Mountain Fish Genomes Reveal How Extra DNA Helps Survive High Altitudes

A new study shows how a mountain fish called Schizothorax nukiangensis has extra copies of its genes that let it thrive in fast‑moving, low‑oxygen rivers. Scientists first measured the fish’s DNA size and chromosome number. They found 100 chromosomes arranged in 25 matching pairs, meaning the fish carries four copies of each gene – a condition called autotetraploidy. Using long‑read DNA sequencing, Hi‑C chromosome mapping and careful separation of the four copies, researchers built a complete genome map. The analysis confirms that this extra DNA appeared about 680, 000 years ago in a single event unique to the fish’s lineage. When the genome was compared with related species, many gene families involved in immune defense, oxidation control and mobile DNA elements had expanded. These changes likely helped the fish fight infections, cope with low oxygen levels and adapt to rapid currents in mountain streams.Population surveys across three altitude zones showed small but clear genetic differences. Genes linked to bone growth, immune protection and cilia movement – the tiny hair‑like structures that help fish swim in strong water – were under selection, suggesting adaptation to the steep, fast rivers. RNA studies revealed that at higher elevations certain genes for immunity and stress resistance are more active, matching patterns seen in other high‑altitude fish. This indicates that different species use similar molecular tricks to survive the same harsh conditions. Overall, the research demonstrates that recent chromosome duplication, along with new gene functions and varied expression of duplicated genes, has enabled this fish to dominate the extreme habitats of the Salween River basin. It offers a clearer picture of how freshwater animals evolve in high‑altitude environments.