International Journal of Molecular Zoology, 2025, Vol.15, No.2, 48-57 http://animalscipublisher.com/index.php/ijmz 54 6.2 Comparative study on the adaptation of catfish populations in different ecological regions to hypoxia Catfish are distributed in rivers, lakes, swamps and artificial breeding environments. There are obvious differences in the ability of catfish populations in different habitats to tolerate hypoxia. This difference has both a genetic basis and the contribution of environmental plasticity factors. Comparison of catfish hypoxia tolerance in river and lake environments reflects the important influence of hydrodynamics and oxygen conditions. In aquaculture environments, different management conditions and ecological patterns affect catfish oxygen tolerance. High-density pond culture often causes hypoxia in the early morning due to the consumption of oxygen by algae and microorganisms at night. Catfish populations that have been in this environment for a long time may have increased their hypoxia tolerance threshold through phenotypic plasticity (Figure 2) (Kumar et al., 2018). Figure 2 An intensively aerated commercial catfish pond in Mississippi (Adopted from Kumar et al., 2018) Catfish populations in different geographical regions have different hypoxia tolerance due to long-term exposure to different climatic and hydrological conditions. Tropical regions have high water temperatures, high biological oxygen consumption, and more frequent hypoxia stress on fish, so catfish there tend to be more "tolerant" than populations in temperate regions. For example, bearded catfish in tropical freshwater environments in Africa generally have excellent hypoxia tolerance, which is related to the fact that the ponds they inhabit are often hypoxic at night. In contrast, channel catfish in large lakes in temperate North America rarely encounter extreme hypoxia, and their tolerance may not be as good as the former. Through cross-population hybridization and introduction, the genetic oxygen tolerance differences of catfish of different lineages can be compared. For example, the hypoxia survival rate and related physiological indicators of the hybrids can be observed by hybridizing African catfish with strong hypoxia tolerance and local catfish (Kang et al., 2017; Borowiec et al., 2018). 7 Application Prospects and Research Prospects 7.1 Hypoxia-tolerant breeding strategies and molecular marker screening In the field of aquaculture, it is of great significance to improve the hypoxia tolerance of cultured varieties. The genetic improvement of hypoxia tolerance in catfish can be started from two aspects: traditional breeding and modern molecular assisted breeding. On the one hand, traditional family selection has been used to screen catfish strains with strong stress resistance. It is reported that after artificial selection, several cultured strains of American channel catfish have increased their hypoxia tolerance survival time by about 20%-30% compared with unselected wild strains (Sun et al., 2014; Wang et al., 2017). On the other hand, molecular marker-assisted breeding and whole genome selection provide more efficient means for the improvement of hypoxia tolerance traits. The breeding of hypoxia-tolerant catfish varieties requires the comprehensive use of traditional and modern technical means: there must be a pressure selection process of repeated screening in a hypoxic environment, and it is also necessary to be good at using molecular markers to lock in dominant genes. With the advancement of catfish whole genome sequencing and functional genomics research, a series of molecular markers related to hypoxia
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