Transcriptional profiling of Bulinus globosus provides insights into immune gene families in snails supporting the transmission of urogenital schistosomiasis

Abstract

Schistosomiasis, urogenital and intestinal, afflicts 251 million people worldwide with approximately two-thirds of the patients suffering from the urogenital form of the disease. Freshwater snails of the genus Bulinus (Gastropoda: Planorbidae) serve as obligate intermediate hosts for Schistosoma haematobium, the etiologic agent of human urogenital schistosomiasis. These snails also act as vectors for the transmission of schistosomiasis in livestock and wildlife. Despite their crucial role in human and veterinary medicine, our basic understanding at the molecular level of the entire Bulinus genus, which comprises 37 recognized species, is very limited. In this study, we employed Illumina-based RNA sequencing (RNAseq) to profile the genome-wide transcriptome of Bulinus globosus, one of the most important intermediate hosts for S. haematobium in Africa. A total of 179,221 transcripts (N50 = 1,235) were assembled and the benchmarking universal single-copy orthologs (BUSCO) was estimated to be 97.7%. The analysis revealed a substantial number of transcripts encoding evolutionarily conserved immune-related proteins, particularly C-type lectin (CLECT) domain-containing proteins (n = 316), Toll/Interleukin 1-receptor (TIR)-containing proteins (n = 75), and fibrinogen related domain-containing molecules (FReD) (n = 165). Notably, none of the FReDs are fibrinogen-related proteins (FREPs) (immunoglobulin superfamily (IgSF) + fibrinogen (FBG)). This RNAseq-based transcriptional profile provides new insights into immune capabilities of Bulinus snails, helps provide a framework to explain the complex patterns of compatibility between snails and schistosomes, and improves our overall understanding of comparative immunology.

Keywords: Bulinus globosus; Fibrinogen-related protein; Immunity; RNAseq-based transcriptome; Schistosomiasis.

Fig. 1.

A workflow of transcript assembly.

Fig. 2.

Length distribution (A) and percentage of size (B) of assembled transcripts.

Fig. 3.

A analysis of Bench-marking-Universal-Single-Copy-Orthologs (BUSCO).

Fig. 4.

Distribution of transcripts hit by BlastX search (A) and distribution of top-hit phyla of non-molluscan species (B).

Fig. 5.

Domain architectures of C-type lectin (CLECT) domain-contaiing proteins. A) shows the structure of the three CREP-like peoteins (SP + CLECT + IG/IG-like). B) show structure of the four proteins (SP + CLECT + IG/IGc2/IG-like). IG: immunologlobiun. IGc2: Immunoglobulin C-2 type. FN3: Fibronectin type 3 domain. Regions in red, pink, and green color represent signal pepptide (SP), low complex region, and coiled coil region, respectively.

Fig. 6.

TIR-containing proteins identified in B. globosus (A) and phylogenetic relationship of TIR across phylum Mollusca (B). SP: signal peptide; TM: transmembrane domain. Bootstrap values of <60% are not shown. The sequence starting with Bu denotes the sequence from B. globusus.