Birth Season and Breed Effects on Newborn Longissimus Thoracis and Semimembranosus Muscles: Insights from the Nero Di Lomellina Piglets

<i>Background</i>: Understanding the factors influencing muscle development is essential for the livestock industry. This study aims to evaluate the effect of birth season and breed on the muscles <i>Longissimus thoracis</i> (LT) and <i>Semimembranosus</i> (SM) from newborn piglets of the local Italian breed Nero di Lomellina (NL) and the Commercial Hybrid Large WhitexDuroc (CH), born in winter (W) and summer (S). <i>Methods</i>: Muscles' morphological features were evaluated, and the expression of Myogenic Regulatory Factors (MRFs: MYF5, MYOD, MYOG, MYF6) and heat and cold shock proteins (HSP27, HSP70, HSP90, CIRBP, RMB3) was assessed by quantitative PCR. <i>Results</i>: Both muscles showed a larger fiber cross-sectional area (LT: NL/S > NL/W, <i>p</i> = 0.035. SM: NL/S > NL/W, <i>p</i> = 0.035; CH/S > CH/W, <i>p</i> = 0.05) and a lower total number of fibers in summer piglets (LT: NL/S < NL/W, <i>p</i> = 0.05. SM: NL/S < NL/W, <i>p</i> = 0.033). In LT, MYF6 was higher, mainly in NL, in S (NL/S > NL/W, <i>p</i> < 0.0001; NL/S > CH/S, <i>p</i> = 0.0002), as well as HSP27 (NL/S > NL/W, <i>p</i> = 0.0001; NL/S > CH/S, <i>p</i> = 0.0018), HSP70 (NL/S > NL/W, <i>p</i> = 0.044. CH/S > CH/W, <i>p</i> = 0.0018), HSP90 (NL/S > NL/W, <i>p</i> < 0.0001; NL/S > CH/S, <i>p</i> = 0.023; CH/S > CH/W, <i>p</i> = 0.027), CIRBP NL/S > NL/W, <i>p</i> = 0.003; CH/S > CH/W, <i>p</i> = 0.0008), and RBM3 (NL/S > NL/W, <i>p</i> = 0.01; NL/S > CH/S, <i>p</i> = 0.036). In SM, MYF5 was higher in W in both breeds (NL/W > NL/S, <i>p</i> = 0.008; CH/W > CH/S, <i>p</i> < 0.0001; CH/W > NL/S, <i>p</i> = 0.012). Similarly, MYOD (NL/W > NL/S, <i>p</i> = 0.045), MYOG (NL/W > NL/S, <i>p</i> = 0.002; CH/W > CH/S, <i>p</i> = 0.025), and CIRBP (NL/W > NL/S, <i>p</i> = 0.003; NL/W > CH/W, <i>p</i> = 0.004) were mainly expressed in winter, while HSP90 was expressed in summer in CH. <i>Conclusions</i>: Our results demonstrate that muscle development in piglets at birth can vary between breeds and along different birth seasons, with an enhanced development observed mainly in summer newborns. These results could be helpful for improvement programs for NL and other local breeds by linking muscle development at birth to seasonal adaptation.