Molecular characterization of QPX (Quahog Parasite Unknown), a pathogen ofMercenaria mercenaria

The taxonomic status of Quahog Parasite Unknown (QPX), a protist causing di sease and high mortality in hardclams (Mercenaria mercenaria) from Canada t o Virginia, has not been firmly established. The comparison of QPX 18s rDNA sequences with small-subunit rRNA (SSU RNA) sequences available in the pub lic domain places this organism firmly in the phylum Labyrinthulomycota. Wi th the limited SSU data currently available for the order Labyrinthulida, p lacement within the family Thraustochytriidae is somewhat more tenuous. Mor phological examination also suggests placement in the Labyrinthulomycota. T he absence of sageno-genetosomes and ectoplasmic nets suggests that QPX is a more primitive member of the phylum. The placement of QPX SSU sequence ba sal to all available Labyrinthulomycota SSU sequences other than that of La byrinthuloides minuta tends to support this assessment.