The characterization of secretory phospholipase A2 group IB and its functional profiles during male pregnancy of lined seahorse Hippocampus erectus

Abstract

Seahorse is a specialized animal due to its male pregnant strategy in the brood pouch, which can evidently provide immune protection, nutrition, and osmoregulation to the offspring. Secretory phospholipase A2 group IB (sPLA2-IB), which encodes a secreted phospholipase A2 that functions for metabolic regulation and antibacterial defense, plays a vital role for immune regulation in teleost, but its potential immune functions in seahorse remain unknown. Here, we aimed to evaluate the characteristics and functional profiles of sPLA2-IBs in lined seahorse (Hippocampus erectus), which is one of the most important breeding seahorse species in the world. Results showed that most teleost species have only one sPLA2-IB gene, while the seahorses presented two tandem repetitive copies (sPLA2-IBa and sPLA2-IBb). This finding showed that the highest expression levels of sPLA2-IBs in seahorses were in the liver, and significant up-regulation could be induced by lipopolysaccharide (LPS), polyinosinic-polycytidylic acid (Poly[I:C]), as well as the seahorse's common pathogen Vibrio parahaemolyticus. Interestingly, the expression levels of sPLA2-IBs in the brood pouch showed an upward trend during pregnancy, compared with that in non-pregnancy stage, and peaked in the late pregnancy stage, implying that seahorse sPLA2-IBs in brood pouch might function for immune defense during pregnancy. In addition, seahorse sPLA2-IBa was significantly highly expressed in early developing embryos in brood pouch, but sPLA2-IBb expression dramatically increased in new-born juveniles, which implies the functional divergence between the two sPLA2-IBs by different ontogenetic expression. In conclusion, this study indicated that seahorse sPLA2-IBs play an important role in the immunoprotection during male pregnancy, and functional co-option of seahorse sPLA2-IBs might be conducive to improving the survival rate of its offspring.