# Assessment of Germplasm Improvement in Three Farmed Grass Carp Populations Based on Genetic Variability

## TL;DR

This study assesses genetic diversity in three farmed grass carp populations to understand how breeding practices affect their genetic health and improvement.

## Contribution

The study reveals how gynogenesis and backcrossing influence genetic variability and proposes a method for improving grass carp breeding.

## Key findings

- The CC population showed reduced genetic variability due to gynogenesis technology.
- The KC population exhibited higher genetic diversity compared to CC and CY populations.
- Combining gynogenesis with backcrossing is vital for genetic improvement in grass carp.

## Abstract

In recent decades, grass carp (Ctenopharyngodon idella) has become an important freshwater aquaculture fish species following a breakthrough in key technologies for the artificial breeding of this species. However, people often neglect the selection and renewal of parent fish in the artificial breeding process, leading to the introduction of inferior traits into the breeding population of parent fish and causing the degeneration of the germplasm resources in grass carp. In this study, three germplasm resources of farmed grass carp were assessed based on genetic variability using the mitochondrial cytochrome b (mtDNA Cyt b) gene and nuclear microsatellite markers (simple sequence repeat, SSR). In the three cultured populations, one consists of gynogenetic grass carp which was produced to use the ultraviolet-irradiated sperm of koi carp (Cyprinus carpio) as a source of sperm stimulation (named the CC population), another comprises common grass carp (named the CY population), and the third is composed of a new disease-resistant grass carp which were gynogenetic grass carp mated with common grass carp. The results demonstrated that a “micro-hybrid” was found in the CC population, and there were differences in genetic variability among the three populations, especially between the CC and CY populations. These findings revealed that gynogenesis technology might cause a certain degree of reduction in the level of genetic variability in the directional selection and then purifies and fixes the maternal traits in grass carp populations. However, the genetic variability of the offspring can be improved using backcrossing technology.

The assessment of genetic improvement was comprehensively analyzed using the mtDNA Cyt b gene and SSR markers among three farmed grass carp populations caught in May 2024. The results of an mtDNA Cyt b gene analysis in 198 individuals showed that the haplotype diversity index (Hi) and nucleotide diversity index (Pi) were 0.555 and 0.00058, respectively. The results of the analysis of SSR marker data in 196 individuals indicated that the unequal dosage amplification at the same locus was found in the CC population. Moreover, the total number of alleles (A: 338), number of alleles per locus (Na: 15.36), observed heterozygosity (Ho: 0.8391), expected heterozygosity (He: 0.8380), and polymorphic information content (PIC: 0.8191) in the KC population was relatively higher than that in the CC (A: 129; Na: 5.86; Ho: 0.0025; He: 0.6191; PIC: 0.5747) and CY (A: 293; Na: 8.77; Ho: 0.821; He: 0.7483; and PIC: 0.5747) populations. The FST and AMOVA analysis showed the existence of a significant differentiation (p < 0.001), with a high genetic differentiation between the CC and CY populations. In summary, a high genetic variability exists in the KC population, while the father (CY) and mother (CC) populations have relatively low genetic variability. This study reveals evidence of the existence of a “micro-hybrid”. Moreover, the results demonstrated that combining both gynogenesis and backcross breeding technology is vital for the genetic improvement of grass carp. Moreover, continuous research into the genetic health of these populations is required as well as support for the protection of germplasm resources and artificial breeding.