A semiquantitative PCR assay for assessing Perkinsus marinus infections in the eastern oyster, Crassostrea virginica.

A 3.2-kb fragment of Perkinsus marinus DNA was cloned and sequenced. A noncoding domain was identified and targeted for the development of a semiquantitative polymerase chain reaction (PCR) assay for the presence of P. marinus in eastern oyster tissues. The assay involves extracting total DNA from oyster hemolymph and using 1 microgram of that DNA as template in a stringent PCR amplification with oligonucleotide primers that are specific for the P. marinus 3.2-kb fragment. With this assay, we can detect 10 pg of total P. marinus DNA per 1 microgram of oyster hemocyte DNA with ethidium bromide (EtBr) staining of agarose gels, 100 fg total P. marinus DNA with Southern blot autoradiography, and 10 fg of total P. marinus DNA with dot-blot hybridizations. We have used the sensitivity of the PCR assay to develop a method for estimating the level of P. marinus DNA in oyster hemolymph and have successfully applied this technique to gill tissues. Our semiquantitative assay uses a dilution series to essentially titrate the point at which a P. marinus DNA target is no longer amplified in a sample. We refer to this technique as "dilution endpoint" PCR. Using hemocytes obtained by withdrawing a 1-ml sample of hemolymph, this assay provides a nondestructive methodology for rapidly screening large numbers of adult oysters for the presence and quantification of P. marinus infection levels. This technique is applicable to other tissues (gills) and could potentially be applied to DNA extracts of whole larvae or spat.