Can oysters Crassostrea virginica develop resistance to dermo disease in the field: The impediment posed by climate cycles
Populations of eastern oysters, Crassostrea virginica, are commonly limited by mortality from dermo disease. Little development of resistance to Perkinsus marinus, the dermo pathogen, has occurred, despite the high mortality rates and frequency of epizootics. Can the tendency of the parasite to exhibit cyclic epizootics limit the oyster's response to the disease despite the presence of alleles apparently conferring disease resistance? We utilize a gene-based population dynamics model to simulate the development of disease resistance in Crassostrea virginica populations exposed to cyclic mortality encompassing periodicities expected of dermo disease over the geographic range at which epizootics have been observed. Cyclic disease reduces the incremental rate of development of disease resistance profoundly, primarily as a consequence of a reduction in the time-integrated population mortality rate, which will be about half the cycle's apogean rate. Cyclicity enhances host survival for more susceptible genotypes at cycle nadir. Moreover, alleles conferring disease resistance typically are rare in the naïve population. Cyclicity permits these rare alleles to drift and most often, that drift is towards lower frequencies because fewer animals carrying these alleles predestines a lower probability of their successful dissemination during sweepstakes reproduction at cycle nadir. Variations in population dynamics, such as differences in abundance, fecundity at size, and in the number of individuals successfully producing recruits varied the outcome little. The large number of loci contributing to disease resistance, the cyclic nature of the exposure relieving the population in predictable time units from selection pressure, and the tendency for conditions that might enhance development of disease resistance such as rapid growth to be counterbalanced by multiple yearly spawnings, hamper the rapid development of disease resistance. Unfortunately, epizootic mortality rates at cycle apogee, twice that observed at cycle nadir or prior to onset of disease, are consequential from the standpoint of population sustainability, but much less consequential for driving selection towards disease resistance. The periodicity of dermo epizootics may doom oyster populations to an extended period of low abundance, during which disease resistance slowly improves; bit by bit limiting the depredations of the disease.
Powell, Eric N., John M. Klinck, Ximing Guo, Eileen E. Hofmann, Susan E. Ford, and David Bushek. 2012. "Can oysters Crassostrea virginica develop resistance to dermo disease in the field: The impediment posed by climate cycles." Journal of Marine Research 70, (2). https://elischolar.library.yale.edu/journal_of_marine_research/344