Hilbish 9708074 The PIs have been conducting a detailed analysis of a hybrid zone between two species of marine mussels, Mytilus edulis and M. galloprovincialis that are sympatric in Europe. The PIs' have shown that 1) strong directional selection occurs within hybrid populations that eliminates Mytilus edulis and M. edulis-like hybrids, 2) directional selection is offset by immigration from pure populations of M. edulis, 3) the balance between dispersal and selection is presently stable, and 4) hybridization has been an important feature in the evolutionary history of these species. These studies focus future research in a new direction: the primary issue in understanding the dynamics of this hybrid zone is to determine how pure populations of M. edulis are maintained. The maintenance of pure populations of Mytilus edulis is an essential element of the dynamic equilibrium between dispersal and directional selection that maintains and shapes this hybrid zone. The PI will propose to test the hypothesis that M. edulis has a superior physiological capacity to cope with silt in its diet and that pure populations of this species are selectively maintained in estuarine environments where silt concentrations are high. This hypothesis was suggested by the observation that pure populations of M. edulis are associated with estuaries that have high silt loads and preliminary results from separate studies that indicate that M. edulis may have a much greater absorption efficiencies when exposed to diet containing high concentrations of silt. The PI will also propose a combination of laboratory and field experiments to test this hypothesis. Laboratory experiments will test for the proposed physiological differences between M. edulis and M. galloprovincialis while the field experiments are necessary to demonstrate that these differ ences have significant consequences in nature.
