Detection of the kinetoplastid Azumiobodo hoyamushi, the causative agent of soft tunic syndrome, in wild ascidians Halocynthia roretzi

Innate immunity in the edible ascidian Halocynthia roretzi developing soft tunic syndrome: Hemolymph can eliminate the causative flagellates and discriminate allogeneic hemocytes

The infection of the kinetoplastid flagellate Azumiobodo hoyamushi causes soft tunic syndrome that often results in mass mortality in the aquaculture of the edible ascidian Halocynthia roretzi. In the diseased ascidian individuals, the flagellates are exclusively found in the tunic matrix that entirely cover the epidermis, and never invade into internal tissues, such as a mantle. The present study for the first time demonstrated that the ascidian blood plasma and hemolymph have an activity to agglutinate and disintegrate the flagellates, suggesting the innate immunity protects the internal tissue from the invasion of A. hoyamushi. This activity is indifferent between the healthy and the diseased individuals. Allo-specific recognition and cytotoxic reaction among ascidian hemocytes, so-called contact reaction, occur among the individuals of healthy-healthy, healthy-diseased, and diseased-diseased combination, and therefore, the hemocytes from diseased individuals still retain the allo-reactivity. Moreover, the allo-reactive combinations are not changed under the presence of the flagellates, indicating the flagellates neither suppress nor induce the effector system of the contact reaction. These results suggest that the infection of A. hoyamushi does not impair the innate immunity in the ascidian hemolymph.

Morphological Study and 3D Reconstruction of the Larva of the Ascidian Halocynthia roretzi

The swimming larva represents the dispersal phase of ascidians, marine invertebrates belonging to tunicates. Due to its adhesive papillae, the larva searches the substrate, adheres to it, and undergoes metamorphosis, thereby becoming a sessile filter feeding animal. The larva anatomy has been described in detail in a few species, revealing a different degree of adult structure differentiation, called adultation. In the solitary ascidian Halocynthia roretzi, a species reared for commercial purposes, embryogenesis has been described in detail, but information on the larval anatomy is still lacking. Here, we describe it using a comparative approach, utilizing 3D reconstruction, as well as histological/TEM observations, with attention to its papillae. The larva is comparable to those of other solitary ascidians, such as Ciona intestinalis. However, it displays a higher level of adultation for the presence of the atrium, opened outside by means of the atrial siphon, and the peribranchial chambers. It does not reach the level of complexity of the larva of Botryllus schlosseri, a phylogenetically close colonial ascidian. Our study reveals that the papillae of H. roretzi, previously described as simple and conform, exhibit dynamic changes during settlement. This opens up new considerations on papillae morphology and evolution and deserves to be further investigated.

Neobodonids are dominant kinetoplastids in the global ocean

Kinetoplastid flagellates comprise basal mostly free-living bodonids and derived obligatory parasitic trypanosomatids, which belong to the best-studied protists. Due to their omnipresence in aquatic environments and soil, the bodonids are of ecological significance. Here, we present the first global survey of marine kinetoplastids and compare it with the strikingly different patterns of abundance and diversity in their sister clade, the diplonemids. Based on analysis of 18S rDNA V9 ribotypes obtained from 124 sites sampled during the Tara Oceans expedition, our results show generally low to moderate abundance and diversity of planktonic kinetoplastids. Although we have identified all major kinetoplastid lineages, 98% of kinetoplastid reads are represented by neobodonids, namely specimens of the Neobodo and Rhynchomonas genera, which make up 59% and 18% of all reads, respectively. Most kinetoplastids have small cell size (0.8-5 µm) and tend to be more abundant in the mesopelagic as compared to the euphotic zone. Some of the most abundant operational taxonomic units have distinct geographical distributions, and three novel putatively parasitic neobodonids were identified, along with their potential hosts.

Tunic extract of the host ascidian attracts the causal agent of soft tunic syndrome, Azumiobodo hoyamushi (Kinetoplastea: Neobodonida)

Azumiobodo hoyamushi, a kinetoplastid flagellate, is the causative agent of soft tunic syndrome, an infectious disease of the edible ascidian Halocynthia roretzi. The flagellate is thought to invade the tunic matrix via a damaged area of the tunic on the siphon wall. We hypothesized that the flagellate locates the tunic entry site by a chemotactic response to soluble substances diffused from the host ascidians. To investigate this hypothesis, we examined whether the flagellate shows a chemotactic response to tissue extracts (tunic and other tissues) from the host ascidian H. roretzi. We tested extracts from 5 tissues as well as hemolymph. Only the tunic extract showed significant positive chemotactic activity, and the activity decreased with increasing dilution. Furthermore, autoclaved tunic extract, extracts from diseased individuals, and extract from the styelid ascidian Styela clava also had chemotactic activity, although the activities were lower than that of tunic extract from healthy H. roretzi. Ultrafiltration of the tunic extract through a 3 kDa cutoff membrane completely abrogated the activity; the ultrafiltration retentate still showed activity. Thus, the soluble factors that attract the flagellate are present exclusively in the tunic extract, and the chemotactic factors are larger than 3 kDa. Our experiments also suggested that the tunic extract contains both heat-stable and heat-labile factors. We conclude that the flagellate locates the tunic entry site by chemotaxis toward soluble factors that diffuse from a damaged area of the tunic on the siphon wall.

Disinfection of fertilized eggs of the edible ascidian Halocynthia roretzi for prevention of soft tunic syndrome

Azumiobodo hoyamushi, the causative agent of soft tunic syndrome, was likely introduced to farming sites of the edible ascidian Halocynthia roretzi via ascidian spat. The source of infection is thought to be cysts of A. hoyamushi that reside in the substrates on which the ascidian spat are attached, but not the spat themselves. Thus, there is a need to develop methods to prevent contamination of the substrates with A. hoyamushi during seed production of the ascidian. We evaluated the protozoacidal effects of sodium hypochlorite and povidone-iodine against the flagellate and temporary cyst forms of A. hoyamushi. Additionally, we evaluated the effects of these disinfectants on the development of fertilized ascidian eggs. The flagellate form of A. hoyamushi was completely inactivated by povidone-iodine (5 ppm, 1 min) and sodium hypochlorite (1 ppm, 1 min). The temporary cysts of A. hoyamushi were completely inactivated by both disinfectants (5 ppm, 1 min). Disinfection with 50 ppm povidone-iodine for 15 min or 5 ppm sodium hypochlorite for 15 min had no effect on ascidian embryogenesis. Thus, horizontal transmission of A. hoyamushi via the substrates can be efficiently prevented by disinfecting ascidian eggs or tools used for spawning with povidone-iodine baths ranging from 5 ppm for 1 min to 50 ppm for 15 min without any side effects.

Encystment and excystment of kinetoplastid Azumiobodo hoyamushi, causal agent of soft tunic syndrome in ascidian aquaculture

Soft tunic syndrome in the edible ascidian Halocynthia roretzi is caused by the kinetoplastid flagellate Azumiobodo hoyamushi, which was found to assume a fusiform cell form with 2 flagella in axenic, pure culture. When the flagellate form was incubated in sterilized artificial seawater (pH 8.4), some of the cells became cyst-like and adhered to the bottom of the culture plate. The cyst-like forms were spherical or cuboidal, and each had 2 flagella encapsulated in its cytoplasm. Encystment was also induced in culture medium alkalified to the pH of seawater (8.4) but not in unmodified (pH 7.2) or acidified media (pH 6.4). More than 95% of the cyst-like cells converted to the flagellate form within 1 d following transfer to seawater containing ascidian tunic extracts from host ascidians. The cyst-like cells were able to survive in seawater with no added nutrients for up to 2 wk at 20°C and for a few months at 5 to 15°C. The survival period in seawater depended on temperature: some cyst-like cells survived 3 mo at 10°C, and ca. 95% of these converted to flagellate forms in seawater containing tunic extracts. Thus, A. hoyamushi is able to persist under adverse conditions in a cyst-like form able to adhere to organic and inorganic substrata for protracted periods of time.

Azumiobodo hoyamushi, the kinetoplastid causing soft tunic syndrome in ascidians, may invade through the siphon wall

The infectious kinetoplastid Azumiobodo hoyamushi causes 'soft tunic syndrome', a serious problem in aquaculture of the edible ascidian Halocynthia roretzi. Infection tests using diseased tunics demonstrated that juvenile (0.8 yr old) individuals never developed soft tunic syndrome, but all individuals in the other age groups (1.8, 2.8, and 3.8 yr old) showed the disease symptoms. In the infection tests, tunic softening was first observed at the tunic around siphons. Based on ultrastructural observation of the inner wall of the branchial siphon, the tunic lining the inner wall in juveniles (0.5 yr old) was completely covered with cuticle, which had a dense structure to prevent bacterial and protist invasion. In contrast, the tunic was often partly damaged and not covered with cuticle in healthy adults (≥2.5 yr old). The damaged tunic in the siphon wall could be an entrance for A. hoyamushi into the tunic of adult hosts.

Development of loop-mediated isothermal amplification targeting 18s ribosomal DNA for rapid detection of Azumiobodo hoyamushi (Kinetoplastea)

Ascidian soft tunic syndrome (AsSTS) caused by Azumiobodo hoyamushi (A. hoyamushi) is a serious aquaculture problem that results in mass mortality of ascidians. Accordingly, the early and accurate detection of A. hoyamushi would contribute substantially to disease management and prevention of transmission. Recently, the loop-mediated isothermal amplification (LAMP) method was adopted for clinical diagnosis of a range of infectious diseases. Here, the authors describe a rapid and efficient LAMP-based method targeting the 18S rDNA gene for detection of A. hoyamushi using ascidian DNA for the diagnosis of AsSTS. A. hoyamushi LAMP assay amplified the DNA of 0.01 parasites per reaction and detected A. hoyamushi in 10 ng of ascidian DNA. To validate A. hoyamushi 18S rDNA LAMP assays, AsSTS-suspected and non-diseased ascidians were examined by microscopy, PCR, and by using the LAMP assay. When PCR was used as a gold standard, the LAMP assay showed good agreement in terms of sensitivity, positive predictive value (PPV), and negative predictive value (NPV). In the present study, a LAMP assay based on directly heat-treated samples was found to be as efficient as DNA extraction using a commercial kit for detecting A. hoyamushi. Taken together, this study shows the devised A. hoyamushi LAMP assay could be used to diagnose AsSTS in a straightforward, sensitive, and specific manner, that it could be used for forecasting, surveillance, and quarantine of AsSTS.

Evolution of parasitism in kinetoplastid flagellates

Kinetoplastid protists offer a unique opportunity for studying the evolution of parasitism. While all their close relatives are either photo- or phagotrophic, a number of kinetoplastid species are facultative or obligatory parasites, supporting a hypothesis that parasitism has emerged within this group of flagellates. In this review we discuss origin and evolution of parasitism in bodonids and trypanosomatids and specific adaptations allowing these protozoa to co-exist with their hosts. We also explore the limits of biodiversity of monoxenous (one host) trypanosomatids and some features distinguishing them from their dixenous (two hosts) relatives.