(Haplosporidia) in pearl oysters Pinctada maxima (Jameson, 1901)

Haplosporosomes, sporoplasmosomes and their putative taxonomic relationships in rhizarians and myxozoans

This paper reviews current knowledge of the structure, genesis, cytochemistry and putative functions of the haplosporosomes of haplosporidians (Urosporidium, Haplosporidium, Bonamia, Minchinia) and paramyxids (Paramyxa, Paramyxoides, Marteilia, Marteilioides, Paramarteilia), and the sporoplasmosomes of myxozoans (Myxozoa - Malacosporea, Myxosporea). In all 3 groups, these bodies occur in plasmodial trophic stages, disappear at the onset of sporogony, and reappear in the spore. Some haplosporidian haplosporosomes lack the internal membrane regarded as characteristic of these bodies and that phylum. Haplosporidian haplosporogenesis is through the Golgi (spherulosome in the spore), either to form haplosporosomes at the trans-Golgi network, or for the Golgi to produce formative bodies from which membranous vesicles bud, thus acquiring the external membrane. The former method also forms sporoplasmosomes in malacosporeans, while the latter is the common method of haplosporogenesis in paramyxids. Sporoplasmogenesis in myxosporeans is largely unknown. The haplosporosomes of Haplosporidium nelsoni and sporoplasmosomes of malacosporeans are similar in arraying themselves beneath the plasmodial plasma membrane with their internal membranes pointing to the exterior, possibly to secrete their contents to lyse host cells or repel haemocytes. It is concluded that these bodies are probably multifunctional within and between groups, their internal membranes separating different functional compartments, and their origin may be from common ancestors in the Neoproterozoic.

Haplosporidium raabein. sp. (Haplosporidia): a parasite of zebra mussels,Dreissena polymorpha(Pallas, 1771)

Extensive connective tissue lysis is a common outcome of haplosporidian infection. Although such infections in marine invertebrates are well documented, they are relatively rarely observed in freshwater invertebrates. Herein, we report a field study using a comprehensive series of methodologies (histology, dissection, electron microscopy, gene sequence analysis, and molecular phylogenetics) to investigate the morphology, taxonomy, systematics, geographical distribution, pathogenicity, and seasonal and annual prevalence of a haplosporidian observed in zebra mussels,Dreissena polymorpha. Based on its genetic sequence, morphology, and host, we describeHaplosporidium raabein. sp. fromD. polymorpha– the first haplosporidian species from a freshwater bivalve.Haplosporidium raabeiis rare as we observed it in histological sections in only 0·7% of the zebra mussels collected from 43 water bodies across 11 European countries and in none that were collected from 10 water bodies in the United States. In contrast to its low prevalences, disease intensities were quite high with 79·5% of infections advanced to sporogenesis.

Detection of Minchinia occulta in samples of pearl oysters Pinctada maxima infected by Haplosporidium hinei

OBJECTIVE

To determine if juvenile pearl oysters (Pinctada maxima) infected with Haplosporidium hinei are also infected with another haplosporidian parasite, Minchinia occulta.

DESIGN

Archived samples of pearl oysters infected with H. hinei were examined using polymerase chain reaction (PCR) assays and in situ hybridisation (ISH) to analyse and identify haplosporidians. A 144-bp and 220-bp region of Minchinia DNA were targeted by PCR and amplified DNA from formalin-fixed H. hinei-infected pearl oyster samples was sequenced. A 25-bp oligonucleotide probe targeting a variable section of the parasite's small subunit rRNA gene was used in ISH.

RESULTS

The results of DNA-based diagnostic assays supported each other. The sequences obtained by PCR were found to be almost identical to M. occulta from rock oysters and the ISH assay demonstrated infection with M. occulta in affected pearl oysters. ISH indicated a prevalence of infection of 26.7% in one of the previous outbreaks.

CONCLUSION

Pearl oyster spat are susceptible to infection by a Minchinia parasite, most likely M. occulta, which was recently identified in rock oysters within the pearl-producing zones of Western Australia and is associated with mortalities of up to 80% in this species. The occurrence of haplosporidian co-infections in pearl oysters suggests the immunocompetence of juvenile oysters may be an important factor in preventing infection and therefore preventing mortalities such as those occurring in the recent outbreaks of pearl oyster oedema disease.

Spore ornamentation of Minchinia occulta n. sp. (Haplosporidia) in rock oysters Saccostrea cuccullata (Born, 1778)

A Minchinia sp. (Haplosporidia: Haplosporidiidae) parasite was identified infecting rock oysters and morphologically described by Hine and Thorne (2002) using light microscopy and transmission electron microscopy (TEM). The parasite was associated with up to 80% mortality in the host species and it is suspected that the parasite would be a major impediment to the development of a tropical rock oyster aquaculture industry in northern Western Australia. However, attempts to identify the parasite following the development of a specific probe for Haplosporidium nelsoni were unsuccessful. The SSU region of the parasite's rRNA gene was later characterized in our laboratory and an in situ hybridization assay for the parasite was developed. This study names the parasite as Minchinia occulta n sp. and morphologically describes the parasite using histology, scanning electron microscopy and transmission electron microscopy. The non-spore stages were unusual in that they consisted primarily of uninucleate stages reminiscent of Bonamia spp. The parasite's spores were ovoid to circular shaped and measured 4·5 μm–5·0 μm×3·5–4·1 μm in size. The nucleus of the sporoplasm measured 1·5–2·3 μm and was centrally located. The spores were covered in a branching network of microtubule-like structures that may degrade as the spore matures.