Small subunit ribosomal RNA gene sequence of the parasitic protozoan Haplosporidium nelsoni provides a molecular probe for the oyster MSX disease

Elucidating the life cycle of Marteilia sydneyi, the aetiological agent of QX disease in the Sydney rock oyster (Saccostrea glomerata)

Marteilia sydneyi (Phylum Paramyxea, Class Marteiliidea, Order Marteiliida) (the causative agent of QX disease) is recognised as the most severe parasite to infect Saccostrea glomerata, the Sydney rock oyster, on the east coast of Australia. Despite its potential impact on industry (>95% mortality), research towards lessening these effects has been hindered by the lack of an experimental laboratory model of infection as a consequence of our incomplete understanding of the life cycle of this parasite. Here, we explored the presence of this parasite in hosts other than a bivalve mollusc from two study sites on the Hawkesbury River, New South Wales, Australia. We employed PCR-based in situ hybridisation and sequence analysis of a portion of the first internal transcribed spacer of rDNA in an attempt to detect M. sydneyi DNA in 21 species of polychaete worm. Marteilia DNA was detected in 6% of 1247 samples examined by PCR; the analysis of all amplicons defined one distinct sequence type for first internal transcribed spacer, representing M. sydneyi. Of the polychaete operational taxonomic units test-positive in PCR, we examined 116 samples via in situ hybridisation DNA probe staining and identified M. sydneyi DNA in the epithelium of the intestine of two specimens of Nephtys australiensis. Two differing morphological forms were identified: a 'primordial' cell that contained a well-defined nucleus but had little differentiation in the cytoplasm, and a 'plasmodial' cell that showed an apparent syncytial structure. This finding represents the first known record of the identification of M. sydneyi being parasitic in an organism other than an oyster, and only the third record of any species of Marteilia identified from non-molluscan hosts. Future work aims at determining if N. australiensis and S. glomerata are the only hosts in the life cycle of this paramyxean, and the development of experimental models to aid the production of QX disease-resistant oysters.

Quantitative PCR assay to determine prevalence and intensity of MSX (Haplosporidium nelsoni) in North Carolina and Rhode Island oysters Crassostrea virginica

The continuing challenges to the management of both wild and cultured eastern oyster Crassostrea virginica populations resulting from protozoan parasites has stimulated interest in the development of molecular assays for their detection and quantification. For Haplosporidium nelsoni, the causative agent of multinucleated sphere unknown (MSX) disease, diagnostic evaluations depend extensively on traditional but laborious histological approaches and more recently on rapid and sensitive (but not quantitative) end-point polymerase chain reaction (PCR) assays. Here, we describe the development and application of a quantitative PCR (qPCR) assay for H. nelsoni using an Applied Biosystems TaqMan® assay designed with minor groove binder (MGB) probes. The assay was highly sensitive, detecting as few as 20 copies of cloned target DNA. Histologically evaluated parasite density was significantly correlated with the quantification cycle (Cq), regardless of whether quantification was categorical (r2 = 0.696, p < 0.0001) or quantitative (r2 = 0.797, p < 0.0001). Application in field studies conducted in North Carolina, USA (7 locations), revealed widespread occurrence of the parasite with moderate to high intensities noted in some locations. In Rhode Island, USA, application of the assay on oysters from 2 locations resulted in no positives.

Detection of Bonamia ostreae based on small subunit ribosomal probe

Bonamia ostreae is a protozoan parasite of the flat oyster, Ostrea edulis, which has caused significant loss of oysters in Europe over the last decade. B. ostreae was purified from infected flat oysters and DNA was extracted. The nearly complete small subunit rDNA gene of B. ostreae was amplified using universal oligonucleotides and the PCR product was cloned and sequenced. BLAST research with this sequence revealed similarities to Haplosporidium nelsoni, Haplosporidium costale, and Minchinia teredinis. These data suggest that B. ostreae may be included in the genus Haplosporidium. Specific B. ostreae primers were designed for labeling, by PCR, a probe. This probe was successfully used by in situ hybridization to detect B. ostreae in infected fiat oysters, thus confirming the accuracy of this SSU rDNA sequence. The probe lead also to the detection of Bonamia sp. in infected Tiostrea chilensis and H. nelsoni in infected Crassostrea virginica but not Mikrocytos mackini infected Crassostrea gigas. These primers were also used to detect B. ostreae from infected oyster tissues by PCR. This B. ostreae SSU rDNA gene sequence provides genetic information as a first step toward elucidation of the taxonomic boundaries among the microcell organisms. Moreover, the development of DNA detection assays will be valuable specific diagnostic tools.

Sponge phylogeny, animal monophyly, and the origin of the nervous system: 18S rRNA evidence

We sequenced 18S rRNA genes of a calcareous sponge, Clathrina cerebrum, a demosponge, Axinella polypoides, and a zoanthid cnidarian, Parazoanthus axinellae. Our phylogenetic analysis supports the monophyly of kingdom Animalia and confirms that choanoflagellate protozoans are their closest relatives. Sponges as a whole are monophyletic, but possibly paraphyletic; demosponges and hexactinellids form a monophyletic group of silicious sponges. Our phylogenetic trees support a monophyletic origin of the nervous system in the immediate common ancestor of Cnidaria and Ctenophora. They weakly suggest that animals with a nervous system may be more closely related to calcareous sponges than to silicious sponges; the nervous system might have originated in an early calcareous sponge. Our trees confirm that Myxozoa and Placozoa are animals that arose by secondary loss of the nervous system, but suggest that Myxozoa may be sisters of, rather than derived from, Bilateria. Kingdom Animalia is divided into four subkingdoms: Radiata (Porifera, Cnidaria, Placozoa, Ctenophora), Myxozoa, Mesozoa, and Bilateria. The 18S rRNA genes of Myxozoa evolved over twice as fast as in Radiata. Comparison with the fossil record reveals a brief 10-fold (or greater) acceleration in the rate of rRNA evolution in early Bilateria followed by normal low rates for about 500 million years.

A sensitive and specific DNA probe for the oyster pathogen Haplosporidium nelsoni

Haplosporidium nelsoni is a pathogen of the eastern oyster, Crassostrea virginica, along the middle Atlantic coast of the U.S. Genomic DNA was extracted from H. nelsoni plasmodia and small subunit (SSU) rDNA was amplified by PCR, cloned and sequenced. The sequence of H. nelsoni SSU rDNA was aligned with that of another haplosporidian, Minchinia teredinis, and with SSU rDNA data of C. virginica and various protists in GenBank. A 21-base oligonucleotide unique to H. nelsoni, designated MSX1347, was commercially synthesized and tested for sensitivity and specificity. In dot blot hybridizations the probe detected 100 pg of cloned H. nelsoni rDNA and the presence of H. nelsoni in 1 microgram of genomic DNA from an infected oyster. It did not hybridize with 1 microgram of genomic DNA from uninfected C. virginica or with cloned SSU rDNA of M. teredinis. The probe was further tested for specificity with in situ hybridizations on AFA-fixed, paraffin-embedded tissue sections. The probe hybridized well with H. nelsoni plasmodia and immature spores, but poorly with mature spores. The probe did not hybridize with oyster tissue, with other common oyster parasites such as P. marinus or Nematopsis sp., or with the haplosporidians Haplosporidium louisiana from mud crabs (Panopeus spp.), Haplosporidium costale from C. virginica or M. teredinis from shipworms (Teredo spp.).