Parasitic Dinoflagellate Research Articles

Stable dominance of parasitic dinoflagellates in Antarctic sponges.

Marine sponges are dominant components of Antarctic benthos and representative of the high endemism that characterizes this environment. All microbial groups are part of the Antarctic sponge holobionts, but microbial eukaryotes have been studied less, and their symbiotic role still needs to be better understood. Here, we characterize the dynamics of microbial eukaryotes associated with Antarctic sponges, focusing on dinoflagellates over three summer periods to better understand the members, interannual variations, and trophic and lifestyle strategies. The analysis revealed that dinoflagellates dominate microeukaryotic communities in Antarctic sponges. The results also showed significant differences in the diversity and composition of dinoflagellate communities associated with sponges compared to those in seawater. Antarctic sponges were dominated by a single dinoflagellate family, Syndiniales Dino-Group-I-Clade 1, which was present in high abundance in Antarctic sponges compared to seawater communities. Despite minor differences, the top microeukaryotic amplicon sequence variants (ASVs) showed no significant interannual abundance changes, indicating general temporal stability within the studied sponge species. Our findings highlight the abundance and importance of parasitic groups, particularly the classes Coccidiomorphea, Gregarinomorphea, and Ichthyosporea, with the exclusive dominance of Syndiniales Dino-Group-I-Clade 1 within sponges. The present study comprehensively characterizes the microbial eukaryotes associated with Antarctic sponges, showing a remarkable stability of parasitic dinoflagellates in Antarctic sponges. These findings underscore the significant role of parasites in these marine hosts, with implications for population dynamics of the microeukaryome and the holobiont response to a changing ocean.

The Sinking Dead—Arctic Deep‐Sea Scavengers' Diet Suggests Nekton as Vector in Benthopelagic Coupling

ABSTRACTMany benthic deep‐sea animals rely on carcasses from the overlying water column that sink to the seafloor and form local organic enrichments known as food falls. This flux of organic carbon from the shallow pelagic to the deep sea is part of the biological carbon pump (BCP) and as such contributes to carbon sequestration. For a complete understanding of local carbon budgets, it is crucial to identify the diversity and distribution of sinking carcasses which are difficult to detect by observational methods. Here, we analyzed the diet of the abundant amphipod scavenger, Eurythenes gryllus, by DNA metabarcoding to assess their potential to identify food falls in the Fram Strait, a gateway to the Arctic. E. gryllus scavenges on nekton but so far it was not certain whether this represents their main diet. We detected dietary taxa (26 in total) in 20 out of 101 analyzed amphipods. We found that amphipods primarily fed on larger nekton including fish, cephalopods, and mammals, with bony fish being the most targeted food source in terms of diversity and abundance. Only one amphipod had fed on a gelatinous organism. These results support the hypothesis that E. gryllus targets mostly nekton food falls. The diversity of dietary taxa differed between the Eastern and Western Fram Strait, which suggests regional variability in food falls availability. We also detected, for the first time in E. gryllus, infections with the parasitic dinoflagellate Hematodinium. This detection demonstrates the potential of metabarcoding for revealing both food web dynamics and host–parasite interactions in the deep sea. E. gryllus seems a promising “natural sampler” to monitor the diversity of deep‐sea food falls which will help to investigate the importance of medium‐sized food falls in local vertical carbon export in a rapidly changing Arctic Ocean.

Biogeographical and biodiversity patterns of planktonic microeukaryotes along the tropical western to eastern Pacific Ocean transect revealed by metabarcoding.

Microeukaryotic plankton (0.2-200 µm), which are morphologically and genetically highly diverse, play a crucial role in ocean productivity and carbon consumption. The Pacific Ocean (PO), one of the world's largest oligotrophic regions, remains largely unexplored in terms of the biogeography and biodiversity of microeukaryotes based on large-scale sampling. We investigated the horizontal distribution of microeukaryotes along a 16,000 km transect from the west to the east of the PO. The alpha diversity indices showed a distinct decreasing trend from west to east, which was highly correlated with water temperature. The microeukaryotic community, which was clustered into the western, central, and eastern PO groups, displayed a significant distance-decay relationship. Syndiniales, a lineage of parasitic dinoflagellates, was ubiquitously distributed along the transect and dominated the community in terms of both sequence and zero-radius operational taxonomic unit (ZOTU) proportions. The prevailing dominance of Syndiniales-affiliated ZOTUs and their close associations with dinoflagellates, diatoms, and radiolarians, as revealed by SparCC correlation analysis, suggested that parasitism may be an important trophic strategy in the surface waters of the PO. Geographical distance and temperature were the most important environmental factors that significantly correlated with community structure. Overall, our study sheds more light on the distribution pattern of both alpha and beta diversities of microeukaryotic communities and highlighted the importance of parasitisms by Syndiniales across the tropical PO.IMPORTANCEUnderstanding the biogeographical and biodiversity patterns of microeukaryotic communities is essential to comprehending their roles in biogeochemical cycling. In this study, planktonic microeukaryotes were collected along a west-to-east Pacific Ocean transect (ca. 16,000 km). Our study revealed that the alpha diversity indices were highly correlated with water temperature, and the microeukaryotic communities displayed a distinct geographical distance-driven pattern. The predominance of the parasitic dinoflagellate lineage Syndiniales and their close relationship with other microeukaryotic groups suggest that parasitism may be a crucial survival strategy for microeukaryotes in the surface waters of the Pacific Ocean. Our findings expand our understanding of the biodiversity and biogeographical pattern of microeukaryotes and highlight the significance of parasitic Syndiniales in the surface ocean.

Histopathology and Phylogeny of the Dinoflagellate Hematodinium perezi and the Epibiotic Peritrich Ciliate Epistylis sp. Infecting the Blue Crab Callinectes sapidus in the Eastern Mediterranean.

Bioinvasions constitute both a direct and an indirect threat to ecosystems. Direct threats include pressures on local trophic chains, while indirect threats might take the form of an invasion of a microorganism alongside its host. The marine dinoflagellate Hematodinium perezi, parasitizing blue crabs (Callinectes sapidus), has a worldwide distribution alongside its host. In Greece, fluctuations in the blue crab population are attributed to overexploitation and the effects of climate change. The hypothesis of the present study was that blue crab population reductions cannot only be due to these factors, and that particular pathogens may also be responsible for the fluctuations. To investigate this hypothesis, both lethargic and healthy blue crab specimens were collected from three different fishing sites in order to assess the health status of this important species. Together with the lethargic responses, the hemolymph of the infested crabs presented a milky hue, indicating the first signs of parasitic infestation with H. perezi. The histopathological results and molecular identification demonstrated the effect of the presence of H. perezi in the internal organs and their important role in the mortality of blue crabs. Specifically, H. perezi, in three different tissues examined (heart, gills, hepatopancreas), affected the hemocytes of the species, resulting in alterations in tissue structure. Apart from this dinoflagellate parasite, the epibiotic peritrich ciliate Epistylis sp. was also identified, infecting the gills. This study represents the first detection of H. perezi in the eastern Mediterranean, demonstrating that this is the main causative agent of blue crab mortality on Greek coastlines.

Morphology of Hobagella saltata n. gen. and n. sp. (Syndiniophyceae, Miozoa) infecting the marine dinoflagellate Cucumeridinium coeruleum (Dinophyceae, Miozoa) and its potential onshore advection

Over the past decade, molecular phylogenies have placed endoparasites of the genus Euduboscquella in a distinct subclade within clade 4 of the Marine Alveolate (MALV) Group I. Recently, however, rRNA gene sequences have become available for four novel Euduboscquella-like species that infect dinoflagellates, with phylogenies including these sequences indicating that the genus Euduboscquella is paraphyletic. Here, we provide a morphological characterization of the intracellular and extracellular life-cycle stages of a novel species that infects Cucumeridinium coeruleum, a warm water pelagic species sometimes found in coastal environments. We formally describe the novel parasite, Hobagella saltata n. gen, n. sp., and identify a constellation of morphological and developmental characters that distinguish it, as well as Euduboscquella melo and E. nucleocola, both parasites of dinoflagellates, from Euduboscquella species that infect ciliates. We recommend the reassignment of E. melo and E. nucleocola as Hobagella melo n. comb. and H. nucleocola n. comb., respectively. We also propose the family Hobagellidae for these three congeners. We anticipate that the character set developed for distinguishing species of Euduboscquella and Hobagella will be valuable for sorting other Euduboscquella-Hobagella-like taxa scattered across the Group I phylogeny and will provide insight into morphological evolutionary patterns within Group I. Lastly, we consider the potential influence of summer-fall typhoons on the occurrence of H. saltata and its host C. coeruleum in near-shore waters along the southeastern coast of Korea and propose a hypothesis regarding the northward transport and onshore advection of host and parasite populations. If future research supports the hypothesized mechanisms, it could help us better understand parasite distribution and potential changes in biogeography associated with ongoing global changes in surface seawater temperature.

Characterization of Piscinoodinium sp. associated with epizootics and mortality in non-native and endemic freshwater fish of the Andaman Islands, India.

Piscinoodinium is a well-known parasitic dinoflagellate genus that causes epizootics in tropical freshwater fish. This study investigated the first outbreak of Piscinoodinium sp. in freshwater fish of the Andaman Islands, India. The infection was noticed in the non-native fish Betta splendens and Poecilia reticulata, and in the endemic killifish Aplocheilus andamanicus. The infection caused mass mortality in larval, sub-adult, and adult fish. The parasite was identified based on microscopy and molecular characterization. The prevalence, parasite density, and mortality percentage were recorded. Infected fish had numerous trophonts attached on the body surface, gills, fins, and eyes. Infected skin and gills showed degeneration of the epithelial cells and clubbed gills. Small subunit rDNA (955 bp) was amplified from the Piscinoodinium sp. infecting B. splendens, revealing genetic differences between it and Piscinoodinium spp. from the USA and from India.

DNA metabarcoding reveals the dietary profiles of a benthic marine crustacean, Nephrops norvegicus.

Norwegian lobster, Nephrops norvegicus, are a generalist scavenger and predator capable of short foraging excursions but can also suspension feed. Existing knowledge about their diet relies on a combination of methods including morphology-based stomach content analysis and stable isotopes, which often lack the resolution to distinguish prey items to species level particularly in species that thoroughly masticate their prey. DNA metabarcoding overcomes many of the challenges associated with traditional methods and it is an attractive approach to study the dietary profiles of animals. Here, we present the diet of the commercially valuable Nephrops norvegicus using DNA metabarcoding of gut contents. Despite difficulties associated with host amplification, our cytochrome oxidase I (COI) molecular assay successfully achieves higher resolution information than traditional approaches. We detected taxa that were likely consumed during different feeding strategies. Dinoflagellata, Chlorophyta and Bacillariophyta accounted for almost 50% of the prey items consumed, and are associated with suspension feeding, while fish with high fisheries discard rates were detected which are linked to active foraging. In addition, we were able to characterise biodiversity patterns by considering Nephrops as natural samplers, as well as detecting parasitic dinoflagellates (e.g., Hematodinium sp.), which are known to influence burrow related behaviour in infected individuals in over 50% of the samples. The metabarcoding data presented here greatly enhances a better understanding of a species' ecological role and could be applied as a routine procedure in future studies for proper consideration in the management and decision-making of fisheries.

Planktoniella sol, a New Host for the Ectoparasitic Dinophyta Paulsenella Chatton from the Northeastern Arabian Sea

Parasitic dinoflagellates, an essential component of the marine ecosystem, are mainly represented by five genera, Myxodinium, Paulsenella, Dubosquella, Coccidinium and Amoebophrya, infecting marine diatoms. All members of dinophyte Paulsenella are ectoparasites of diatoms and presently include three species. The marine diatom Planktoniella sol infection by a parasitic dinoflagellate Paulsenella chaetoceratis from the northeastern Arabian Sea was observed during the spring intermonsoon 2012. ~17 parasitic cells represented by their dinospores infect the host diatom. The diatom P. sol forms a new host record for the parasite, and the report is the first of its kind from the Indian waters.

Outbreak of Parasitic Dinoflagellate Piscinoodinium sp. Infection in an Endangered Fish from India: Arulius Barb (Dawkinsia arulius).

Freshwater velvet disease is caused by the dinoflagellate parasite, Piscinoodinium sp. This parasite has been reported in tropical and subtropical fishes, and it can cause devastating losses. Moreover, Piscinoodinium sp. is identified as one of the least studied finfish parasites, and the available molecular information about this parasite is meager. Recently, Piscinoodinium sp. was responsible for the 100% cumulative mortality of the captive-bred F1 generation of Arulius barb (Dawkinsia arulius), an endangered freshwater fish native to India. The trophont stages of the parasite were observed in the skin and gills of the affected fish. The total DNA was extracted from the trophonts collected from the affected Arulius barb and the partial nucleotide sequence of the rDNA complex region (2334 bp) was amplified using PCR. The amplified PCR product exhibited a high sequence identity (97.61%) with Piscinoodinium sp. In the phylogenetic analysis of the SSU rDNA, Piscinoodinium sp. emerged as a separate clade from other dinoflagellate species. This is the first report of the infection of Piscinoodinium sp. in Arulius barb and the molecular information generated from this study can serve as a baseline to study the diversity of the parasite in India. Furthermore, the impact of this parasite among wild fish stock is not known, and this parasite needs further research focus to generate more molecular information and to understand the host-pathogen interaction.

Hematodinium perezi naturally infects Asian brush-clawed crab (Hemigrapsus takanoi).

The parasitic dinoflagellates of the genus Hematodinium have been considered one of the most important emerging pathogens for a broad range of marine crustaceans around the world. In China, frequent outbreaks of Hematodinium infections have caused serious economic losses for local farmers since 2004. Wild crabs were recently indicated to play a vital role in the transmission and spreading of the Hematodinium disease in polyculture pond systems. Based on PCR amplification and histopathological examination, we demonstrated that H. perezi can naturally infect a wild crab species, Hemigrapsus takanoi, which were collected from the waterways located on the coast of Rizhao or Weifang, Shandong Peninsula, China. According to the sequence similarity analysis and phylogenetic analysis, the Hematodinium isolates were identified as H. perezi and belonged to genotype II. The prevalence of H. perezi ranged from 3.3% to 5.7% in H. takanoi originating from Rizhao (n=165 wild crabs) and from 0.9% to 20.0% in that originating from Weifang (n=1386 wild crabs), respectively. To our knowledge, H. takanoi is, for the first time, reported as a new host for Hematodinium. Given the wide distribution of H. takanoi on the coasts along the Shandong Peninsula and the relative high prevalence of infection we monitored in our study, we speculate that H. takanoi contributes to the introducing and spreading parasitic Hematodinium between ponds via waterways in a poly-culturing system. Findings in this study broaden the host range of this parasite and expand the scope of our surveillance for Hematodinium disease in China.

Integrative omics analysis highlights the immunomodulatory effects of the parasitic dinoflagellate Hematodinium on crustacean hemocytes

Parasitic dinoflagellates in genus Hematodinium have caused substantial economic losses to multiple commercially valuable marine crustaceans around the world. Recent efforts to better understand the life cycle and biology of the parasite have improved our understanding of the disease ecology. However, studies on the host-parasite interaction, especially how Hematodinium parasites evade the host immune response are lacking. To address this shortfall, we used the comprehensive omics approaches (miRNA transcriptomics, iTRAQ-based proteomics) to get insights into the host-parasite interaction between hemocytes from Portunus trituberculatus and Hematodinium perezi in the present study. The parasitic dinoflagellate H. perezi remodeled the miRNome and proteome of hemocytes from challenged hosts, modulated the host immune response at both post-transcriptional and translational levels and caused post-transcriptional regulation to the host immune response. Multiple important cellular and humoral immune-related pathways (ex. Apoptosis, Endocytosis, ECM-receptor interaction, proPO activation pathway, Toll-like signaling pathway, Jak-STAT signaling pathway) were significantly affected by Hematodinium parasites. Through modulation of the host miRNome, the host immune responses of nodulation, proPO activation and antimicrobial peptides were significantly suppressed. Cellular homeostasis was imbalanced via post-transcriptional dysregulation of the phagosome and peroxisome pathways. Cellular structure and communication was seriously impacted by post-transcriptional downregulation of ECM-receptor interaction and focal adhesion pathways. In conclusion, H. perezi parasites could trigger striking changes in the miRNome and proteome of crustacean hemocytes, and this parasite exhibited multifaceted immunomodulatory effects and potential immune-suppressive mechanisms in crustacean hosts.

Occurrence of parasitic infections in a Margalefidinium polykrikoides (Dinophyceae) bloom in the Taiwan Strait in 2018

The harmful dinoflagellate species Margalefidinium polykrikoides is comprised of several ribotypes and is responsible for recurrent blooms worldwide. Parasitic dinoflagellates have been suggested to play a role in the decline of M. polykrikoides blooms; however, information on the host–parasite interaction remains limited. We report a bloom caused by M. polykrikoides ribotype I in the Taiwan Strait in June 2018, as confirmed by microscopic observation and molecular characterization. Amoebophrya ceratii infecting M. polykrikoides was identified by single-cell polymerase chain reaction and subsequent direct sequencing of the LSU rRNA gene from the bloom samples, which shared a 94.72% similarity with Amoebophrya ceratii ex Levanderina fissa. DNA metabarcoding targeting SSU rRNA genes of the V4 region revealed a high diversity of Amoebophrya infecting various hosts in the bloom samples. Amoebophrya ceratii accounted for approximately 45% of Syndiniales Group II from the bloom samples, but less than 2.5% in the non-bloom samples. In addition to two known Amoebophrya spp. ex M. polykrikoides from South Korea, our results provide evidence of infection of M. polykrikoides by a third Amoebophrya species; however, its exact role in the bloom decline of M. polykrikoides remains to be determined.

Protistan plankton communities in the Galápagos Archipelago respond to changes in deep water masses resulting from the 2015/16 El Niño

The Galápagos Archipelago lies within the Eastern Equatorial Pacific Ocean at the convergence of major ocean currents that are subject to changes in circulation. The nutrient-rich Equatorial Undercurrent upwells from the west onto the Galápagos platform, stimulating primary production, but this source of deep water weakens during El Niño events. Based on measurements from repeat cruises, the 2015/16 El Niño was associated with declines in phytoplankton biomass at most sites throughout the archipelago and reduced utilization of nitrate, particularly in large-sized phytoplankton in the western region. Protistan assemblages were identified by sequencing the V4 region of the 18S rRNA gene. Dinoflagellates, chlorophytes and diatoms dominated most sites. Shifts in dinoflagellate communities were most apparent between the years; parasitic dinoflagellates, Syndiniales, were highly detected during the El Niño (2015) while the dinoflagellate genus, Gyrodinium, increased at many sites during the neutral period (2016). Variations in protistan communities were most strongly correlated with changes in subthermocline water density. These findings indicate that marine protistan communities in this region are regimented by deep water mass sources and thus could be profoundly affected by altered ocean circulation.

Chloroplast Ultrastructure and Photosynthetic Response of the Dinoflagellate Akashiwo sanguinea Throughout Infection by Amoebophrya sp.

The endoparasitic dinoflagellate Amoebophrya infects a number of marine dinoflagellates, including toxic and harmful algal bloom-forming species. The parasite kills its host and has been proposed to be a determining factor in the demise of dinoflagellate blooms in restricted coastal waters. Previous studies have mainly focused on the occurrence, prevalence, and diversity of Amoebophrya, while the interactions between the parasite and its host have received limited attention. Herein, an Amoebophrya sp.-Akashiwo sanguinea co-culture was established from Chinese coastal waters, and morphological, physiological, and transcriptional changes throughout an infection cycle of the parasite were systemically studied. The parasitic dinoflagellate was very infectious, resulting in an infection rate up to 85.83% at a dinospore:host ratio of 10:1. Infected host cells died eventually and released approximately 370 dinospores/cell. The host nuclear structures were rapidly degraded by Amoebophrya infection, and the chloroplasts of parasitized host cells remained intact until the parasite filled the almost entire cell structure. Nevertheless, infected cultures showed sustained but lower levels of photosynthetic performance (∼64% of control cultures), and the photosynthesis-related genes were significantly down-regulated. These findings provide a better understanding of the biological basis of the complex parasite-host interactions, which will be helpful to further elucidate the ecological significance of parasitic dinoflagellates in marine ecosystems.

Dinoflagellate-targeted PCR reveals highly abundant and diverse communities of parasitic dinoflagellates in and near Zhubi Reef, South China Sea

Dinoflagellate-targeted PCR reveals highly abundant and diverse communities of parasitic dinoflagellates in and near Zhubi Reef, South China Sea

A Novel Parasitic, Syndinean Dinoflagellate Euduboscquella triangula Infecting the Tintinnid Helicostomella longa

A tintinnid species, Helicostomella longa, infected by the parasitic dinoflagellate Euduboscquella triangula n. sp. was discovered from the southern coast of Korea in August of 2015 and 2016. Parasite morphology and development were analyzed by observation of live cells and protargol-stained specimens. The parasite was determined to be a new species in the genus Euduboscquella based on morphological and molecular data. A representative sequence of the novel species clustered in Euduboscquella group I. The morphological and developmental features of E. triangula were distinguished from those of its congeners by: (1) numerous shallow and intertwining grooves on an inconspicuous shield; (2) sporocytes initially forming a short chain, but separating after the second or third division regardless of spore type; (3) production of motile mushroom-shaped dinospores, non-motile spherical spores, and non-motile triangular spores. Dinospores were formed by ca. 28% of infections, while both non-motile spherical and triangular spores occurred at a frequency of ca. 36%. All spore types showed completely identical 18S rDNA sequences. Parasite prevalence was 15.5 and 8.3% on 17 and 24 August of 2015, respectively, with infection intensity on both dates being 1.3.

First detection of Hematodinium sp. In spiny king crab Paralithodes brevipes, and new geographic areas for the parasite in tanner crab Chionoecetes bairdi, and red king crab Paralithodes camtschaticus

A parasitic dinoflagellate of the genus Hematodinium was found off the Pacific coast of Kamchatka in three species of crabs: red king crab Paralithodes camtschaticus, tanner crab Chionoecetes bairdi, and spiny king crab Paralithodes brevipes. This is the first detection of Hematodinium in spiny king crab. The results of the genetic analysis showed that the pathogen found in P. brevipes, P. camtschaticus, and C bairdi from the Avacha and Kronotsky bays off the Pacific coast of Kamchatka was the same or very close to the Hematodinium sp., which infects many species of crustaceans in the Northern Hemisphere. The prevalence of infection was 0.2% for tanner crabs and 2.7% for red king crabs. Due to a limited sample size, we were unable to calculate the prevalence for spiny king crabs and female red king crabs. Both the macroscopic and microscopic signs of the pathology were similar in all diseased crabs. The differences in the micromorphology of the Hematodinium cells we found in the three crab species, including the presence or absence of trichocysts, the shape of the plasmodia, and the structure of pore complexes, are most likely related to the life cycle and the physiology of the parasite. The results of the genetic analysis showed that the pathogen found in P. brevipes, P. camtschaticus, and C. bairdi from the Avacha and Kronotsky bays of the Pacific coast of Kamchatka was the same or very close to the Hematodinium sp., which infects many species of crustaceans in the Northern Hemisphere.

Impacts of Seagrass on Benthic Microalgae and Phytoplankton Communities in an Experimentally Warmed Coral Reef Mesocosm

The effects of seagrass on microalgal assemblages under experimentally elevated temperatures (28°C) and CO2 partial pressures (pCO2; 800 μatm) were examined using coral reef mesocosms. Concentrations of nitrate, ammonium, and benthic microalgal chlorophyll a (chl-a) were significantly higher in seagrass mesocosms, whereas phytoplankton chl-a concentrations were similar between seagrass and seagrass-free control mesocosms. In the seagrass group, fewer parasitic dinoflagellate OTUs (e.g., Syndiniales) were found in the benthic microalgal community though more symbiotic dinoflagellates (e.g., Cladocopium spp.) were quantified in the phytoplankton community. Our results suggest that, under ocean acidification conditions, the presence of seagrass nearby coral reefs may (1) enhance benthic primary productivity, (2) decrease parasitic dinoflagellate abundance, and (3) possibly increase the presence of symbiotic dinoflagellates.

Transmission pattern of the parasitic dinoflagellate Hematodinium perezi in polyculture ponds of coastal China

Since 2012, Hematodinium perezi epizootics have occurred frequently in cultured swimming crabs Portunus trituberculatus in polyculture ponds along the coast of Shandong Province, China. These outbreaks have resulted in significant economic losses to local farmers. The cryptic transmission pattern of H. perezi has hampered the prevention and control of the pathogen in affected farms. The mudflat crab Helice tientsinensis is one of the dominant wild crabs in the region with considerable prevalence of the parasitic infection covering the full culture period of P. trituberculatus. Mudflat crabs can migrate in and out of polyculture ponds during water exchange and may act as an important reservoir host and source of H. perezi for newly seeded P. trituberculatus. During 2019, we carried out continuous field investigations and sentinel experiments in the polyculture pond system, as well as a series of laboratory infection trials, to study whether waterborne transmission of H. perezi can occur in polyculture ponds, and to determine whether H. perezi can be transmitted from H. tientsinensis to cultured P. trituberculatus. The sentinel experiment verified waterborne transmission of H. perezi to cultured P. trituberculatus in the polyculture pond. The cohabitation trial further indicated that H. perezi was transmittable from H. tientsinensis to P. trituberculatus through waterborne transmission, whereas P. trituberculatus was not able to acquire H. perezi infection by feeding on tissues of infected H. tientsinensis. The results highlighted the transmission pattern of the parasitic dinoflagellate H. perezi in the polyculture pond systems widely used along the coast line of China. In addition, the mudflat crab H. tientsinensis was confirmed as a reservoir host of H. perezi in the epidemic area, and confirmed as the likely source of the parasite during the intensive culture of P. trituberculatus.

Rapid protein evolution, organellar reductions, and invasive intronic elements in the marine aerobic parasite dinoflagellate Amoebophrya spp

BackgroundDinoflagellates are aquatic protists particularly widespread in the oceans worldwide. Some are responsible for toxic blooms while others live in symbiotic relationships, either as mutualistic symbionts in corals or as parasites infecting other protists and animals. Dinoflagellates harbor atypically large genomes (~ 3 to 250 Gb), with gene organization and gene expression patterns very different from closely related apicomplexan parasites. Here we sequenced and analyzed the genomes of two early-diverging and co-occurring parasitic dinoflagellate Amoebophrya strains, to shed light on the emergence of such atypical genomic features, dinoflagellate evolution, and host specialization.ResultsWe sequenced, assembled, and annotated high-quality genomes for two Amoebophrya strains (A25 and A120), using a combination of Illumina paired-end short-read and Oxford Nanopore Technology (ONT) MinION long-read sequencing approaches. We found a small number of transposable elements, along with short introns and intergenic regions, and a limited number of gene families, together contribute to the compactness of the Amoebophrya genomes, a feature potentially linked with parasitism. While the majority of Amoebophrya proteins (63.7% of A25 and 59.3% of A120) had no functional assignment, we found many orthologs shared with Dinophyceae. Our analyses revealed a strong tendency for genes encoded by unidirectional clusters and high levels of synteny conservation between the two genomes despite low interspecific protein sequence similarity, suggesting rapid protein evolution. Most strikingly, we identified a large portion of non-canonical introns, including repeated introns, displaying a broad variability of associated splicing motifs never observed among eukaryotes. Those introner elements appear to have the capacity to spread over their respective genomes in a manner similar to transposable elements. Finally, we confirmed the reduction of organelles observed in Amoebophrya spp., i.e., loss of the plastid, potential loss of a mitochondrial genome and functions.ConclusionThese results expand the range of atypical genome features found in basal dinoflagellates and raise questions regarding speciation and the evolutionary mechanisms at play while parastitism was selected for in this particular unicellular lineage.