Enzyme protein polymorphism in the slug Arion ater

SUMMARY Populations of the slug Arion ater rufus sampled from several localities in England and the Netherlands are polymorphic for different electro-phoretic variants of digestive-gland esterase and salivary-gland tetra-zolium oxidase. Samples of Arion ater ater , which is endemic to the British Isles, were found to be monomorphic for these enzymes. Localized populations of A. a. rufus in Britain could have originated from specimens imported accidentally from continental Europe. There is evidence for gene flow between overlapping populations of the two subspecies. In one of the English populations of A. a. rufus , studied in detail, polymorphism for esterase and tetrazolium oxidase is shown to be balanced. Heterosis may be an important factor contributing to the maintenance of balanced polymorphism for tetrazolium oxidase in this population.