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

## TL;DR

This study identifies a gene, ZSWIM7, linked to male fertility in a type of carp hybrid, offering a potential tool for breeding programs.

## Contribution

The study discovers a specific SNP in ZSWIM7 associated with improved testis development and sperm production in male allotetraploid carp.

## Key findings

- SNP3 (c.23T/C) in ZSWIM7 correlates with higher gonadosomatic index and mature sperm counts in male 4nAT carp.
- ZSWIM7 is highly expressed in testis and localized in early meiotic primary spermatocytes.
- ZSWIM7 is proposed as a candidate gene for marker-assisted selection to maintain male fertility in allotetraploid breeding.

## Abstract

The allotetraploid crucian–carp hybrid is a valuable germplasm because it can be used to produce sterile triploids with superior growth performance. However, maintaining male fertility has become a challenge after many generations. In this study, based on genome resequencing, we focused on the ZSWIM7 gene and discovered an SNP molecular marker associated with better testis development and high mature sperm counts. Further spatiotemporal expression analysis demonstrated that ZSWIM7 is mainly active in the testis, specifically in early meiotic primary spermatocytes. Our findings highlight the potential of ZSWIM7 as a candidate gene for marker-assisted selection to maintain male fertility in allotetraploid breeding.

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 (Carassius auratus red var., 2n = 100) and a male common carp (Cyprinus carpio 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), ZSWIM7 (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 ZSWIM7 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 ZSWIM7 was highly expressed in the testis and specifically localized to the nuclei of early meiotic primary spermatocytes. Collectively, these results establish ZSWIM7 as a promising biomarker for 4nAT testicular development, offering a potential molecular tool for maintaining male fertility in allotetraploid fish breeding.