Host-parasite interactions among broadly distributed populations of the eastern oyster Crassostrea virginica and the protozoan Perkinsus marinas

The protozoan oyster parasite Perkinsus marinus causes extensive mortality in eastern oyster (Crassostrea virginica) populations during summer and fall across much of the oyster's distribution. Despite more than 40 yr of research on this particular parasite, no study has unequivocally demonstrated a genetic basis for host resistance to P. marinus nor has it been determined whether or not there are races of Fl marinus that vary in virulence. Using recently developed techniques to culture P. marinus in vitro, we examined the resistance of 4 genetically distinct oyster populations that had different natural histories of exposure to P. marinus and the virulence of 4 geographically distinct isolates of P. marinus. Offspring were produced from each oyster population and reared in a common environment, then exposed to each isolate of P. marinus. Oysters showed levels of resistance roughly corresponding to the duration parental populations had been exposed to P. marinus (Texas > Virginia > New Jersey = Maine), indicating that those populations which have been exposed to P. marinus for more than 40 yr have developed some resistance. Parasites isolated from the Atlantic coast (Mobjack Bay, VA and Delaware Bay, NJ, USA) produced heavier infections than those isolated from the Gulf of Mexico coast (Barataria Bay, LA and South Bay Laguna Madre, TX, USA), indicating that Atlantic isolates were more virulent than Gulf isolates. These data indicate that resistant races of the eastern oyster exist, and imply the existence of virulent parasite races. No statistically significant interaction was detected between oyster populations and parasite isolates. Relative infection intensities among oyster populations remained more or less constant across parasite isolates and vice versa. The lack of a significant interaction between host populations and parasite isolates indicated that mechanisms of resistance and virulence were general, not race-specific.