The genetic of welfare : gene expression and heat stress in chicken

High environmental temperature is one of the most important stressors associated with economic losses to the poultry industry. It causes poor growth performance, immunosuppression, and high mortality, contributing thus to a production decrease. Broilers are in general more sensitive to high environmental temperatures than other domestic animals but the responses to heat differ between different chicken genetic backgrounds. We used One Step Taqman real time RT-PCR technology to evaluate the effect of heat stress on the expression of six genes codifying for the antioxidative enzymes (SOD and CAT), heat shock protein (HSP) 70 and HSP90, glucocorticoid receptor (NR3C1), and Caspase 6 (CASP6) in the liver of two chicken strains: Red JA Cou Nu Hubbard (CN) and Ross 508 Aviagen (RO). A total of 120 RO and 120 CN broilers, 4 weeks of age, were divided into 4 groups of 60 animals, which were then reared for 4 weeks at two different environmental temperatures: 34°C and 22°C. Corticosterone (CORT) concentration was measured in blood samples using enzyme immunoassay method. We also analyzed the association between transcriptomic response and specific SNPs in each genetic strain of chickens. The results demonstrated that there was both a genotype and a temperature effect on the chickens’ growth, given that a different trend was seen in the four combinations genetic strain-environmental temperature from the 6th week to the end of the experiment. Chronic heat stress caused a significant increase in CORT levels due to activation of the hypothalamic-pituitary-adrenal axis in both genetic strains. Carcass yield at slaughter were similar in all groups, ranging from 86.5 to 88.6%, whereas carcass weight was negatively influenced by heat stress in both breeds. Heat stress affected the expression of quite all target genes and the differences found in the mRNA copies of two (CASP6 and HSP70) out of the six genes could be partly explained by SNPs. In particular, we found three SNPs for the HSP70 gene (HSP70-3, -7, -9) with different allelic frequencies between the two target breeds, and a SNP polymorphism in the CASP6 gene (CASP6-9). The evidence given in this study, on gene expression and genome polymorphisms will be useful in the future marker assisted selection of chickens more tolerant to heat stress.