Establishment and Biological Characteristics Analysis of a Hybrid Culter Lineage from Megalobrama amblycephala (♀) and Culter alburnus (♂)

Culter alburnus (topmouth culter, TC) is extensively distributed in various rivers and lakes in China. As a widely adaptive fish species, they have significant economic value and special ecological roles. Intergeneric hybridization is a pivotal strategy for generating novel hybrid lineages and species. In a previous study, we obtained an improved bisexual hybrid culter, BTBTF1, derived from the hybrid lineage of Megalobrama amblycephala (blunt snout bream, BSB, 2n = 48, ♀) × Culter alburnus (2n = 48, ♂). In this study, we established an improved hybrid culter lineage by the self-crossing of BTBTF1 and evaluated the biological characteristics regarding cytology, morphology, and genetics. DNA content and chromosome analyses confirmed that BTBTF1-F2 was a diploid lineage (2n = 48), with morphological traits exhibiting intermediate values between parental species, except for significantly TC-biased full-length-to-body length (FL/BL) and body length-to-head length (BL/HL) ratios (p < 0.05). ITS sequencing analysis revealed that BTBTF1-F2 inherited ITS1 sequences from BSB and TC. The global methylation level in BTBTF1-F2 was substantially reduced compared to progenitors, characterized by elevated full and diminished hemimethylation states. Transcriptomic analysis identified 7877 differentially expressed genes (DEGs), displaying 9.05%/8.30% maternal (BSB)-dominant and 17.01%/18.95% paternal (TC)-dominant expression patterns in BTBTF1 and F2. Remarkable intergenerational similarity in phenotypic and molecular profiles, coupled with bidirectional inheritance of progenitor characteristics, confirmed BTBTF1-F2 as a genetically stable allodiploid lineage. Remarkably, ITS sequencing analysis, methylation patterns, and DEG expression collectively demonstrated significant TC-oriented bias (p < 0.05). This study reports a novel stabilized allodiploid culter lineage after a comprehensive assessment at cytology, morphology, and genetic levels, and provides new insights into genetic bias in hybrid progeny.