Genomic structure and expression pattern of MHC IIα and IIβ genes reveal an unusual immune trait in lined seahorse Hippocampus erectus

The major histocompatibility complex (MHC) genes are crucial in the adaptive immune system, and the gene duplication of MHC in animals can generally result in immune flexibility. In this study, we found that the lined seahorse (Hippocampus erectus) has only one gene copy number (GCN) of MHC II alpha and II beta, which is different from that in other teleosts. Together with the lack of spleen and gut-associated lymphatic tissue (GALT), the seahorse may be referred to as having a partial but natural immunodeficiency. Highly variable amino acid residues were found in the II alpha and 1113 domains, especially in the alpha 1 and beta 1 domains with 9.62% and 8.43% allelic variation, respectively. Site models revealed seven and ten positively selected positions in the alpha 1 and beta 1 domains, respectively. Real-time PCR experiments showed high expression levels of the MHC II genes in intestine (In), gill (Gi) and trunk kidney (TK) and medium in muscle (Mu) and brood pouch (BP), and the expression levels were significantly up-regulated after bacterial infection. Specially, relative higher expression level of both MHC II alpha and II beta was found in Mu and BP when compared with other fish species, in which MHC II is expressed negligibly in Mu. These results indicate that apart from TIC, Gi and In, MU and BP play an important role in the immune response against pathogens in the seahorse. In conclusion, high allelic variation and strong positive selection in PBR and relative higher expression in MU and BP are speculated to partly compensate for the immunodeficiency. (C) 2016 Elsevier Ltd. All rights reserved.