133. Impact of Breed Type on Gene Expression During Muscle Development in Bos Taurus or Bos Indicus-influenced Cattle.

Abstract Beef cattle production in the U.S. is increasingly challenged by climate change and continued decline in natural resources and arable land. To sustainably meet the demands of a growing global population, strategies that enhance production efficiency and environmental resilience are essential. Increasing the proportion of Bos indicus (BI) genetics within the U.S. beef herd represents one potential approach, as BI cattle exhibit greater tolerance to heat stress and nutritional limitations compared to Bos taurus (BT) cattle. However, despite their resilience, BI cattle often possess less desirable meat quality attributes including reduced marbling and tenderness. Crossbreeding may combine favorable BT and BI traits. Currently, limited research exists on evaluating the molecular mechanisms underlying meat quality in BT×BI crossbred cattle. The objective of this study was to characterize molecular differences in gene expression during skeletal muscle development between purebred BT and BI-influenced steers. Thirty-five yearling steers of two breed types were utilized: Angus (AN; 100% BT; n = 18) and Santa Gertrudis-influenced (SG; 19% BI, 81% BT; n = 17). Three biopsies were collected from the Longissimus lumborum at one-month intervals, with the first biopsy being collected when the steers were an average of 12 months old. Samples were flash-frozen in liquid nitrogen, pulverized, and total RNA was extracted using TRIzol before reverse transcription to cDNA. Relative expression of 92 genes associated with muscle development, adipogenesis, metabolism, stress response, and inflammation were quantified using a Standard BioTools Custom Delta Gene Assay. Data were analyzed using the MIXED procedure of SAS with Tukey-Kramer adjustments to evaluate the fixed effects of breed, time point (TP), and breed × TP. Of the 51 genes related to muscle metabolism and development, one main breed effect was observed such that AN steers tended to exhibit greater relative expression of CKM (P = 0.069) compared to SG steers. Two additional genes related to muscle metabolism and development exhibited breed × TP interactions: MYH1 (P = 0.013) and MYH2 (P < 0.0001). Among 23 genes associated with adipogenesis and marbling, AN steers had lower relative expression of GATA2 (P = 0.048) and tended to have lower NDUFA5 expression (P = 0.057). Breed × TP interactions were observed for FGF21 (P = 0.03) and SLC27A1 (P = 0.0069). Of 15 stress and inflammation related genes, a significant breed × TP interaction was detected for CRYAB (P = 0.0007). Time point influenced the expression (P ≤ 0.05) for all but 14 of the genes evaluated. Collectively, these results indicate that incorporating 19% BI genetics has no notable impact on expression levels of majority of genes examined in this study related to muscle growth, lipid metabolism, and stress responsiveness. However, gene expression did vary over time as animals were developing, so further investigation is warranted to better understand these results.