A chromosomal inversion polymorphism in Scandinavian populations of the seaweed fly, Coelopa frigida.

Abstract

British populations of the seaweed fly Coelopa frigida are polymorphic for the large α/β inversion on chromosome I(Butlin etal., Heredity 48: 45–55). Flies from Norway, Sweden and Denmark are shown to possess the same inversion polymorphism, and to exhibit similar though not identical polymorphisms at two enzyme-determining loci associated with this inversion. There is a cline in inversion frequencies from the North Sea, through the Skagerrak and Kattegat, into the Baltic Sea, This cline correlates with salinities, the algal composition of seaweed beds in which Coelopa breeds, and with the presence of the sibling species C. pilipes. It is suggested that heterokaryotypic advantage is the major selective force maintaining this polymorphism, and that genotype-related differences in generation times, and the longevity of seaweed beds, may constitute lesser forces acting on the inversion frequencies.