Genetic population structure and breeding systems in arionid slugs (Mollusca: Pulmonata)

Abstract

Electrophoresis and biochemical staining were used to analyse genetic variation in enzymes in nine species of terrestrial slugs of the genus Arion from 58 localities in the British Isles. The species fall into three categories on the basis of the genetic structure of their populations and their breeding systems. Arion lusitanicus, A. hortensis, A. distinctus and A. owenii are highly polymorphic and heterozygous, and apparently reproduce predominantly, if not completely, by outcrossing. Arion circumscriptus, A. silvaticus, and A. intermedius consist of one or a few monogenic strains. Their monogenicity apparently results from an automictic mode of reproduction, most probably self-fertilization. Arion ater and A. subfuscus consist of both a polymorphic, heterozygous form and a monogenic strain, between which hybridization occurs. The monogenic strain and the polymorphic form of A. subfuscus occur in pure populations only in Ireland and England, respectively, but each has been introduced throughout the British Isles. In both A. ater and A. subfuscus, hybrid populations show a marked deficiency of heterozygotes as a result of self-fertilization. The genetic structures and breeding systems of populations of the species of Arion introduced to North America are similar to those of native populations in Britain.