International Journal of Aquaculture, 2025, Vol.15, No.3, 99-115 http://www.aquapublisher.com/index.php/ija 108 Geographic isolation restricts gene flow, causing local populations of snakehead to evolve along independent trajectories and gradually form mitochondrial differences. This isolation includes both large scales (such as the Nanling Mountains separating the north and the south, and China and the Southeast Asian continent), and local scales (such as the isolation of different river basins). The longer the isolation and the more difficult the barriers are to overcome, the greater the lineage differences tend to be. In the snakehead genus, it can be seen that the lineages of snakeheads in the Yangtze River and Pearl River basins have been clearly distinguished, while there is no significant difference in the scorpionfish between the tributaries of the Pearl River, which is a reflection of the degree of isolation. Under the effect of geographic isolation, not only does the mitochondrial genome drift and differential selection occur, but snakeheads in different regions also gradually differentiate in morphology and ecology. For example, the northern black snakehead is more cold-resistant and large in size, while the southern scorpionfish is more adapted to warm water and slightly smaller in size. These regional variations in turn consolidate the independence of the lineages. Of course, geographic isolation is not absolute, and in some cases human activities can break natural barriers. For example, hybrid snakeheads (the first generation of the hybrid of black snakehead and scorpionfish) in the south have been introduced and cultured in the northern region. Once they escape, they may cause the original lineage to mix. But in general, the geographical correspondence of each lineage of the genus Snakehead under natural conditions is clear, and isolation has shaped the current genetic pattern. 5.3 Regional lineage markers and adaptation clues Through regional comparison, we can try to identify some lineage-specific molecular markers and preliminarily explore the adaptive significance of the evolution of the lineage of the genus Snakehead. The so-called lineage marker refers to genetic variation that only appears in a specific regional lineage, such as a specific mitochondrial haplotype or mutation site, which can be used to trace the origin and spread of the lineage. Taking the northern lineage of Channa argus as an example, its mitochondrial D-loop region has a unique deletion mutation, which is only found in populations north of the Yellow River, but not in Channa argus in the Yangtze River Basin and south of it. This deletion mutation can be used as a marker of the northern lineage to identify the source of suspicious released populations. Similarly, the South China lineage of Channa maculata carries a unique microsatellite repeat sequence that does not exist in other snakeheads in Southeast Asia, which can help distinguish the mitochondrial types of Channa maculata in South China and the closely related species Channa annan. In terms of adaptive evolution, the environments of lineages in different regions are significantly different, so they may have accumulated genetic characteristics adapted to them. Although the nuclear genome plays a more important role in adaptive traits, the mitochondrial genome may also be involved in environmental adaptation because it controls energy metabolism. For example, the lineage of black snakehead in high-latitude areas may select more efficient energy metabolism mutations in mitochondrial genes to maintain activity in cold water temperatures. In contrast, snakeheads in tropical hypoxic waters (such as some small snakeheads in Thailand) may have substitutions in mitochondrial complex genes that are conducive to hypoxic respiration. Studies on plateau schizothorax have found that certain mutations in the mitochondrial ND1 gene improve the efficiency of electron transfer, allowing it to effectively produce energy in a low-oxygen, high-altitude environment (Jin et al., 2018; Li et al., 2018). Similar mechanisms may also exist in the genus Snakehead: for example, the broad-fronted snakehead lives in a mountain stream environment where the water temperature is relatively low and food is scarce. It is speculated that its mitochondria may have evolved a more energy-efficient respiratory chain configuration. This needs to be further verified by comparing the mitochondrial gene functions of the broad-fronted snakehead and the plain snakehead. The adaptive evolution of regional lineages is also reflected in behavior and life history. Most snakehead species in the Indochina Peninsula reproduce in the monsoon rainy season and have a strong adaptation to seasonal rhythms; while the black snakehead in northern China can survive under the ice in winter and have significant tolerance to low temperatures. The genetic basis behind these differences may partly come from mitochondrial
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