Exploring Evolutionary Relationships within Neodermata Using Putative Orthologous Groups of Proteins, with Emphasis on Peptidases

Adaptive evolution of stress response genes in parasites aligns with host niche diversity

BACKGROUND

Stress responses are key the survival of parasites and, consequently, also the evolutionary success of these organisms. Despite this importance, our understanding of the evolution of molecular pathways dealing with environmental stressors in parasitic animals remains limited. Here, we tested the link between adaptive evolution of parasite stress response genes and their ecological diversity and species richness. We comparatively investigated antioxidant, heat shock, osmoregulatory, and behaviour-related genes (foraging) in two model parasitic flatworm lineages with contrasting ecological diversity, Cichlidogyrus and Kapentagyrus (Platyhelminthes: Monopisthocotyla), through whole-genome sequencing of 11 species followed by in silico exon bait capture as well as phylogenetic and codon analyses.

RESULTS

We assembled the sequences of 48 stress-related genes and report the first foraging (For) gene orthologs in flatworms. We found duplications of heat shock (Hsp) and oxidative stress genes in Cichlidogyrus compared to Kapentagyrus. We also observed positive selection patterns in genes related to mitochondrial protein import (Hsp) and behaviour (For) in species of Cichlidogyrus infecting East African cichlids-a host lineage under adaptive radiation. These patterns are consistent with a potential adaptation linked to a co-radiation of these parasites and their hosts. Additionally, the absence of cytochrome P450 and kappa and sigma-class glutathione S-transferases in monogenean flatworms is reported, genes considered essential for metazoan life.

CONCLUSIONS

This study potentially identifies the first molecular function linked to a flatworm radiation. Furthermore, the observed gene duplications and positive selection indicate the potentially important role of stress responses for the ecological adaptation of parasite species.

Hooked on fish blood: the reliance of a gill parasite on haematophagy

Parasitism involves diverse evolutionary strategies, including adaptations for blood feeding, which provides essential nutrients for growth and reproduction. Sparicotyle chrysophrii (Polyopisthocotyla: Microcotylidae), an ectoparasitic flatworm, infects the gills of gilthead seabream (Sparus aurata), significantly affecting fish health, welfare and Mediterranean cage farm profitability. Despite its impact, limited information exists on its feeding behaviour. This study demonstrates the presence of blood and exogenous haem groups in S. chrysophrii and explores its digestive tract using light and electron microscopy, elucidating its internal morphology and spatial arrangement. Elemental analysis of the digestive haematin cells shows residual oxidized haem depots as haematin crystals. Additionally, we studied the impact of the blood feeding on the host by estimating the average volume of blood intake for an adult parasite (2.84 ± 2.12µl·24h-1) and we described the significant drop of the plasmatic free iron levels in infected hosts. Overall, we demonstrate the parasite's reliance on its host blood, the parasite's buccal and digestive morphological adaptations for blood feeding and the provoked effect on the fish host's health.

Proteomic analysis of Neobenedenia sp. and Rhabdosynochus viridisi (Monogenea, Monopisthocotylea): Insights into potential vaccine targets and diagnostic markers for finfish aquaculture

Monogeneans are parasitic flatworms that represent a significant threat to the aquaculture industry. Species like Neobenedenia melleni (Capsalidae) and Rhabdosynochus viridisi (Diplectanidae) have been identified as causing diseases in farmed fish. In the past years, molecular research on monogeneans of the subclass Monopisthocotylea has focused on the generation of genomic and transcriptomic information and the identification in silico of some protein families of veterinary interest. Proteomic analysis has been suggested as a powerful tool to investigate proteins in parasites and identify potential targets for vaccine development and diagnosis. To date, the proteomic dataset for monogeneans has been restricted to a species of the subclass Polyopisthocotylea, while in monopisthocotyleans there is no proteomic data. In this study, we present the first proteomic data on two monopisthocotylean species, Neobenedenia sp. and R. viridisi, obtained from three distinct sample types: tissue, excretory-secretory products (ESPs), and eggs. A total of 1691 and 1846 expressed proteins were identified in Neobenedenia sp. and R. viridisi, respectively. The actin family was the largest protein family, followed by the tubulin family and the heat shock protein 70 (HSP70) family. We focused mainly on ESPs because they are important to modulate the host immune system. We identified proteins of the actin, tubulin, HSP70 and HSP90 families in both tissue and ESPs, which have been recognized for their antigenic activities in parasitic flatworms. Furthermore, our study uncovered the presence of proteins within ESPs, such as annexin, calcium-binding protein, fructose bisphosphate aldolase, glutamate dehydrogenase, myoferlin, and paramyosin, that are targets for immunodiagnostic and vaccine development and hold paramount relevance in veterinary medicine. This study expands our knowledge of monogeneans and identified proteins that, in other platyhelminths are potential targets for vaccines and drug discovery.

An inside out journey: biogenesis, ultrastructure and proteomic characterisation of the ectoparasitic flatworm Sparicotyle chrysophrii extracellular vesicles

BACKGROUND

Helminth extracellular vesicles (EVs) are known to have a three-way communication function among parasitic helminths, their host and the host-associated microbiota. They are considered biological containers that may carry virulence factors, being therefore appealing as therapeutic and prophylactic target candidates. This study aims to describe and characterise EVs secreted by Sparicotyle chrysophrii (Polyopisthocotyla: Microcotylidae), a blood-feeding gill parasite of gilthead seabream (Sparus aurata), causing significant economic losses in Mediterranean aquaculture.

METHODS

To identify proteins involved in extracellular vesicle biogenesis, genomic datasets from S. chrysophrii were mined in silico using known protein sequences from Clonorchis spp., Echinococcus spp., Fasciola spp., Fasciolopsis spp., Opisthorchis spp., Paragonimus spp. and Schistosoma spp. The location and ultrastructure of EVs were visualised by transmission electron microscopy after fixing adult S. chrysophrii specimens by high-pressure freezing and freeze substitution. EVs were isolated and purified from adult S. chrysophrii (n = 200) using a newly developed ultracentrifugation-size-exclusion chromatography protocol for Polyopisthocotyla, and EVs were characterised via nanoparticle tracking analysis and tandem mass spectrometry.

RESULTS

Fifty-nine proteins involved in EV biogenesis were identified in S. chrysophrii, and EVs compatible with ectosomes were observed in the syncytial layer of the haptoral region lining the clamps. The isolated and purified nanoparticles had a mean size of 251.8 nm and yielded 1.71 × 108 particles · mL-1. The protein composition analysis identified proteins related to peptide hydrolases, GTPases, EF-hand domain proteins, aerobic energy metabolism, anticoagulant/lipid-binding, haem detoxification, iron transport, EV biogenesis-related, vesicle-trafficking and other cytoskeletal-related proteins. Several identified proteins, such as leucyl and alanyl aminopeptidases, calpain, ferritin, dynein light chain, 14-3-3, heat shock protein 70, annexin, tubulin, glutathione S-transferase, superoxide dismutase, enolase and fructose-bisphosphate aldolase, have already been proposed as target candidates for therapeutic or prophylactic purposes.

CONCLUSIONS

We have unambiguously demonstrated for the first time to our knowledge the secretion of EVs by an ectoparasitic flatworm, inferring their biogenesis machinery at a genomic and transcriptomic level, and by identifying their location and protein composition. The identification of multiple therapeutic targets among EVs' protein repertoire provides opportunities for target-based drug discovery and vaccine development for the first time in Polyopisthocotyla (sensu Monogenea), and in a fish-ectoparasite model.

Strong mitonuclear discordance in the phylogeny of Neodermata and evolutionary rates of Polyopisthocotylea

The genomic evolution of Polyopisthocotylea remains poorly understood in comparison to the remaining three classes of Neodermata: Monopisthocotylea, Cestoda, and Trematoda. Moreover, the evolutionary sequence of major events in the phylogeny of Neodermata remains unresolved. Herein we sequenced the mitogenome and transcriptome of the polyopisthocotylean Diplorchis sp., and conducted comparative evolutionary analyses using nuclear (nDNA) and mitochondrial (mtDNA) genomic datasets of Neodermata. We found strong mitonuclear discordance in the phylogeny of Neodermata. Polyopisthocotylea exhibited striking mitonuclear discordance in relative evolutionary rates: the fastest-evolving mtDNA in Neodermata and a comparatively slowly-evolving nDNA genome. This was largely attributable to its very long stem branch in mtDNA topologies, not exhibited by the nDNA data. We found indications that the fast evolution of mitochondrial genomes of Polyopisthocotylea may be driven both by relaxed purifying selection pressures and elevated levels of directional selection. We identified mitochondria-associated genes encoded in the nuclear genome: they exhibited unique evolutionary rates, but not correlated with the evolutionary rate of mtDNA, and there is no evidence for compensatory evolution (they evolved slower than the rest of the genome). Finally, there appears to exist an exceptionally large (≈6.3 kb) nuclear mitochondrial DNA segment (numt) in the nuclear genome of newly sequenced Diplorchis sp. A 3'-end segment of the 16S rRNA gene encoded by the numt was expressed, suggesting that this gene acquired novel, regulatory functions after the transposition to the nuclear genome. In conclusion, Polyopisthocotylea appears to be the lineage with the fastest-evolving mtDNA sequences among all of Bilateria, but most of the substitutions were accumulated deep in the evolutionary history of this lineage. As the nuclear genome does not exhibit a similar pattern, the circumstances underpinning this evolutionary phenomenon remain a mystery.

An insight into the functional genomics and species classification of Eudiplozoon nipponicum (Monogenea, Diplozoidae), a haematophagous parasite of the common carp Cyprinus carpio

BACKGROUND

Monogenea (Platyhelminthes, Neodermata) are the most species-rich class within the Neodermata superclass of primarily fish parasites. Despite their economic and ecological importance, monogenean research tends to focus on their morphological, phylogenetic, and population characteristics, while comprehensive omics analyses aimed at describing functionally important molecules are few and far between. We present a molecular characterisation of monogenean representative Eudiplozoon nipponicum, an obligate haematophagous parasite infecting the gills of the common carp. We report its nuclear and mitochondrial genomes, present a functional annotation of protein molecules relevant to the molecular and biochemical aspect of physiological processes involved in interactions with the fish hosts, and re-examinate the taxonomic position of Eudiplozoon species within the Diplozoidae family.

RESULTS

We have generated 50.81 Gbp of raw sequencing data (Illumina and Oxford Nanopore reads), bioinformatically processed, and de novo assembled them into a genome draft 0.94 Gbp long, consisting of 21,044 contigs (N50 = 87 kbp). The final assembly represents 57% of the estimated total genome size (~ 1.64 Gbp), whereby repetitive and low-complexity regions account for ~ 64% of the assembled length. In total, 36,626 predicted genes encode 33,031 proteins and homology-based annotation of protein-coding genes (PCGs) and proteins characterises 14,785 (44.76%) molecules. We have detected significant representation of functional proteins and known molecular functions. The numbers of peptidases and inhibitors (579 proteins), characterised GO terms (16,016 unique assigned GO terms), and identified KEGG Orthology (4,315 proteins) acting in 378 KEGG pathways demonstrate the variety of mechanisms by which the parasite interacts with hosts on a macromolecular level (immunomodulation, feeding, and development). Comparison between the newly assembled E. nipponicum mitochondrial genome (length of 17,038 bp) and other diplozoid monogeneans confirms the existence of two distinct Eudiplozoon species infecting different fish hosts: Cyprinus carpio and Carassius spp.

CONCLUSIONS

Although the amount of sequencing data and characterised molecules of monogenean parasites has recently increased, a better insight into their molecular biology is needed. The E. nipponicum nuclear genome presented here, currently the largest described genome of any monogenean parasite, represents a milestone in the study of monogeneans and their molecules but further omics research is needed to understand these parasites' biological nature.