importance. We detected that the SNP associated with longevity in the SERPINI2 gene is located in an exon. This nucleotide substitution leads to an amino acid change resulting in the tryptophan presence in the long-lived European bisons, while leucine is present in the reference bisons.
Substitution of one amino acid can lead to a change in the structure of the protein produced, which can then cause a change in its function. Analysis of the structure of such proteins will be the subject of our further research.
The SERPINI 2 gene (SERPIN family I member 2) encodes a member of a family of proteins that acts as an inhibitor of serine protease. These proteins act in the regulation of a variety of physiological processes including coagulation, fibrinolysis, development, malignancy and inflammation . Expression of the encoded protein is downregulated in pancreatic and breast cancer and it is associated with acinar cell apoptosis and pancreatic insufficiency when absent in mice . SERPINI2 deficient mice are growth retarded, have abnormal immunity and reduced lifespan . Association of the SERPINI2 gene with lifespan that was also found in the mice may suggest that the association of this gene with longevity is more general.
This study has its limits due to the low number of sampled European bisons. It will be useful to confirm these results in a study with a larger number of samples. However, the limited sample size still allowed statistical analyses to be performed and to determine the statistical significance of our findings.