International Journal of Molecular Ecology and Conservation, 2025, Vol.15, No.3, 123-133 http://ecoevopublisher.com/index.php/ijmec 127 more mixed. At present, in Western Europe (such as Spain and Italy), multiple alien European catfish populations are derived from mixed releases of seedlings from different origins. It is speculated that there is multi-source hybridization, so the genetic diversity within the alien population is higher. In North America, native catfish are concentrated in the family Ictaluridae, including species such as channel catfish, blue catfish, and flathead catfish. Large North American catfish are widely distributed in connected water systems such as the Mississippi River and its tributaries. Under natural conditions, gene exchange between populations is relatively smooth, and the overall genetic structure is not very different (Ondračková et al., 2025). Janzen and Blouin-Demers (2023) studied North American channel catfish and found that although the species inhabits different types of habitats (rivers and lakes), it generally shows low genetic differentiation, and there is no obvious geographical gradient in neutral genetic variation among populations in various places. However, local structures have also been observed in certain specific environments. For example, differences in habitat preferences may cause channel catfish to form substructures in large rivers and adjacent lakes. Blue catfish, another important species in North America, has been introduced in large numbers into the Atlantic coastal basin as a sport fish, and has produced invasive populations in places such as the Chesapeake Bay. The blue catfish populations in these areas are established by exogenous individuals and may be mixed with different genetic strains, causing the genetic composition of the new population to change compared with the place of origin. 4 Dynamic Mechanism and Diffusion Path of Global Catfish Gene Flow 4.1 The limiting effect of geographical and ecological barriers on gene flow Geographical barriers are the primary factor limiting gene flow. For most catfish, land and mountains constitute a hard barrier to the spread of freshwater fish. Catfish populations in different basins are often isolated for a long time and cannot exchange genes through natural channels. For example, there is no direct connection between the major basins of the Nile River, Congo River, and Niger River in Africa, so catfish populations in different rivers have evolved independently (Popoola, 2022). The Mekong River catfish can flow downstream to the coast, then migrate horizontally along the coastline, and then enter another tributary, thus realizing a migration route that "bypasses the estuary". Studies have observed that the populations of catfish in different tributaries "converge" and "recombine" after entering the sea, and migrate parallel long distances along the coast, resulting in genetic mixing of the populations in each tributary (Duong et al., 2023). This shows that for catfish with strong salt tolerance, seawater is not an insurmountable barrier. In contrast, even if strictly freshwater species are not far apart geographically, gene flow is also difficult to achieve if there are saltwater or completely dry areas in between. Habitat fragmentation (such as wetland drying and seasonal interruption of rivers) can also form ecological isolation, temporarily or permanently isolating catfish populations (Valenzuela-Aguayo et al., 2019). In addition, reproductive habits and behaviors can also act as barriers to gene flow. Even if some catfish live in connected water bodies, differences in spawning site preferences or reproductive migration routes may lead to "sympatric isolation". For example, if two populations spawn in different tributaries of the same river, and the migration of young fish has an imprinting effect, then over time, the genetic exchange between the two populations will decrease. However, this mechanism has been less studied in catfish and more observations are needed. 4.2 The role of paleoclimate change and geological events Historically, climate and geological changes have shaped the current distribution pattern of catfish populations and have had a profound impact on gene flow. During periods of glaciation and sea level change, different basins may have been connected or separated. For example, the Amazon and Orinoco basins in South America have been temporarily connected and separated by tributary spillovers many times in geological history, resulting in complex interweaving of catfish lineages in the two basins. There is evidence that some catfish (such as species of the genus Hypophthalmus) exchanged genes during the historical connection period of the Amazon-Orinoco, and then evolved separately after the rivers separated, forming similar but independent species today (Lujan et al., 2017; Collins et al., 2018). At present, there are no shared Pseudoplatystoma catfish species in the Amazon and Orinoco, which is believed to be caused by geological isolation.
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