Characterization and expression analysis of key genes of pathogen-associated molecular pattern-triggered Toll-like receptor 5 signaling in the lined seahorse responses to bacterial and parasitic infection

The innate immune system plays an important role in the immune responses of fish, particularly in immunodeficient seahorses, which are vulnerable to infection. In this study, we investigated the evolutionary adaptation of the Toll-like receptor 5 signaling in the lined seahorse (Hippocampus erectus) and the physiological and molecular responses of seahorse to bacterial and parasitic tapeworm infections. Four sites under positive selection among three genes, hsp70, tlr5, and tak1, showed Bayes Empirical Bayes values >0.95. The respiratory rate of H. erectus increased after being challenged with Vibrio and Scalithrium. Stress-induced expression of hsp60 and hsp70, which can bind PAMP molecules and modulate PAMP-induced Toll-like receptor (TLR) signaling, was significantly upregulated in immune organs such as the gill, intestine, kidney, and liver. Additionally, the MyD88-dependent TLR signaling pathway of H. erectus was activated in response to both bacterial and parasitic infections. The expression profiles of TLR signaling molecules revealed that these molecules mediate immune processes and act against various pathogens. These results improve our understanding of the functional role of key genes involved in PAMP-mediated signaling and provide theoretical support for the prevention and control of diseases in cultured seahorses.