Molecular diagnostics, field validation, and phylogenetic analysis of Quahog Parasite Unknown (QPX), a pathogen of the hard clam Mercenaria mercenaria

Diseases of Aquatic Organisms

K. S. Reece¹  N. A. Stokes¹  L. M. Ragone Calvo¹  E. M. Burreson¹ 

关键词: Hard clams;    Disease;    QPX;    Molecular techniques;    PCR;    Probe;   

DOI  :  10.3354/dao052233

学科分类：生物科学（综合）

来源: Inter-Research

【 摘 要 】

ABSTRACT: Quahog Parasite Unknown (QPX) is a protistan parasite that causes disease and mortality in the hard clam Mercenaria mercenaria. PCR primers and DNA oligonucleotide probes were designed and evaluated for sensitivity and specificity for theQPX organism specifically and for the phylum Labyrinthulomycota in general. The best performing QPX-specific primer pair amplified a 665 bp region of the QPX small-subunit ribosomal DNA (SSU rDNA) and detected as little as 1 fg cloned QPX SSU rDNA and 20fg QPX genomic DNA. The primers did not amplify DNA of uninfected hard clams M. mercenaria or of the thraustochytrids Schizochytrium aggregatum, Thraustochytrium aureum, and T. striatum. The general labyrinthulomycete primers,which were designed to offer broader specificity than the QPX primers, amplified a 435 bp region of SSU rDNA from QPX, and a 436 to 437 bp region of SSU rDNA from S. aggregatum, T. aureum, and T. striatum, but did not amplify that of theclam M. mercenaria. Field validation of the QPX-specific primer pair, through comparative sampling of 224 clams collected over a 16 mo period from a QPX endemic site in Virginia, USA, indicated that the PCR assay is equivalent to histologicaldiagnosis if initially negative PCR products are reamplified. Oligonucleotide DNA probes specific for QPX and the phylum Labyrinthulomycota were evaluated for in situ hybridization assays of cell smears or paraffin-embedded tissues. Two DNA probesfor QPX offered limited sensitivity when used independently; however, when used together as a probe cocktail, sensitivity was greatly enhanced. The probe cocktail hybridized to putative QPX organisms in tissues of hard clams collected from Virginia, NewJersey, Massachusetts and Canada, suggesting that the QPX organisms in these areas are either very closely related or the same species. The QPX probe cocktail did not hybridize with clam tissue or with the thraustochytrids S. aggregatum, T.aureum, and T. striatum. The labyrinthulomycete DNA probe hybridized with QPX and the 3 thraustochytrids, with no background hybridization to clam tissue. SSU rDNA sequences were obtained for the putative QPX organisms from geographicallydistinct sites. Phylogenetic analyses based on the QPX and Labyrinthulomycota sequences confirmed earlier reports that QPX is a member of this phylum, but could not definitively demonstrate that all of the QPX organisms were the same species.

【 授权许可】

Unknown