Transcriptome Analysis of the Sydney Rock Oyster, Saccostrea glomerata: Insights into Molluscan Immunity

Abstract

BackgroundOysters have important ecological functions in their natural environment, acting as global carbon sinks and improving water quality by removing excess nutrients from the water column. During their life-time oysters are exposed to a variety of pathogens that can cause severe mortality in a range of oyster species. Environmental stressors encountered in their habitat can increase the susceptibility of oysters to these pathogens and in general have been shown to impact on oyster immunity, making immune parameters expressed in these marine animals an important research topic.ResultsPaired-end Illumina high throughput sequencing of six S. glomerata tissues exposed to different environmental stressors resulted in a total of 484,121,702 paired-end reads. When reads and assembled transcripts were compared to the C. gigas genome, an overall low level of similarity at the nucleotide level, but a relatively high similarity at the protein level was observed. Examination of the tissue expression pattern showed that some transcripts coding for cathepsins, heat shock proteins and antioxidant proteins were exclusively expressed in the haemolymph of S. glomerata, suggesting a role in innate immunity. Furthermore, analysis of the S. glomerata ORFs showed a wide range of genes potentially involved in innate immunity, from pattern recognition receptors, components of the Toll-like signalling and apoptosis pathways to a complex antioxidant defence mechanism.ConclusionsThis is the first large scale RNA-Seq study carried out in S. glomerata, showing the complex network of innate immune components that exist in this species. The results confirmed that many of the innate immune system components observed in mammals are also conserved in oysters; however, some, such as the TLR adaptors MAL, TRIF and TRAM are either missing or have been modified significantly. The components identified in this study could help explain the oysters’ natural resilience against pathogenic microorganisms encountered in their natural environment.

Reproductive neuropeptides that stimulate spawning in the Sydney Rock Oyster (Saccostrea glomerata)

“…Due to the physical proximity of the visceral ganglia with the gonad, it was assumed to be most likely for neuroendocrine signalling, and indeed many of the other neuropeptides were detected in it. Fortunately, a transcriptome generated from mixed S. glomerata tissues (hemolymph, mantle, gill, gonad, digestive tissue and adductor muscle) enabled the identification of ELH and GnRH, although it is unclear which tissue and what sex contained the transcripts (Ertl et al, 2016). ELH is known to be involved in the reproduction of gastropods (Morishita et al, 2010, Li et al, 1999, where it induces egg laying behaviour in sexually mature Aplysia (Scheller et al, 1983, Nambu and Scheller, 1986, Nagle et al, 1988.…”

“…Although the majority of previously known molluscan neuropeptides were identified, no ELH or GnRH peptide precursor was found by LC-MS/MS. However, the ELH and GnRH precursors were identified within additional transcriptomic data, (Ertl et al 2016), and a draft S. glomerata genome, respectively. Several of the identified neuropeptides were synthesised for the in vivo bioassay (Table 1).…”

Immune Transcripts of the Pacific Oyster, Crassostrea gigas Larvae that Change in Expression as a Result of Bacteria Challenge

“…C. gigas TLRs, peptidoglycan recognition proteins (PGRPs), scavenger receptors (SRs), brinogen-like proteins (FREPs), calciumdependent (C-type) mannose receptor, macrophage mannose receptor (MMR), lectins, C-type receptor, protein toll, collectin-12 and hemolymph lipopolysaccharide-binding protein (LPS-BP) were found in higher abundance in challenged groups compared to controls (see Additional le 5: Table S1). Besides TLRs, which are discussed in detail below, other PRRs found in this transcriptome study have also been identi ed in C. gigas [18], Eriocheir sinensis [22], the Sydney Rock Oyster Saccostrea glomerata [23], Limulus polyphemus [24] from PGRPs, SRs, FREPs, C-type mannose receptor, MMR, lectins, and protein toll to collectin-12 and LPS-BP. PGRPs which can recognize peptidoglycan and peptidoglycan-containing bacteria play an indispensable role in innate immunity for invertebrates and vertebrates, due to its outstanding ability in detecting and eliminating invasive bacteria.…”

“…Since the rst description of Toll and its role in the immune response against fungal infection in Drosophila melanogaster [38], a large number of TLRs have been reported and functionally characterized in various species. As an ancient family of evolutionary conserved PRRs, TLRs, which are found in many species including mammals, ies, crustaceans and molluscs, are playing as crucial roles in immune system [23]. They play a key role in early host defense against invading pathogens by recognizing conserved PAMPs and activating downstream signaling pathways which can induce the production of in ammatory cytokines and/or type I interferon to clear invading pathogens.…”

Herpes virus OsHV-1 and the protistPerkinsus marinusmodify the expression of the Down syndrome cell adhesion molecule gene in gill and mantle ofCrassostreaspp.

“…Dscam transcripts have been reported in bivalve molluscs, such as Saccostrea glomerata (Ertl, O´connor, Papanicolaou, Wiegand, & Elizur, ), Mytilus edulis (Gorbushin & Iakovleva, ), and C. gigas (Gorbushin & Iakovleva, ). Furthermore, Dscam distribution may be tissue specific since in C gigas , Dscam isoforms are absent in haemocytes (Gorbushin & Iakovleva, ); however, to our knowledge, no information on experimental conditions or differential expression in tissues of M. edulis and C. gigas is available.…”