Polymorphisms and Expression Characteristics of the ZSWIM7 Gene Are Associated with the Fertility of Male Allotetraploid of Red Crucian Carp × Common Carp

Allotetraploid fish produced by distant hybridization are valuable germplasm for the mass production of sterile triploids. The allotetraploid crucian-carp hybrid (4nAT, 4n = 200) is derived from the intergeneric cross between a female red crucian carp (<i>Carassius auratus</i> red var., 2n = 100) and a male common carp (<i>Cyprinus carpio</i> L., 2n = 100). However, after 33 successive generations, this lineage faces a critical bottleneck in maintaining male fertility. The present study aimed to develop new biomarkers for testicular development and characterize the associated functional gene expression profile in 4nAT. Following whole-genome resequencing and selection signature analysis of 15 male 4nAT individuals from each of the high-development group (HDG) and low-development group (LDG), <i>ZSWIM7</i> (Zinc Finger SWIM-Type Containing 7), a gene implicated in reproductive development, was selected as a candidate for further fertility association studies. Seven SNPs were screened in the coding region of <i>ZSWIM7</i> of 70 4nAT males; among these, SNP3 (c.23T/C) exhibited a significant correlation between genotypes and testicular development: individuals with the CT genotype showed a higher gonadosomatic index (1.17 ± 0.68 vs. 0.65 ± 0.50) and greater counts of mature spermatozoa (2537.67 ± 283.95 vs. 341.56 ± 121.66) compared to those with the TT genotype. Further quantitative PCR and immunofluorescence assays demonstrated that <i>ZSWIM7</i> was highly expressed in the testis and specifically localized to the nuclei of early meiotic primary spermatocytes. Collectively, these results establish <i>ZSWIM7</i> as a promising biomarker for 4nAT testicular development, offering a potential molecular tool for maintaining male fertility in allotetraploid fish breeding.