Intron-targeted PCR: a new approach to survey neutral DNA polymorphism in bivalve populations

PCR (polymerase chain reaction) amplification of non-coding introns in phylogenetically widespread genes, using DNA primers based on the conserved exon sequences, provides a widely applicable strategy for finding DNA polymorphisms in eukaryotic genomes. Polymorphisms in introns provide a new source of potentially neutral genetic markers for use in population biology. Here we use this approach to design PCR primers for an intron of calmodulin genes. We show that there are at least two calmodulin genes in mussels of the Mytilus edulis species complex, and using gene- and species-specific primers we resolve two alleles at a calmodulin intron locus. Population surveys using PCR of adult mussel DNA reveal that genotype frequencies at most sites surveyed in England, Scotland and Italy, conform to Hardy-Weinberg equilibrium, suggesting that this is a novel neutral genetic marker. The data also provide preliminary evidence for restricted gene flow between mussel populations on the west and northeast coasts of Britain, and for local effects around the Thames estuary.