We obtained oral biopsies of the Marsican brown bear (small population) and the European brown bear (here the large Slovakian population was used as a control) to proliferate fibroblasts (live cells) in laboratory. Cell cultures were successfully established and maintained. Standard canine cell cultures and the original brown bear cell cultures described above were used to test in vitro the metabolic impact of mutations in the mitochondrion that are present only in the Marsican bear and at the homozygous state in all of its individuals and were predicted to be deleterious according to bioinformatic analyses. These tests were performed transfecting the cells with plasmids carrying the mutation(s) of interests; mitochondrial bioenergetic and cellular functions were measured as fitness proxies. Our results indicated that the standard canine cells with and without an overexpression of the ND5 gene carrying the mutations typical of the European Brown bear (large population) show a similar calcium concentration, membrane potential, and reactive oxygen production, whereas cell cultures transfected with the Marsican mutations show a decrease of the first two parameters and an increase of the last one (Figure). These results point towards a less efficient mitochondrial respiration and energy production in the smaller Marsican population. Similar differences were observed when cell cultures obtained from the Marsican and the European bears were directly compared. We are continuing these cellular experiments in bears to investigate functional mitochondrial parameters and the specific effect of single deleterious mutations on fitness more in detail. We will keep you posted on this blog.