DISEASE-RESISTANCE AND IMPROVED PERFORMANCE FOR Genetically Improved and CROSS-BRED EASTERN OYSTERS Crassostrea virginica. Results from a decade of field trials in New England.

Over five decades of effort to develop genetically improved lines of eastern oysters (Crassostrea virginica) has resulted in the production of several lines that demonstrate enhanced growth and survival under specific disease pressures. In two separate common garden field trials, we investigated the relative performance of the Rutgers University NEH (diploid and triploid), University of Maine UMFS, and Clinton lines, and interline hybrids when grown at sites from New Jersey to Maine. Line performance in both trials was highly site-specific; while there were significant differences in growth between sites, there was only subtle growth variation among lines. In contrast, line performance was heavily dependent on survival. Survival was, in turn, heavily dependent on site-specific disease pressure due to MSX, Dermo, or Roseovarius Oyster Disease and line-specific disease resistance. Hybrid lines displayed very little improvement in shell height or final wet weight relative to parental lines. However, the hybrid lines inherited varying degrees of disease resistance that was dependent on the particular disease pressure experienced at individual sites. Not surprisingly, the variation final yield was heavily dependent on line-specific survival at sites were disease pressure was high and more strongly associated with variation in growth at sites where disease pressure was light. The differences in line performance are indicative of a high degree of genotype by environment interaction for survival, growth, and yield among the parental and hybrid lines. The prevalence of such interactions suggest that regional breeding programs for eastern oysters should take advantage of the superior characteristics of the extant lines, but will need to consider alternatives to mass selection in order to efficiently breed lines of oysters that benefit growers and hatcheries throughout the region.