Identification of genomic regions and candidate genes for chicken meat ultimate pH by combined detection of selection signatures and QTL.

BACKGROUND: The understanding of the biological determinism of meat ultimate pH, which is strongly related to muscle glycogen content, is a key point for the control of muscle integrity and meat quality in poultry. In the present study, we took advantage of a unique model of two broiler lines divergently selected for the ultimate pH of the pectoralis major muscle (PM-pHu) in order to decipher the genetic control of this trait. Two complementary approaches were used: detection of selection signatures generated during the first five generations and genome-wide association study for PM-pHu and Sartorius muscle pHu (SART-pHu) at the sixth generation of selection. RESULTS: Sixty-three genomic regions showed significant signatures of positive selection. Out of the 10 most significant regions (detected by HapFLK or FLK method with a p-value below 1e-6), 4 were detected as soon as the first generation (G1) and were recovered at each of the four following ones (G2-G5). Another four corresponded to a later onset of selection as they were detected only at G5. In total, 33 SNPs, located in 24 QTL regions, were significantly associated with PM-pHu. For SART-pHu, we detected 18 SNPs located in 10 different regions. These results confirmed a polygenic determinism for these traits and highlighted two major QTL: one for PM-pHu on GGA1 (with a Bayes Factor (BF) of 300) and one for SART-pHu on GGA4 (with a BF of 257). Although selection signatures were enriched in QTL for PM-pHu, several QTL with strong effect haven't yet responded to selection, suggesting that the divergence between lines might be further increased. CONCLUSIONS: A few regions of major interest with significant selection signatures and/or strong association with PM-pHu or SART-pHu were evidenced for the first time in chicken. Their gene content suggests several candidates associated with diseases of glycogen storage in humans. The impact of these candidate genes on meat quality and muscle integrity should be further investigated in chicken.

Genetic parameters of white striping in relation to body weight, carcass composition, and meat quality traits in two broiler lines divergently selected for the ultimate pH of the pectoralis major muscle.

BACKGROUND: White striping (WS) is an emerging quality defect with adverse consequences for the sensorial, technological, and nutritional qualities of breast meat in broiler chickens. The genetic determinism of this defect is little understood and thus the aim of the study presented here was to estimate the genetic parameters of WS in relation to other traits of economic importance such as body weight, carcass composition, and technological meat quality in an experimental population consisting of two divergent lines selected for high (pHu + line) or low (pHu- line) ultimate pH (pHu) of the pectoralis major (p. major) muscle. RESULTS: The incidence of WS in the whole population was 50.7%, with 36.7% of broilers being moderately and 14% being severely affected. A higher incidence of moderate (p < 0.001) and severe (p < 0.0001) WS was observed in the pHu + line, and strong genetic determinism (h(2) = 0.65 ± 0.08) was evidenced for WS in the studied lines. In addition, WS was significantly genetically correlated with body weight (rg = 0.33 ± 0.15), and breast meat yield (0.68 ± 0.06), but not with the percentage of leg or abdominal fat. Increased body weight and breast muscle yield were significantly associated with increased incidence and severity of WS regardless of the line. Significant rg were observed between WS and several meat quality traits, including breast (0.21 ± 0.08) and thigh (0.31 ± 0.10) pHu, and breast cooking loss (0.30 ± 0.15). WS was also strongly genetically correlated with the intramuscular fat content of the pectoralis major muscle (0.64 ± 0.09), but not with the lipid oxidation index of this muscle. CONCLUSIONS: This study highlighted the role of genetics as a major determinant of WS. The estimated genetic correlations showed that WS was more highly related to muscle development than to the overall growth of the body. The positive genetic association reported in this study between WS and muscle pHu indicated a possible relationship between the ability of muscle to store energy as a carbohydrate and its likelihood of developing WS. Finally, the strong genetic determinism of WS suggested that selection can be an efficient means of reducing the incidence of WS and of limiting its undesirable consequences on meat quality in broiler chickens.

Ovalbumin-related protein X is a heparin-binding ov-serpin exhibiting antimicrobial activities.

Ovalbumin family contains three proteins with high sequence similarity: ovalbumin, ovalbumin-related protein Y (OVAY), and ovalbumin-related protein X (OVAX). Ovalbumin is the major egg white protein with still undefined function, whereas the biological activity of OVAX and OVAY has not yet been explored. Similar to ovalbumin and OVAY, OVAX belongs to the ovalbumin serine protease inhibitor family (ov-serpin). We show that OVAX is specifically expressed by the magnum tissue, which is responsible for egg white formation. OVAX is also the main heparin-binding protein of egg white. This glycoprotein with a predicted reactive site at Lys(367)-His(368) is not able to inhibit trypsin, plasmin, or cathepsin G with or without heparin as a cofactor. Secondary structure of OVAX is similar to that of ovalbumin, but the three-dimensional model of OVAX reveals the presence of a cluster of exposed positive charges, which potentially explains the affinity of this ov-serpin for heparin, as opposed to ovalbumin. Interestingly, OVAX, unlike ovalbumin, displays antibacterial activities against both Listeria monocytogenes and Salmonella enterica sv. Enteritidis. These properties partly involve heparin-binding site(s) of the molecule as the presence of heparin reverses its anti-Salmonella but not its anti-Listeria potential. Altogether, these results suggest that OVAX and ovalbumin, although highly similar in sequence, have peculiar sequential and/or structural features that are likely to impact their respective biological functions.