Protistan parasite QPX of hard-shell clam Mercenaria mercenaria is a member of Labyrinthulomycota

Abstract

Biomass of the protistan parasite QPX (quahaug parasite X) of hard-shell clam Mercenaria mercenaria was enriched from in vitro culture. The nuclear gene encoding the 18S RNA of the small-subunit ribosomal (ssu-rDNA) was recovered using the polymerase chain reaction (PCR) and sequenced. Phylogenetic analysis clearly showed that QPX is a member of phylum Labyrinthulomycota, within which it appears as a specific relative of Thraustochytrium pachydermum. These results confirm the provisional assignment of QPX to the Labyrinthulomycota made previously on the basis of morphological and ultrastructural characters found in some, but not all, geographic isolates.KEY WORDS: QPX, quahaug parasite X · Mercenaria mercenaria · Hard-shell clam · Small-subunit ribosomal DNA · Labyrinthulomycota · Thraustochytrium pachydermum Resale or republication not permitted without written consent of the publisherDis Aquat Org 42: [185][186][187][188][189][190] 2000 trids are found in similar environments, although more typically in association with decomposing plant material or organic detritus than on living plants or algae. A few are of economic importance: Labyrinthula macrocystis has been implicated in the decline of shellfish fisheries along the Atlantic coasts of North America and Europe during the 1930s, due to its (perhaps secondary) role in the 'wasting disease' of the eelgrass Zostera marina (Pokorny 1967), while Labyrinthuloides haliotidis is a pathogen of the abalones Haliotis kamtschatkana and H. rufescens, causing up to 100% mortality in juveniles (Bower 1987a,b, Bower et al.1994.Using the culture method developed by Kleinschuster et al. (1998), we enriched biomass of QPX from infected Mercenaria mercenaria isolated from New Brunswick, and have used the polymerase chain reaction (PCR) to amplify the nuclear gene encoding the small-subunit ribosomal RNA (ssu-rDNA) from this enriched fraction. METHODS AND MATERIALSBiological samples. Infected Mercenaria mercenaria ranging in size from 40 to 87 mm were collected from Sam Orr Pond, St Andrews, New Brunswick, on 16 July 1998. Within 24 h of removal from the water, a dorso-ventral cross-section of tissues was removed from each clam and preserved in 1G4F (Howard & Smith 1983) for histological examination. At the same time, approximately 1 mm 3 samples of gill, mantle, digestive gland, gonad and foot were collected for culture in Minimal Essential Medium (MEM; Sigma M0644).Separation of parasites from tissues. The methods we employed are slight modifications of those used by Bower (1987c) for culture of Labyrinthuloides haliotidis in juvenile abalone (Haliotis rufescens and H. kamtschatkana). The combined tissues were macerated in a 10 ml Potter Elvehjem tissue grinder with 1 ml sterile artificial seawater (SAS). The homogenate was transferred to a 15 ml capped centrifuge tube (Fisher) with an additional 2 ml SAS containing antibiotics (10 000 U penicillin, 10 000 µg streptomycin and 25 µg amphotericin B ml -1 SAS). The homogenate was centrifuged at 1750 × g in a bencht...

Isolation and Characterization of a Novel Single-Stranded RNA Virus Infectious to a Marine Fungoid Protist, Schizochytrium sp. (Thraustochytriaceae, Labyrinthulea)

“…In addition, thraustochytrids are known to produce large amounts of polyunsaturated fatty acids (PUFA), such as docosahexaenoic acid and docosapentaenoic acid (44), which are considered important food resources for higher organisms in marine systems (30,34,50). Furthermore, some species of thraustochytrids are known to be pathogens of mollusks, such as octopuses and bivalves (1,46,49). Because of these distinctive features, the ecological significance of thraustochytrids in the coastal ecosystems has been highlighted (50,51).…”

Friend or foe: the association of Labyrinthulomycetes with the Caribbean sea fan Gorgonia ventalina

“…Preliminary histological evaluation of these multi-focal purple lesions re vealed the presence of a stramenophile (ovoid parasites distributed within a mucus net; Petes et al 2003) similar in appearance to the quahog parasite unknown (QPX) parasite of clams (Ragone Calvo et al 1998, Ragan et al 2000 and Labyrinthula zosterae that infects marine grasses (Durako & Kuss 1994, Ralph & Short 2002. Laby rin thulomycetes (Labyrinthulomycota) produce a very distinctive, basophilic-staining ectoplasmic network matrix with continuity from the network matrix to the cell cytoplasm (Porter 1990).…”

“…Laby rin thulomycetes (Labyrinthulomycota) produce a very distinctive, basophilic-staining ectoplasmic network matrix with continuity from the network matrix to the cell cytoplasm (Porter 1990). Although Laby rinthulomycetes are common in estuarine and marine habitats, only a few of the identified Labyrinthulomycetes appear pathogenic and have been reported to be the primary infectious agent implicated in damaging epizootics, especially in the sea grasses Zostera marina and Thalassia testudinum (Muehlstein et al 1991, Durako & Kuss 1994, Ralph & Short 2002, octopus (Polglase 1980), nudibranch (McLean & Porter 1982), abalone (Bower 1987), and the quahog clam Mercenaria mercenaria (Ragone Calvo et al 1998, Ragan et al 2000, Lyons et al 2007). …”

Influence of Host Genetic Origin and Geographic Location on QPX Disease in Northern Quahogs (=hard Clams), Mercenaria Mercenaria

“…The epizootiology of QPX is poorly understood. Based on morphological and molecular analyses, QPX is a protist that has been tentatively classified as a member of the Thraustochytriidae family within the phylum Labyrinthulomycota (Whyte et al 1994, Maas et al 1999, Ragan et al 2000, Stokes et al 2002. Thraustochytrids are widely distributed in marine and estuarine environments and typically are associated with detrital sediments, benthic algae, and marine plants.…”