A review: Molecular breeding related techniques 

of Larimichthys crocea

Abstract: Large yellow croaker (Larimichthys crocea) is an important mariculture fish in China, and its annual output ranks first in cage culture in China. In order to further promote highquality development of Larimichthys crocea culture industry, many scholars in China have successively carried out research on the selection and breeding of Larimichthys crocea , and have made good progress. The excellent varieties of Larimichthys crocea “Minyou No. 1”, “Donghai No. 1” and “Yongdai No. 1” came out successively, which promoted the construction of the integrated industrial system of “breedingreproductionpromotion” of Larimichthys crocea in China. Although varieties with excellent economic traits can be discovered through population and family selection, traditional breeding techniques have shortcomings such as long cycle, low efficiency and poor predictability, which greatly limit the development of Larimichthys crocea breeding industry. 

As early as the 1960s, some scientists put forward the idea of breeding based on molecular markers, but this advanced point of view did not receive extensive attention from the academic circles. In the 1990s, the DNA molecular markers had been gradually valued and excavated, and after decades of rapid development, molecularassisted breeding technology has gradually become the focus of guiding global fish breeding innovation. Early application of DNA molecular markers mainly included restriction fragment length polymorphism (RFLP), amplified fragment length polymorphism (AFLP), simple sequence repeat (SSR) and single nucleotide polymorphism (SNP) markers, etc. SSR and SNP markers have gradually become the main technologies in the field of global molecular markerassisted breeding due to their high polymorphism, easy detection, and high genetic stability. After entering the 21st century, SSR and SNP markers began to be used for genetic improvement research of Larimichthys crocea . With the rapid development of molecular biology and sequencing technology, as well as the deciphering of Larimichthys crocea genome, new genotyping and analysis techniques have emerged, and have been widely used in Larimichthys crocea breeding research. RADseq technique, started in 2007, used restriction enzyme in genome cutting and high throughput sequencing. RADseq does not need the whole genome, thus it can be applied in all aquatic animals, and it has the advantage of low cost. WGR can be used for whole genome sequencing of different species, fully interpreting the genetic variation information and evaluating the correlation between the variant information and economic traits. Using whole genome resequencing (WGR) and restrictionsiteassociated DNA sequencing (RADSeq) technology can mine a large amount of SNP information, which lays the foundation for SNP typing. Then, based on the genotype data, a highdensity genetic linkage map can be constructed and used for quantitative trait locus (QTL) mapping and genomewide association study (GWAS). A series of SNP loci related to economic traits of Larimichthys crocea  have been identified, including growth characteristics, disease resistance, gonadal development and stress resistance traits. Based on these SNPs, markerassisted selection (MAS) and genomewide selection (GS) have been used for Larimichthys crocea breeding. Now, with the rapid development of highthroughput sequencing technology, rapid reduction of sequencing cost, and more accurate genome assembly technology, a large number of candidate genes and genetic loci have been discovered and reported in Larimichthys crocea , providing a solid foundation for molecular breeding of Larimichthys crocea . Some molecular breeding techniques have been used in the breeding of Larimichthys crocea and achieved remarkable results. The development of modern gene sequencing technology has promoted the innovation of breeding technology. The breeding technology of Larimichthys crocea is bound to make continuous progress with the development of genetic theory and technology. Larimichthys crocea molecular breeding in China starts late, so the foundation is weak. However, our country has unique industrial conditions such as abundant germplasm resources and huge market demand, which has promoted the development of molecular breeding of Larimichthys crocea . At present, the research and application of biological breeding technology presents a trend of contention in China, with broad prospects for development. The continuous maturation and breakthrough of molecular breeding technology will lead to the great reform of traditional breeding methods.

This paper summarizes the research progress of major molecular selection techniques in breeding field of Larimichthys crocea , including RADSeq, genetic linkage map and QTL mapping, GWAS, WGR, SNP array, GS and gene editing technology. The development and application of breeding technology of Larimichthys crocea are prospected. Germplasm is the foundation for the sustainable development of auqaculture, also the key part in the aquaculture field in the 14th Five Year Plan period and the future. Currently, the innovation and research of Larimichthys crocea  germlpasm is facing a advantageous situation. Though china has made important progresses in genetic breeding field of Larimichthys crocea, it still has limitations in cuttingedge breeding techniques, efforts should be made in hot spot fields and techniques.First, the gene editing breeding of Larimichthys crocea is still in its infancy. A large number of major genes related to economic traits have now been discovered. In the future, gene editing technology will be used to accurately analyze the function of target genes, and to carry out germplasm creation to achieve the purpose of optimizing traits. Second, GS technology is an efficient breeding method based on genomewide molecular markers for breeding value calculation and selection. It can be combined with today’s hot omics technology and artificial intelligence to design breeding programs and build a theoretical system for precise control of complex traits, thereby bringing new vitality to the scientific and technological innovation of omics breeding industry. Third, as an emerging research field, intestinal microbiota is crucial for the healthy development of Larimichthys crocea , and is significantly related to host genes and economic traits. In the future, the integration of the model “economic traitsintestinal microbiotagenetic genes” can be constructed, to provide a new theoretical perspective and technical support for the innovation of Larimichthys crocea breeding.