Revealing Natural Hybridization Between Hypophthalmichthys molitrix and Hypophthalmichthys nobilis Within Their Native Range (Xiangjiang River of China) Through Morphological and Molecular Approaches

Hypophthalmichthys molitrix and Hypophthalmichthys nobilis are economically important freshwater fish species in China. During resource surveys in the Xiangluzhou section of Xiangjiang River, Changsha, in 2020, fish suspected to be natural hybrids of H. molitrix and H. nobilis were identified. To confirm this, we employed morphological and molecular analyses of H. molitrix, H. nobilis, and their suspected hybrids. Morphometric results revealed that most countable and measurable traits of suspected hybrids (n = 29) were intermediate between those of H. molitrix (n = 30) and H. nobilis (n = 26). However, the traits were slightly biased toward those of H. molitrix, representing an intermediate, transitional form indicative of hybridization. Genome resequencing followed by principal component analysis and population structure analysis indicated that the genetic composition of the suspected hybrids (n = 9) was intermediate between H. molitrix (n = 10) and H. nobilis (n = 9), containing nuclear genetic material from both species. These results confirmed the suspected hybrids as true hybrids of H. molitrix and H. nobilis. COI gene barcoding results showed that the suspected hybrids (n = 28) formed a single clade in the haplotype network and shared haplotypes exclusively with H. molitrix (n = 29), indicating that H. molitrix was the maternal parent. Furthermore, the asymmetric hybridization pattern suggested potential influence of physicochemical factors, such as water temperature, water quality, and flow velocity, which affect the affinity between sperm and egg cells. We propose that the natural hybridization of H. molitrix and H. nobilis originates from genetic exchange. The influence of Xiangjiang River’s unique aquatic ecological environment and hydraulic engineering projects, which have led to changes in flow regimes and the degradation of spawning grounds, have further altered the species’ ecological reproductive habits and disrupted reproductive isolation and natural hybridization. This study provides direct empirical evidence of natural hybridization between H. molitrix and H. nobilis within their native ranges in China through morphological and molecular research methods, thereby complementing previous hybridization inferences based on genomic data. These findings offer a novel view on fish hybridization theory.