Descriptions of Mikrocytos veneroïdes n. sp. and Mikrocytos donaxi n. sp. (Ascetosporea: Mikrocytida: Mikrocytiidae), detected during important mortality events of the wedge clam Donax trunculus Linnaeus (Veneroida: Donacidae), in France between 2008 and 2011

A Mitosome With Distinct Metabolism in the Uncultured Protist Parasite Paramikrocytos canceri (Rhizaria, Ascetosporea)

Ascetosporea are endoparasites of marine invertebrates that include economically important pathogens of aquaculture species. Owing to their often-minuscule cell sizes, strict intracellular lifestyle, lack of cultured representatives and minimal availability of molecular data, these unicellular parasites remain poorly studied. Here, we sequenced and assembled the genome and transcriptome of Paramikrocytos canceri, an endoparasite isolated from the European edible crab Cancer pagurus. Using bioinformatic predictions, we show that P. canceri likely possesses a mitochondrion-related organelle (MRO) with highly reduced metabolism, resembling the mitosomes of other parasites but with key differences. Like other mitosomes, this MRO is predicted to have reduced metabolic capacity and lack an organellar genome and function in iron-sulfur cluster (ISC) pathway-mediated Fe-S cluster biosynthesis. However, the MRO in P. canceri is uniquely predicted to produce ATP via a partial glycolytic pathway and synthesize phospholipids de novo through the CDP-DAG pathway. Heterologous gene expression confirmed that proteins from the ISC and CDP-DAG pathways retain mitochondrial targeting sequences that are recognized by yeast mitochondria. This represents a unique combination of metabolic pathways in an MRO, including the first reported case of a mitosome-like organelle able to synthesize phospholipids de novo. Some of these phospholipids, such as phosphatidylserine, are vital in other protist endoparasites that invade their host through apoptotic mimicry.

Evaluation of histopathology, PCR, and qPCR to detect Mikrocytos mackini in oysters Crassostrea gigas using Bayesian latent class analysis

Latent class analysis (LCA) is a common method to evaluate the diagnostic sensitivity (DSe) and specificity (DSp) for pathogen detection assays in the absence of a perfect reference standard. Here we used LCA to evaluate the diagnostic accuracy of 3 tests for the detection of Mikrocytos mackini in Pacific oysters Crassostrea gigas: conventional polymerase chain reaction (PCR), real-time quantitative PCR (qPCR), and histopathology. A total of 802 Pacific oysters collected over 12 sampling events from 9 locations were assessed. Preliminary investigations indicated that standard LCA assumptions of test independence and constant detection accuracy across locations were likely unrealistic. This was mitigated by restructuring the LCA in a Bayesian framework to include test-derived knowledge about pathogen prevalence and load for categorizing populations into 2 classes of infection severity (low or high) and assessing separate DSe and DSp estimates for each class. Median DSp estimates were high (>96%) for all 3 tests in both population classes. DSe estimates varied between tests and population classes but were consistently highest for qPCR (87-99%) and lowest for histopathology (21-51%). Acknowledging that detection of M. mackini may be fitted to multiple diagnostic and management purposes, qPCR had the highest DSe while maintaining similar DSp to both conventional PCR and histopathology and thus is generally well-suited to most applications.

Mikrocytos mytilicoli n.sp. (Cercozoa, Mikrocytida, Mikrocytiidae) infecting the copepod Mytilicola intestinalis (Arthropoda, Cyclopoida, Mytilicolidae), a symbiont of Mytilus galloprovincialis in Galicia (NW Spain)

During a histopathological survey of Mytilus galloprovincialis in Galicia (NW Spain), microcells were observed infecting several organs of the symbiont copepod Mytilicola intestinalis. Positive results of PCR assay with specific primers for genus Mikrocytos and a clear signal of in situ hybridization with MACKINI-1 digoxigenin- labelled DNA probe (DIG-ISH) indicated a protozoan parasite of Mikrocytos genus. The ultrastructural study revealed intra and extracellular locations, polymorphic nuclei, intracellular round vesicles in the cytoplasm and absence of mitochondria. The present paper reports the characterization of the Mikrocytos sp. infecting M. intestinalis and proposes a novel species in the genus: Mikrocytos mytilicoli n. sp. A sequence of 18S-28S rDNA was obtained with 95.6% maximum identity (query cover 100%) with Mikrocytos mackini. Phylogenetic analysis showed that M. mytilicoli n. sp. and M. mackini share a common ancestor. However, comparison of the ITS1 rDNA region showed low similarity (75.8%) with M. mackini, which, combined with differences in ultrastructural details, host and geographic location, support the designation of a new species. This is the first description of a microcytid parasite of the genus Mikrocytos from a non-bivalve host.

Comparative Genomics Reveals a Significant Sequence Variability of Myticin Genes in Mytilus galloprovincialis

Myticins are cysteine-rich antimicrobial peptides highly expressed in hemocytes of Mytilus galloprovincialis. Along with other antimicrobial peptides (AMPs), myticins are potent effectors in the mussel immune response to pathogenic infections. As intertidal filter-feeders, mussels are constantly exposed to mutable environmental conditions, as well as to the presence of many pathogens, and myticins may be key players in the great ability of these organisms to withstand these conditions. These AMPs are known to be characterized by a remarkable sequence diversity, which was further explored in this work, thanks to the analysis of the recently released genome sequencing data from 16 specimens. Altogether, we collected 120 different sequence variants, evidencing the important impact of presence/absence variation and positive selection in shaping the repertoire of myticin genes of each individual. From a functional point of view, both the isoelectric point (pI) and the predicted charge of the mature peptide show unusually low values compared with other cysteine-rich AMPs, reinforcing previous observations that myticins may have accessory functions not directly linked with microbe killing. Finally, we report the presence of highly conserved regulatory elements in the promoter region of myticin genes, which might explain their strong hemocyte-specific expression.

Host Range of the Mikrocytid Parasite Paramikrocytos canceri in Decapod Crustaceans

Mikrocytids are a widespread but rather neglected group of parasites of aquatic invertebrates. One such parasite is Paramikrocytos canceri—discovered to infect the antennal gland of the juvenile edible crab, Cancer pagurus, taken from several intertidal sites across the United Kingdom. To determine if this parasite is also present in other species of decapod crustaceans, we surveyed crabs (n = 330) across two contrasting sites in Pembrokeshire (UK). Using a histopathological approach, P. canceri infection was confirmed in variable numbers of edible crabs from both survey sites, 7–44%. No measurable signs of infection were encountered in four other co-located species, including European shore crabs (Carcinus maenas), Montagu’s crabs (Xantho hydrophilus), velvet swimming crabs (Necora puber) and broad-clawed porcelain crabs (Porcellana platycheles). These data imply that P. canceri has a more limited host range than suggested by molecular diagnosis alone.

Baseline pathological data of the wedge clam Donax trunculus from the Tyrrhenian Sea (Mediterranean Basin)

In recent years, a collapse in Donax trunculus fishing yields has occurred in the Tyrrhenian Sea (Mediterranean Basin). There is little information available on the impact disease may have had on D. trunculus populations. For the first time, a pathological survey was performed on the natural beds of the bivalve on the Campania and Lazio coasts, western Italy. Detected pathogens and related diseases were analysed, and their prevalence and mean intensity values were calculated. Viral particles, Chlamydia-like organisms, ciliates, coccidians, microcells and trematodes were observed. An unknown ciliate was linked to severe inflammatory and necrotic lesions in the digestive gland. Metacercariae of the trematode Postmonorchis sp. were also strongly represented in almost all samples, reaching high levels of infection; however, none of the pathogens described required the World Organisation for Animal Health to be notified. Initial results indicated that further surveys related to environmental data are necessary in order to assess the relevance of these early observations in managing the declining D. trunculus population in the Tyrrhenian Sea.