Candidate genes promising for marker-assisted selection in aquaculture (review)

Modern aquaculture is a rapidly developing sector of food production that serves as a source of animal protein, essential amino acids, fats, vitamins, minerals, enzymes and is important for food security. In Russia, commercial fish farming is still significantly inferior in volume to industrial fish farming. A promising approach in the scientific support of commercial aquaculture is the search for polymorphic loci in candidate genes and the identification of reliable associations between various genotypes and productivity indicators for subsequent marker-assisted selection (MAS) of commercial aquaculture objects. The purpose of this review was to summarize and analyze publications concerning single nucleotide polymorphism (SNP) in genes affecting size and weight in fish. Body weight is one of the economically important characteristics for which selection is carried out in fish farms. It depends on the growth of skeletal muscle, so genes that influence the growth and development of muscle tissue are considered as potential candidate genes. The most important of them include the genes for myostatin ( MSTN ), insulin-like growth factors I and II ( IGF-I , IGF-II ), growth hormone ( GH ) and growth hormone receptor ( GHR ) (X.Y. Dai et al., 2015; D.L. Li et al., 2014). When assessing the effect of candidate genes on a particular trait, polymorphisms in those genes are first examined, and then the relationship between specific alleles/genotypes and phenotypic expression of the trait of interest is statistically assessed. If significant associations are found, this is considered evidence that the gene is either directly involved in the genetic control of the trait, or the functional polymorphism is located sufficiently close to the marker and the two loci are in linkage disequilibrium (M. Lynch and B. Walsh, 1997; D.L. Yowe and R. J. Epping, 1995). Myostatin plays an important role in inhibiting muscle growth and development. In most mammals, the loss or inactivation of myostatin ( MSTN -/-) causes an increase in the size and number of myofibers, which leads to an increase in muscle mass (A. Clop et al., 2006; L. Grobet et al., 1997; D.S. Mosher et al. al., 2007; S. Rao et al., 2016). The genes for insulin-like growth factors I and II encode the corresponding polypeptide hormones which have a molecular structure similar to proinsulin and play an important role in regulation of growth, development and differentiation of cells and tissues in vertebrates (J.I. Jones et al., 1995; M Codina et al., 2008). Insulin-like growth factors I and II are the most important endocrine mediators of the action of growth hormone; they are synthesized in the liver, skeletal muscles and other tissues (W.J. Tao and E.G. Boulding, 2003; K.M. Reindl et al., 2011). Growth hormone, or somatotropin, is a polypeptide hormone that is synthesized in the somatotropic cells of the pituitary gland and participates in the regulation of somatic growth in fish (J.I. Johnsson and B.T. Björnsson, 1994; B. Cavari et al., 1993). The growth hormone receptor is a transmembrane protein that belongs to the class 1 cytokine receptor superfamily and serves as an important regulator of growth and metabolism (T. Zhu et al., 2001). GHR as a receptor mediates the biological effects of growth hormone on target cells by transmitting a stimulatory signal across the cell membrane with subsequent induction of transcription of many genes, including IGF-I (Y. Kobayashi et al., 1999). SNPs in the genes MSTN , IGF-I , IGF-II , GH , RGH can affect the size and weight in various fish species and can be an auxiliary tool in breeding programs (D. Gencheva and S. Stoyanova, 2018; C. De-Santis and D.R. Jerry, 2007; Y. Sun et al., 2012). The functional characterization and associations of growth and development indicators with genetic polymorphisms in the genes of myostatin, insulin-like growth factors I and II, growth hormone and growth hormone receptor considered in the review allow us to recommend these genes as the most promising candidates for searching polymorphic loci with subsequent statistical assessment of the genotype-trait relationship. The reliable associations can be used in marker selection to replace broodstocks and improve the efficiency of commercial aquaculture.