Variation in mitochondrial DNA in a cline of allele frequencies and shell phenotype in the dog-whelkNucella lapillus(L.)

Genetic constitution in the intertidal gastropodNucella lapillus(L.) influences shell shape, growth rate and physiology. Clinal variation in these traits along a 5 km stretch of coastline in south Devon can be related to environmental variation in temperature and desiccation stress. We have examined mtDNA variation along this shore to investigate whether the cline represents primary or secondary contact. Two distinct mtDNA haplotypes were found which exhibit coincident step clines with karyotypic, allozymic and phenotypic variation and covary with the environmental pressures of temperature and desiccation. These results are interpreted in the context of the wider scale distribution of genetic and phenotypic variation inN. lapillus. It is suggested that the shore studied may represent one of a number of regions of secondary contact within a mosaic hybrid zone inN. lapillus, where coadapted phenotypic variation correlates with habitat and the position of the clines represents an environmental transition.