Immunoglobulin gene expression as a biomarker for Streptococcosis resistance in Nile tilapia (Oreochromis niloticus)

Streptococcus agalactiae is a significant pathogen in Nile tilapia (Oreochromis niloticus) culture, causing economic losses in Thailand. Formalin-killed S. agalactiae vaccine in tilapia can be used in the prevention of diseases and mortalities. In this study, we administered a formalin-killed S. agalactiae vaccine to individual tilapia broodstock to activate immunoglobulin (IgM) gene expression. The IgM gene expression levels differed among individual broodstock and were grouped into four categories: high IgM males (HM), low IgM males (LM), high IgM females (HF), and low IgM females (LF). Using these categories, we designed matings to create four breeding groups: HM × HF, HM × LF, LM × HF, and LM × LF, generating three fry families per group. The tilapia fry was then orally challenged with S. agalactiae for 14 days. The results showed a significant difference (p < 0.05) in survival rates among the breeding groups. The HM × HF group had the highest survival at 93.33 ± 6.20 %, while the LM × LF group had the lowest at 21.11 ± 6.50 %. Gene expression analysis of toll-like receptor 2, hepcidin, immunoglobulin, MHC class II, and granzyme genes at 24, 48, 72, and 96 h post S. agalactiae challenge revealed significant temporal changes. IgM and granzyme expression levels varied notably among groups, with IgM peaking at 72-96 h and showing a significant difference (p < 0.05) between groups. The HM × HF, HM × LF, and LM × HF breeding groups displayed significantly higher IgM expression levels compared to the LM × LF group. Granzyme gene expression also showed significant differences (p < 0.05) from 48 h post-challenge, with the highest expression observed in the HM × HF group and the lowest in the LM × LF group. No significant difference (p > 0.05) was observed between the HM × LF and LM × HF groups for granzyme expression. This experiment concludes that high levels of IgM gene expression of broodstock tilapia induced by formalin-killed S. agalactiae vaccine could be used as a gene marker to select the S. agalactiae-resistant tilapia in breeding program.