Large-scale automated measurement and genetic parameters analysis of early-life myofiber characteristics in broilers

The meat yield and quality of broilers are largely determined by myofiber hyperplasia and hypertrophy. However, the inability to accurately quantify myofiber traits on a large scale has limited their application in breeding programs. Here, we utilized whole-slide imaging (WSI) and deep learning tool to quantify the total number (TNM) and cross-sectional area (CSA) of myofibers in the pectoralis major (PM) and gastrocnemius (GAS) muscles of 1,032 chicks, and subsequently estimated the genetic parameters for these traits. The results revealed substantial phenotypic variation in these traits, with coefficients of variation ranging from 9.82 % to 28.85 %. Significant differences were detected between sexes and muscles. The PM muscle contained about 1.5-fold more myofibers but nearly half the CSA compared with the GAS. Meanwhile, in the PM, females had approximately 22 % more myofibers but 15 % smaller CSA than males, whereas in the GAS, males showed 17 % more myofibers but 9 % smaller CSA than females. Heritability estimates were high for TNM (0.41 in the PM and 0.67 in the GAS) and considerable for CSA (0.30 and 0.24, respectively), indicating strong genetic control over myofiber characteristics. We further found a strong negative genetic correlation between TNM and CSA within the PM muscle (-0.89), and a moderate negative correlation in the GAS (-0.34), which may reflect developmental constraints due to limited physical space during embryonic growth. Moreover, strongly negative correlations for the same trait across muscles (-0.84 for TNM and -0.46 for CSA) suggest tissue-specific regulation. These findings indicate that myofiber traits show promise for early selection, and emphasize the need to simultaneously consider both breast and leg muscles to achieve balanced improvement in broiler meat yield and quality.