The cnidarian parasite Ceratonova shasta utilizes inherited and recruited venom-like compounds during infection

Background Cnidarians are the most ancient venomous organisms. They store a cocktail of venom proteins inside unique stinging organelles called nematocysts. When a cnidarian encounters chemical and physical cues from a potential threat or prey animal, the nematocyst is triggered and fires a harpoon-like tubule to penetrate and inject venom into the prey. Nematocysts are present in all Cnidaria, including the morphologically simple Myxozoa, which are a speciose group of microscopic, spore-forming, obligate parasites of fish and invertebrates. Rather than predation or defense, myxozoans use nematocysts for adhesion to hosts, but the involvement of venom in this process is poorly understood. Recent work shows some myxozoans have a reduced repertoire of venom-like compounds (VLCs) relative to free-living cnidarians, however the function of these proteins is not known. Methods We searched for VLCs in the nematocyst proteome and a time-series infection transcriptome of Ceratonova shasta, a myxozoan parasite of salmonid fish. We used four parallel approaches to detect VLCs: BLAST and HMMER searches to preexisting cnidarian venom datasets, the machine learning tool ToxClassifier, and structural modeling of nematocyst proteomes. Sequences that scored positive by at least three methods were considered VLCs. We then mapped their time-series expressions in the fish host and analyzed their phylogenetic relatedness to sequences from other venomous animals. Results We identified eight VLCs, all of which have closely related sequences in other myxozoan datasets, suggesting a conserved venom profile across Myxozoa, and an overall reduction in venom diversity relative to free-living cnidarians. Expression of the VLCs over the 3-week fish infection varied considerably: three sequences were most expressed at one day post-exposure in the fish’s gills; whereas expression of the other five VLCs peaked at 21 days post-exposure in the intestines, coinciding with the formation of mature parasite spores with nematocysts. Expression of VLC genes early in infection, prior to the development of nematocysts, suggests venoms in C. shasta have been repurposed to facilitate parasite invasion and proliferation within the host. Molecular phylogenetics suggested some VLCs were inherited from a cnidarian ancestor, whereas others were more closely related to sequences from venomous non-Cnidarian organisms and thus may have gained qualities of venom components via convergent evolution. The presence of VLCs and their differential expression during parasite infection enrich the concept of what functions a “venom” can have and represent targets for designing therapeutics against myxozoan infections.

The Teleost Thymus in Health and Disease: New Insights from Transcriptomic and Histopathological Analyses of Turbot, Scophthalmus maximus

The thymus is a primary lymphoid organ that plays a pivotal role in the adaptive immune system. The immunobiology of the thymus in fish is considered to be similar to that of mammals, but it is actually poorly characterized in several cultured teleost species. In particular, while investigations in human and veterinary medicine have highlighted that the thymus can be affected by different pathological conditions, little is known about its response during disease in fish. To better understand the role of the thymus under physiological and pathological conditions, we conducted a study in turbot (Scophthalmus maximus), a commercially valuable flatfish species, combining transcriptomic and histopathological analyses. The myxozoan parasite Enteromyxum scophthalmi, which represents a major challenge to turbot production, was used as a model of infection. The thymus tissues of healthy fish showed overrepresented functions related to its immunological role in T-cell development and maturation. Large differences were observed between the transcriptomes of control and severely infected fish. Evidence of inflammatory response, apoptosis modulation, and declined thymic function associated with loss of cellularity was revealed by both genomic and morphopathological analyses. This study presents the first description of the turbot thymus transcriptome and provides novel insights into the role of this organ in teleosts’ immune responses.

In vitro and in vivo assays reveal that cations affect nematocyst discharge in Myxobolus cerebralis (Cnidaria: Myxozoa)

Myxozoans are parasitic, microscopic cnidarians that have retained the phylum-characteristic stinging capsules called nematocysts. Free-living cnidarians, like jellyfish and corals, utilize nematocysts for feeding and defence, with discharge powered by osmotic energy. Myxozoans use nematocysts to anchor to their fish hosts in the first step of infection, however, the discharge mechanism is poorly understood. We used Myxobolus cerebralis, a pathogenic myxozoan parasite of salmonid fishes, and developed two assays to explore the nature of its nematocyst discharge. Using parasite actinospores, the infectious stage to fish, we stimulated discharge of the nematocysts with rainbow trout mucus in vitro, in solutions enriched with chloride salts of Na+, K+, Ca2+ and Gd3+, and quantified discharge using microscopy. We then used quantitative polymerase chain reaction to evaluate the in vivo effects of these treatments, plus Mg2+ and the common aquaculture disinfectant KMnO₄, on the ability of M. cerebralis actinospores to infect fish. We found that Mg2+ and Gd3+ reduced infection in vivo, whereas Na+ and K+ over-stimulated nematocyst discharge in vitro and reduced infection in vivo. These findings align with nematocyst discharge behaviour in free-living Cnidaria, and suggest phylum-wide commonalties, which could be exploited to develop novel approaches for controlling myxozoan diseases in aquaculture.

Acanthocephalan parasites collected from Austrian fishes: molecular barcoding and pathological observations

Acanthocephalan parasites were collected from the intestinal tracts of 137 predominantly wild fish (1 barbel Barbus barbus, 3 European chub Squalius cephalus, 13 rainbow trout Oncorhynchus mykiss and 120 brown trout Salmo trutta) from 12 localities. The condition factor, intensity of acanthocephalan infection and pathological lesions, if applicable, were documented. Routine bacteriology and virology were performed, and the brown trout were additionally tested for the presence of the myxozoan parasite Tetracapsolioides bryosalmonae by PCR. In total, 113 acanthocephalans were barcoded by sequencing a section of the mitochondrial cytochrome oxidase subunit I (COI) gene. Barcoding of the acanthocephalan tissues resulted in 77 sequences, of which 56 were assigned to Echinorhynchus truttae (3 genotypes), 11 to Pomphorhynchus tereticollis (9 genotypes), 9 to Acanthocephalus sp. (5 genotypes) and 1 to Neoechinorhynchida. Most of these genotypes were detected for the first time. Statistically, the acanthocephalan infection did not have an impact on the condition factor of the brown trout. Infection with P. tereticollis caused more severe pathological changes in the digestive tract than E. truttae. The present study provides new data regarding the distribution of acanthocephalan species in Austria and their impact on individual fish. In addition, new barcoding data from acanthocephalan parasites are presented, and the occurrence of P. tereticollis in European chub in Austria and in brown and rainbow trout in general was confirmed for the first time.

First transcriptome analysis of bryozoan Fredericella sultana, the primary host of myxozoan parasite Tetracapsuloides bryosalmonae

Bryozoans are aquatic invertebrate moss animals that are found worldwide. Fredericella sultana is a freshwater bryozoan and is the most common primary host of myxozoan parasite, Tetracapsuloides bryosalmonae. However, limited genomic resources are available for this bryozoan, which hampers investigations into the molecular mechanisms of host-parasite interactions. To better understand these interactions, there is a need to build a transcriptome dataset of F. sultana, for functional genomics analysis by large-scale RNA sequencing. Total RNA was extracted from zooids of F. sultana cultivated under controlled laboratory conditions. cDNA libraries were prepared and were analyzed by the Illumina paired-ends sequencing. The sequencing data were used for de novo transcriptome assembly and functional annotation. Approximately 118 million clean reads were obtained, and assembled into 85,544 contigs with an average length of 852 bp, an N50 of 1,085 bp, and an average GC content 51.4%. A total of 23,978 (28%) contigs were annotated using BLASTX analysis. Of these transcripts, 4,400 contigs had highest similarity to brachiopod species Lingula anatina. Based on Gene ontology (GO) annotation, the most highly scored categories of biological process were categorized into cellular process (27%), metabolic process (24%), and biological regulation (8%) in the transcriptome of F. sultana. This study gives first insights into the transcriptome of F. sultana and provides comprehensive genetic resources for the species. We believe that the transcriptome of F. sultana will serve as a useful genomic dataset to accelerate research of functional genomics and will help facilitate whole genome sequencing and annotation. Candidate genes potentially involved in growth, proteolysis, and stress/immunity-response were identified, and are worthy of further investigation.

The strength and form of natural selection on transcript abundance in the wild

Gene transcription variation is known to contribute to disease susceptibility and adaptation, but we currently know very little about how contemporary natural selection shapes transcript abundance. We estimated selection on transcript abundance in cohort of a wild salmonid fish (Salmo trutta) affected by a myxozoan parasite through mark-recapture field sampling and the integration of RNA-seq with classical regression-based selection analysis. We show, based on fin transcriptomes of the host, that infection by an extracellular myxozoan parasite (Tetracapsuloides bryosalmonae) and subsequent host survival is linked to the upregulation of mitotic cell cycle genes. We also detect a widespread signal of disruptive selection; intermediate transcription levels were frequently associated with reduced host survival. Our results provide insights how selection can be measured at the transcriptome level to dissect the molecular mechanisms of contemporary natural selection driven by climate change and emerging anthropogenic threats.