PSXIII-30 - Using conventional and slow-growing broilers to determine differences in early life developmental physiology of muscle and adipose tissue.

Abstract: Optimizing growth and body composition while maintaining health, welfare, and meat quality challenges the broiler industry. The objective of this study is to identify molecular pathways in developing adipose and muscle that underlie differential growth rate and body composition in broiler chickens. A conventional (Cobb700) line and a slow-growing (RedBro) line were compared to represent divergent growth and body composition within the context of commercial broiler genetics. Eggs (n=60) from each line were obtained from commercial hatcheries and incubated until embryonic day 17 (E17), or until hatch and raised to 5 (D5) or 10d (D10) of age. At each age, breast muscle (BM) and subcutaneous (SQ) fat pad (n=10) were collected, weighed, and snap-frozen. Neck and abdominal (ABD) fat pads (n=10) were collected at D10 and D14. Adipocyte size at E17 and D10 was determined in H&E-stained tissue sections using ImageJ. T-tests were used to compare body and tissue weights and adipocyte size between lines (α=0.05).  RNA (n=5/tissue/age) from adipose and breast was isolated for transcriptomic analyses using 3’ RNAseq. Raw reads were mapped to the chicken genome (bGalGal1.mat.broiler.GRCg7b). Differentially expressed genes (DEGs; fdr P-value < 0.1) were identified using the R package DEseq2 R (version 4.3.1). Sets of DEGs were analyzed for functional annotation using Database for Annotation, Visualization, and Integrated Discovery (david.ncifcrf.gov). Body weight was significantly higher in Cobb vs. Redbro chicks at each age (p < 0.05). The most notable differences in relative tissue weight occurred for adipose (SQ) at E17 and D5, and for BM at D10. Relative fat pad weight (SQ) was significantly higher in Cobb vs. Redbro at E17 and D5. In the embryo, this difference was associated with larger adipocytes, suggesting increased capacity for energy storage in fast-growing broilers. In contrast, at D5, Cobb exhibited a significant increase in abundance of small adipocytes, indicating a shift towards adipocyte hyperplasia and/or enhanced mobilization of stored energy post-hatch. Differences in adiposity and adipocyte size were associated with 103 and 153 DEGs at E17 and D5, respectively. These gene sets were enriched in various growth-related functions, including tissue morphogenesis, peptide transport, and mitogenesis. Relative breast weight diverged significantly at D10, with heavier BM in Cobb chicks. The set of 278 DEGs at this age were enriched in multiple GO categories related to lipid metabolism (16% of genes) and protein metabolism. Although relative breast muscle weight did not differ at E17, 362 DEGs were identified at this age, and were enriched in functions related to skeletal muscle development (n=43) and mitogenesis, among others. In conclusion, these results identify both phenotypic and transcriptomic differences that underlie, and in some cases precede, divergent growth in broiler chickens. Further investigation is required to determine how these differences affect mature body composition.