Expression of genes associated with economic traits of broiler chickens (Gallus gallus domesticus), as influenced by various paratypical factors (review)

Commercial production of broiler chicken meat is based on the use of early maturing high-yielding crosses created by geneticists and breeders. The original lines of modern broiler chickens were obtained through artificial selection, primarily in terms of feed efficiency, conversion and growth rate (W. Fu et al., 2016). Progressive genetic research, breeding and feeding techniques combined with effective veterinary control ensure production of high quality poultry meat (A.A. Grozina, 2014). From 1957 to 2001, the time for broiler chickens to reach market weight decreased 3-fold, while feed intake decreased too (M. Georges et al., 2019). Expression study of genes involved in broiler growth and development, nutrient assimilation, and resistance to pathogens is necessary for successful selection of birds with desirable qualities (K. Lassiter et al., 2019). The aim of the review is to analyze the diversity of genes and their activity in the formation of economically useful traits of broiler chickens and factors influencing their expression. The article presents an overview of the genes involved in growth and development ( GH , IGF-1 , GHR , MYOD1 , MYOG , MSTN ), nutrient assimilation ( SLC2A1 , SLC2A2 , SLC2A3 , SLC2A8 , SLC2A9 , SLC2A12 , SLC6A19 , SLC7A1 , SLC7A2 , SLC7A5-7 , SLC15A1 , SLC38A2 ), immune response ( IL1B , IL6 , IL8L2 , IL16 , IL17A , IL18 , TNF- a, AvBD1-AvBD14 ). A somatotropic growth hormone (GH)-insulin-like growth factor 1 (IGF-1)-growth hormone receptor (GHR) axis is a pathway to regulate skeletal growth rate and body size (L.E. Ellestad et al., 2019). Analysis of the gene GH, GHR, and IGF-1 expression and selection for high growth rate in broiler chickens can increase growth hormone binding activity, IGF-1 synthesis in the liver, and therefore body weight (S. Pech-Pool et al., 2020). Myogenesis is mediated by a number of factors and genes, including myogenic regulatory factors ( MRF ), myogenic differentiation factor 1 ( MYOD1 ), myogenin ( MYOG ) the expression of which may vary depending on the feed ingredient and specific additives. Dietary proteases significantly increase the expression of MYOD1 and MYOG genes in pectoral muscle, GH and IGF-1 in liver and improve growth performance (Y. Xiao et al., 2020). Genes associated with nutrient absorption and their expression affect transport proteins, leading to accelerated nutrient delivery to the intestinal epithelium, circulatory system, and then to all organs and tissues. In turn, their expression can depend on various feed additives. Solute carrier family (SLC) proteins involved in amino acid transport comprises SLC6A19 (B0AT1) and SLC38A2 (SNAT2) sodium-dependent carriers of neutral amino acids; SLC7A1 and SLC7A2 carriers of cationic amino acids (cationic amino acid transporter - CAT: CAT1, CAT2); SLC7A5-7 L-type amino acid transporter (LAT: LAT1, gLAT2) (J.A. Payne et al, 2019; C.N. Khwatenge et al., 2020; N.S. Fagundes et al., 2020). Immunity gene expression ( IL1B , IL6 , IL8L2 , IL16 , IL17A , IL18 , TNF- a, AvBD1-AvBD14 ) initiating the synthesis of immune response factors is affected by Escherichia coli , Salmonella spp., Pseudomonas aeruginosa , Clostridium perfringens , Listeria monocytogenes , Eimeria spp. infections (G.Y. Laptev et al., 2019; T. Nii et al., 2019). The modulating effect of temperature on gene expression was also revealed. Increased rearing temperature (39 °C) leads to a significant increase in expression of IL6 , IL1b , TNF- a, TLR2 , TLR4 , NFkB50 , NFkB65 , Hsp70 and HSF3 genes in spleen and liver tissues (M.B. Al-Zghoul et al., 2019). Various feed additives (prebiotics, probiotics, synbiotics, phytobiotics and amino acids) are being sought that act via modulation of gene expression and may maintain the physiological condition of birds, prevent the development of diseases, promote faster growth without compromising health and thus improve poultry productivity.