International Journal of Aquaculture, 2025, Vol.15, No.3, 99-115 http://www.aquapublisher.com/index.php/ija 105 temperate East Asia, the Asiatica group (Channa argus sinensis, etc.) is confined to the subtropical waters of southern China, and the Striata group (Channa tanguta, etc.) is concentrated in the plain waters of tropical Southeast Asia. This correspondence between geographical distribution and systematic lineages suggests that different lineages may have evolved independently under regional isolation and environmental selection. Taking Chinese waters as an example, at least three major lineages can be identified: the northern temperate lineage (Channa argus belongs to), the South China subtropical lineage (Channa argus maculata and Channa argus sinensis belong to), and the southwestern mountain lineage (Channa argus brevis belongs to). The northern lineage of Channa argus is widely distributed in a large area north of the Yangtze River. It is the most widely distributed species in the genus Snakehead, with relatively rich genetic diversity, and presents several branches on the phylogenetic tree, corresponding to populations in different regions. In contrast, the distribution range of Channa maculata and Channa sinensis of the South China lineage is relatively limited, especially Channa sinensis, which is mainly found in the middle and lower reaches of the Yangtze River and local water systems to the south, and is considered to be an evolutionarily independent branch. Small species such as Channa brevis have multiple closely related species in mountain streams in South China and Southeast Asia to form a lineage group (Gachua Group), the internal subdivision of which is not completely clear and may contain several cryptic species. The geographical units corresponding to these lineages (northern plains, southern hills, mountain streams, etc.) reflect the impact of geological events and ecological environment on the evolution of Snakehead. Lineage differentiation analysis can also quantify the genetic distance between different lineages. The genetic distance calculated from the whole mitochondrial genome sequence shows that the genetic differences between species of Snakehead are large. For example, the sequence difference between Channa brevis and Channa maculata is about 8%~10%, while some distant species of the same genus can reach more than 15% (Zhou et al., 2019; Wang et al., 2021; Wang and Zhao, 2024). This shows that the main lineages of Snakehead are anciently differentiated and lack genetic exchange between them. On the other hand, the genetic differentiation between different geographical populations within the species is relatively small. For example, the mitochondrial differences between populations in different regions of China are usually <2%, indicating that a certain degree of genetic exchange or recent common ancestor is still maintained within the species. However, regional branches have also been detected in some widespread species. For example, the North China, Northeast China and Central China populations of the black fish form sub-lineages, corresponding to the historical differentiation caused by glacial climate and river basin isolation. 4.3 Divergence time estimation and evolutionary branching The molecular clock method is used to calibrate the phylogenetic tree of the genus Snakehead, and the time of occurrence of each major divergence event can be estimated. The results of Rüber et al. (2020) using mitochondrial gene data calibration showed that the origin of the genus Snakehead can be traced back to the late Oligocene to the early Miocene, about 20-30 million years ago. Since then, the genus has undergone several radiation differentiations: for example, the divergence between the Asian continental group and the South Asian/Southeast Asian group such as Argus/Asiatica is estimated to have occurred about 10 Ma~15 Ma ago, which coincides with the period of geological changes and climate changes in the Himalayas and Southeast Asia. Particularly noteworthy is that the East Himalayan biodiversity hotspot is considered to be one of the important cradles of the diversification of the genus Snakehead. As the Indian Plate and the Eurasian Plate collided and uplifted, a variety of water systems and climate environments were created, which contributed to the rapid differentiation of snakehead species in the region (Kamran et al., 2020). Analysis of each branch node found that the most recent common ancestor of some living species may have appeared in the Pliocene to the Pleistocene. For example, the phylogenetic nodes of the two major Chinese snakehead species, Channa argus and Channa maculata, are about 5 Ma~8 Ma, suggesting that they had already embarked on their own independent evolutionary paths in the Pliocene. At that time, the climate in East Asia gradually became dry and cold, which may have led to the separation of the north and south water systems, thus giving rise to the cold-resistant Channa argus and the warm-water Channa maculata lineages, respectively. This is
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