TLR2 gene in seahorse brood pouch plays key functional roles in LPS-induced antibacterial responses

TLR21 is involved in the NF-κB and IFN-β pathways in largemouth bass (Micropterus salmoides) and interacts with TRIF but not with the Myd88 adaptor

Toll-like receptors (TLRs) are pattern recognition receptors that trigger host immune responses against various pathogens by detecting evolutionarily conserved pathogen-associated molecular patterns (PAMPs). TLR21 is a member of the Toll-like receptor family, and emerging data suggest that it recognises unmethylated CpG DNA and is considered a functional homologue of mammalian TLR9. However, little is known regarding the role of TLR21 in the fish immune response. In the present study, we isolated the cDNA sequence of TLR21 from the largemouth bass (Micropterus salmoides) and termed it MsTLR21. The MsTLR21 gene contained an open reading frame (ORF) of 2931 bp and encodes a polypeptide of 976 amino acids. The predicted MsTLR21 protein has two conserved domains, a conserved leucine-rich repeats (LRR) domain and a C-terminal Toll-interleukin (IL) receptor (TIR) domain, similar to those of other fish and mammals. In healthy largemouth bass, the TLR21 transcript was broadly expressed in all the examined tissues, with the highest expression levels in the gills. After challenge with Nocardia seriolae and polyinosinic polycytidylic acid (Poly[I:C]), the expression of TLR21 mRNA was upregulated or downregulated in all tissues tested. Overexpression of TLR21 in 293T cells showed that it has a positive regulatory effect on nuclear factor-kappaB (NF-κB) and interferons-β (IFN-β) activity. Subcellular localisation analysis showed that TLR21 was expressed in the cytoplasm. We performed pull-down assays and determined that TLR21 did not interact with myeloid differentiation primary response gene 88 (Myd88); however, it interacted with TIR domain-containing adaptor inducing interferon-β (TRIF). Taken together, these findings suggest that MsTLR21 plays important roles in TLR/IL-1R signalling pathways and the immune response to pathogen invasion.

Gene loss and co-option of toll-like receptors facilitate paternal immunological adaptation in the brood pouch of pregnant male seahorses

Male pregnancy in syngnathids (seahorses, pipefishes, and sea dragons) is an evolutionary innovation in the animal kingdom. Paternal immune resistance to the fetus is a critical challenge, particularly in seahorses with fully enclosed brood pouches and sophisticated placentas. In this study, comparative genomic analysis revealed that all syngnathid species lost three vertebrate-conserved Toll-like receptors (TLR1, TLR2, and TLR9), of which all play essential roles in immune protection and immune tolerance in the uterus and placenta. Quantitative real-time PCR (qRT-PCR) analysis showed that the TLR paralog genes including TLR18, TLR25, and TLR21 were highly expressed in the placenta inside the seahorse brood pouch and changed dynamically during the breeding cycle, suggesting the potentially important role of the TLRs during male pregnancy. Furthermore, the immune challenge test in vitro showed a remarkable expression response from all three TLR genes to specific pathogenic antigens, confirming their immune function in seahorse brood pouches. Notably, the altered antigen recognition spectrum of these genes appeared to functionally compensate in part for the lost TLRs, in contrast to that observed in other species. Therefore, we suggest that gene loss and co-option of TLRs may be a typical evolutionary strategy for facilitating paternal immunological adaptation during male pregnancy.

Immunological tolerance in the evolution of male pregnancy

The unique male pregnancy in pipefishes and seahorses ranges from basic attachment (pouch-less species: Nerophinae) of maternal eggs to specialized internal gestation in pouched species (e.g. Syngnathus and Hippocampus) with many transitions in between. Due to this diversity, male pregnancy offers a unique platform for assessing physiological and molecular adaptations in pregnancy evolution. These insights will contribute to answering long-standing questions of why and how pregnancy evolved convergently in so many vertebrate systems. To understand the molecular congruencies and disparities in male pregnancy evolution, we compared transcriptome-wide differentially expressed genes in four syngnathid species, at four pregnancy stages (nonpregnant, early, late and parturition). Across all species and pregnancy forms, metabolic processes and immune dynamics defined pregnancy stages, especially pouched species shared expression features akin to female pregnancy. The observed downregulation of adaptive immune genes in early-stage pregnancy and its reversed upregulation during late/parturition in pouched species, most notably in Hippocampus, combined with directionless expression in the pouch-less species, suggests immune modulation to be restricted to pouched species that evolved placenta-like systems. We propose that increased foeto-paternal intimacy in pouched syngnathids commands immune suppression processes in early gestation, and that the elevated immune response during parturition coincides with pouch opening and reduced progeny reliance. Immune response regulation in pouched species supports the recently described functional MHC II pathway loss as critical in male pregnancy evolution. The independent co-option of similar genes and pathways both in male and female pregnancy highlights immune modulation as crucial for the evolutionary establishment of pregnancy.

Hepcidin Gene Co-Option Balancing Paternal Immune Protection and Male Pregnancy

Viviparity has originated independently more than 150 times in vertebrates, while the male pregnancy only emerged in Syngnathidae fishes, such as seahorses. The typical male pregnancy seahorses have closed sophisticated brood pouch that act as both uterus and placenta, representing an excellent model system for studying the evolutionary process of paternal immune protection. Phylogenetic analysis indicated that the hampII gene family has multiple tandem duplicated genes and shows independent lineage-specific expansion in seahorses, and they had the highest ratio of nonsynonymous substitutions to synonymous substitutions (dN/dS) in the seahorse phylogenetic branch. The expression levels of hampIIs in the brood pouch placenta were significantly higher during pregnancy than non-pregnancy. Both LPS stimulation test in vivo and cytotoxicity test in vitro proved the immunological protection function of hampIIs against pathogen infection in seahorse. Besides, seahorse hampII peptides exhibit weaker antibacterial function, but stronger agglutination and free endotoxin inhibition. We assumed that the modified immunological function seemed to be a trade-off between the resistance to microbial attack and offspring protection. In brief, this study suggests that the rapid co-option of hampIIs contributes to the evolutionary adaption to paternal immune care during male pregnancy.