Gene Expression Differences in Muscle and Adipose Tissue Help Explain Variation in Meat Tenderness Across USDA Carcass Grades in Beef and Fat Classes in Sheep

Meat tenderness is a critical quality trait that varies even among carcasses of similar USDA quality grades. In this comparative study of beef cattle and sheep, we investigated how gene expression profiles in muscle and adipose tissues are associated with carcass quality grades and Fat Classes. Fifteen steers and fifteen wethers were raised to produce carcasses spanning Standard, Select, and Choice USDA quality grades (beef) or varying Fat Classes (sheep). In beef, higher quality grade was associated with significantly improved tenderness (lower shear force), while sheep tenderness did not differ consistently across Fat Classes. Exploratory RNA sequencing identified hundreds to thousands of differentially expressed genes (DEGs) between high- and low-quality or fatness groups, particularly in sheep muscle. DEGs were enriched in pathways related to muscle growth, lipid metabolism, and immune signaling. While these transcriptomic differences suggest biological processes associated with carcass traits, the findings are correlational, should be interpreted as hypothesis-generating, and require further validation in larger independent populations. These exploratory findings suggest species-specific differences in the relationship between fatness and tenderness and identify candidate molecular pathways for future investigation.