Gene flow versus local adaptation in the Northern acorn barnacle Semibalanus balanoides: insights from mtDNA control region polymorphisms

Alice F. Brown, Lisa M. Kann and David M. Rand

Evolution, in press.

Marine organisms with planktonic larval dispersal are generally believed to show little population structure due to extensive effective gene flow. However, population structure could arise if habitat-specific selection is sufficiently strong to overcome the homogenizing effects of gene flow. Reciprocal transplant experiments have shown survivorship differences among transplanted Semibalanus balanoides juveniles between coastal and estuary localities (Bertness and Gaines 1993). The observed pattern of survivorship lead to the claim that local adaptation may result from a combination of limited gene flow between, and strong selection within, these habitats. Using variable sequences from the non-coding control region of the mitochondrial DNA of S. balanoides, we test the hypothesis that limited gene flow has led to habitat-specific population differentiation. Samples were analyzed from replicated coastal and estuary localities in both Narragansett Bay, Rhode Island and Damariscotta River, Maine. There is some phylogenetic structure to the sequences analyzed, but there is no correlation of mtDNA haplotype with habitat (coast vs. estuary), nor do the data reject neutrality (Tajima's D = 0.29, P > 0.1). The patterns of Fst indicate that gene flow between coast and estuary is extensive (Nm > 100) and is not lower in the estuary with lower flushing rates (Narragansett Bay). While habitat-specific selection seems likely in the acorn barnacle, this is probably ephemeral since any loci that respond to selection in one generation are essentially homogenized by the next seasons¹ settlement. Given the high estimate of genetic exchange, adaptations for unpredictable environments seem more likely than local adaptation in New England samples of this species. These estimates of neutral gene flow can help identify the strength of selection necessary for local adaptation to accumulate in Semibalanus and point to either strong selection at specific candidate genes in New England samples, or a broader set of loci between ocean basin where gene flow may be restricted