Local adaptation of a marine invertebrate with a high dispersal potential: evidence from a reciprocal transplant experiment of the eastern oyster Crassostrea virginica

We examined the role of local adaptation in structuring the stable genetic step-cline of the eastern oyster Crassostrea virginica along an environmental gradient in the lagoon system of eastern Florida, USA. Reciprocally transplanted progeny, produced by a 10 x 10 genetic cross of wild brood stock from northern and southern genetic lineages yielded significant evidence of local adaptation (interaction of genes and environment) in variables related to fitness, including survival, wet meat weight, and reproductive maturation. The strength of local adaptation was asymmetric, with greater effects on the northern compared to the southern genetic lineage. To a lesser extent, we found evidence of both the role of environment (in particular, adverse effects on both genetic crosses in the southern region), and the role of genetic differences between the 2 crosses independent of environment, with higher initial growth of the southern genetic lineage and higher condition of the northern lineage. These differences suggest that maintenance of the genetic step-cline involves natural selection. We discuss the potential role of temperature and phytoplankton community composition between the northern and southern regions. Our study is the first to determine the genetic basis for fitness-related phenotypes, and to relate this to local adaptation of the eastern oyster. Understanding the role of the environment in structuring the eastern oyster throughout its range is critical for effective management, and the results of this study also suggest that small environmental changes may have significant effects on conservation of the eastern oyster, particularly in the northern genetic lineage.