A comparative study of the genetically improved Abbassa Nile tilapia strain (GIANT-G9) and a commercial strain in Egypt: growth vs. stress response.

Selective breeding is a potent method for developing strains with enhanced traits. This study compared the growth performance and stress responses of the genetically improved Abbassa Nile tilapia strain (G9; GIANT-G9) with a local commercial strain over 12 weeks, followed by exposure to stressors including high ammonia (10 mg TAN/L), elevated temperature (37 °C), and both for three days. The GIANT-G9 showed superior growth, including greater weight gain, final weight, length gain, specific growth rate, and protein efficiency ratio, as well as a lower feed conversion ratio and condition factor compared to the commercial strain. The expression of growth hormone in the brain of the GIANT-G9 increased significantly after 6 weeks, although it slightly decreased after 12 weeks. Growth hormone receptor 1 expression also increased significantly after 6 weeks. Muscle insulin-like growth factors (igf1 and igf2) levels up-regulated significantly only after 12 weeks in the GIANT-G9. Under stress, serum enzymes (alanine aminotransferase, aspartate aminotransferase, and alkaline phosphatase (ALP)) were significantly higher in the GIANT-G9, while the commercial strain had lower levels. No significant changes were observed in liver ALP activity among stressed strains. Under stress, the GIANT-G9 exhibited marked upregulation of splenic Toll-like receptors (tlr2, tlr9, tlr21), myeloid differentiation primary response protein 88 (myd88), nuclear factor kappa B (nf-κB), interleukin (il) 1β, and il6. Notably, il6 expression was higher than il1β in the spleen, with the opposite pattern in the head kidney. In response to immune stimulation, globulin levels significantly increased in the GIANT-G9 but with similar values to the stressed commercial strain. Myostatin expression increased in the spleen of the stressed GIANT-G9. The commercial strain exhibited the best liver catalase and superoxide dismutase activities under stress, while the GIANT-G9 showed increased liver glutathione-S-transferase (GST) activity after exposure to ammonia and temperature stress. Serum lysozyme activity increased in the stressed commercial strain and under temperature stress in the GIANT-G9 but decreased under other stress conditions. Overall, the stressed commercial strain demonstrated higher survivability than the stressed GIANT-G9. The study revealed significant interactions between strains and stress factors. The GIANT-G9 exhibited higher growth rates but lower antioxidant and immune capacities compared to the commercial strain at the juvenile stage of life and production cycle.