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Strains of oysters Crassostrea virginica resistant to mortality caused by the parasite Haplosporidium nelsoni (MSX) were developed and tested through 6 generations. In addition, strains in each generation were followed for up to 6 yr of continuous exposure to the parasite in nature. Selected strains responded to challenge by the parasite with gradually improved survival in successive generations. They were slower to develop patent infections than were unselected groups and were able to delay mortality after infections did develop, but under repeated exposure most oysters eventually died with H. nelsoni parasitism. Many selected strains, however, reached market size before significant mortalities occurred. The data suggest that resistance to H. nelsoni mortality is under the influence of many genes. No clear defense mechanism has been described and we hypothesize that resistance to H. nelsoni may, in part, involve a physiological state in which selected oysters temporarily fail to provide a suitable habitat for the parasite. Temporary insusceptibility would, in this view, be followed by an increased ability to tolerate the parasite when conditions for its development are present. Selection would then favor individuals that are able to prolong periods of insusceptibility and/or to carry out basic life processes while parasitized.
Ford et al. (Wed,) studied this question.