Gene flow versus local adaptation in the northern acorn barnacle, Semibalanus balanoides: Insights from mitochondrial DNA variation

In reciprocal transplant experiments, Bertness and Gaines (1993) found that Semibalanus balanoides juveniles that had settled in an upper Narragansett Bay estuary survived better in that estuary that did juveniles from coasta l localities. The observed pattern of survivorship led to the claim that lo cal adaptation may result from a combination of limited gene flow between a nd strong selection within these habitats. Here we test the hypothesis that limited gene flow has led to habitat-specific population differentiation u sing sequence and restriction fragment length polymorphism analyses of the mitochondrial DNA D-loop region of S. balanoides. Samples were analyzed fro m replicated coastal and estuary localities in both Narragansett Bay, Rhode Island, and Damariscotta River, Maine. The patterns of F-ST indicate that gene flow between coast and estuary is extensive (Nm > 100) and is not lowe r in the estuary, with lower flushing rates (Narragansett Bay). Given the h igh estimate of genetic exchange, adaptations for unpredictable environment s seem more likely than local adaptation in this species because loci that respond to selection in one generation are essentially homogenized by the n ext seasons' settlement. Nevertheless, these estimates of neutral gene flow can help identify the strength of selection necessary for local adaptation to accumulate in Semibalanus.