Sea cucumbers lack vision and rely on chemical sensing to reproduce and survive. However, how they recognize and respond to environmental cues remains unknown. possible candidates are the odorant receptors (oRs), a diverse family of G protein-coupled receptors (GpcRs) involved in olfaction. The present study aimed at characterizing the chemosensory GPCRs in sea cucumbers. At least 246 distinct GpcRs, of which ca. 20% putative ORs, were found in a transcriptome assembly of putative chemosensory (tentacles, oral cavity, calcareous ring, and papillae/tegument) and reproductive (ovary and testis) tissues from Holothuria arguinensis (57 ORs) and in the Apostichopus japonicus genome (79 ORs). The sea cucumber ORs clustered with those of sea urchin and starfish into four main clades of gene expansions sharing a common ancestor and evolving under purifying selection. However, the sea cucumber oRs repertoire was the smallest among the echinoderms and the olfactory receptor signature motif LxxPxYxxxxxLxxxDxxxxxxxxP was better conserved in cluster OR-l1 which also had more members. oRs were expressed in tentacles, oral cavity, calcareous ring, and papillae/tegument, supporting their potential role in chemosensing. This study is the first comprehensive survey of chemosensory GpcRs in sea cucumbers, and provides the molecular basis to understand how they communicate. All living organisms perceive and respond to chemical cues in their environment, which mediate a variety of activities such as feeding, predator avoidance, mating and social behaviours 1,2. These cues can be detected over long and short distances, and include a large diversity of molecules ranging from amino acids and nucleic acids, to small volatile compounds, peptides and proteins (see review 3). To detect and discriminate chemical cues, animals have developed complex chemosensory organs, including the olfactory organs of vertebrates, which contain a large repertoire of chemosensory receptors 4. Although well characterized in some animals 5-7 , chemosensory receptors remain largely undescribed in many metazoan lineages. A large group of chemosensory receptors belong to the G protein-coupled receptors (GPCRs), one of the largest superfamilies of seven transmembrane domain receptors found in metazoans 8. GPCRs convert extracellular stimuli, ranging from small molecules and photons to peptides and proteins, into intracellular biochemical signals via multiple signalling cascades (mostly cAMP and calcium secondary messengers) 9. The large variety of ligands is reflected in the structural diversity of GPCRs which are classified into five main families based on their sequence similarity (GRAFS system): glutamate (G), rhodopsin (R), adhesion (A), frizzled (F) and secretin (S) 10. Chemosensory functions have been associated with the glutamate-receptor family and the rhodopsin-type family 11,12. The latter contains the largest number and the most diverse repertoire of GPCRs involved in vertebrate olfaction 8. Rhodopsin family members involved in vertebrate olfaction incl...

Toll-like receptors (TLRs) recognize conserved pathogen-associated molecular patterns (PAMPs) and are an ancient and well-conserved group of pattern recognition receptors (PRRs). The isolation of the Antarctic continent and its unique teleost fish and microbiota prompted the present investigation into Tlr evolution. Gene homologues of tlr members in teleosts from temperate regions were present in the genome of Antarctic Nototheniidae and the non-Antarctic sister lineage Bovichtidae. Overall, in Nototheniidae apart from D. mawsoni, no major tlr gene family expansion or contraction occurred. Instead, lineage and species-specific changes in the ectodomain and LRR of Tlrs occurred, particularly in the Tlr11 superfamily that is well represented in fish. Positive selective pressure and associated sequence modifications in the TLR ectodomain and within the leucine-rich repeats (LRR), important for pathogen recognition, occurred in Tlr5, Tlr8, Tlr13, Tlr21, Tlr22, and Tlr23 presumably associated with the unique Antarctic microbiota. Exposure to lipopolysaccharide (Escherichia coli O111:B4) Gram negative bacteria did not modify tlr gene expression in N. rossii head–kidney or anterior intestine, although increased water temperature (+4°C) had a significant effect.

Streptococcus agalactiae (group B streptococcus, GBS), a broad-spectrum pathogen, causes great economic losses in fish aquaculture, especially the industry of tilapia. Until now, the knowledge of the immune response mechanism against S. agalactiae infection in tilapia has been limited. In the present study, the gill transcriptome of the tilapia from the GBS and the phosphate buffered saline (PBS) groups were sequenced. The transcriptomic analysis results presented the differentially expressed genes (DEGs) at different time points (DEGs number, 6 h: 2122, 9 h: 1851, 15 h: 1791, and 18 h: 2395) after GBS injection, and significantly enriched immune-related gene ontology (GO) terms such as the innate immune response. The significantly enriched immune pathways included the Toll-like receptor signaling pathway, the nucleotide oligomerization domain (NOD)-like receptor signaling pathway, the cytosolic-DNA sensing pathway, and the intestinal immune network for Immunoglobulin A (IgA) production. Most of the DEGs in Toll-like receptor signaling, NOD-like receptor signaling, and cytosolic-DNA sensing pathways presented upregulations at 18 h, which indicated that the innate immune pathways were activated. Two immune-related pathways (phagosome and cell adhesion molecules) were significantly enriched at all time points, suggesting that these two pathways might also play important roles in the immune response against the GBS infection. The results of HE staining showed that the gills of tilapia were damaged seriously at 9 h post-infection, which might be due to the possibility of pyroptosis resulting from the changes of DEGs in the NOD-like receptor signaling pathway. This study provided new insight into the mechanisms of gill damage in fish infected with S. agalactiae.