SELECTIVE GRAZING OF HAIRLESS SILENE DIOICA PLANTS BY LAND GASTROPODS

Silene dioica (Caryophyllaceae) has evolved different morphotypes that vary in hairiness in different habitats in the Swedish mountains. A h airy form, var. serpertinicola and a hairless (glabrous) form, var, sm ithii grow on serpentine, i.e. a dry soil rich in heavy metals. Var. l apponica is a densely hairy morphotype on subalpine meadows. Crosses b etween these varieties showed that glabrousness has a simple Mendelian inheritance and that glabrousness is determined by a recessive allele . The crosses further suggested that the recessive allele has been pre sent in the original population that invaded the mountain area. Glabro us plants have, however, never been found on subalpine meadows, even t hough an earlier study has shown that gene flow is relatively common a mong serpentine and nearby meadow populations. In addition to the gene tic background of glabrousness, we studied the grazing on glabrous smi thii and hairy serpentinicola and Iapponica plants by the slugs Arion fasciatus and Arion subfuscus and the snail Arianta arbustorum in gree nhouse and in nature. Arion slugs, abundant on subalpine meadows, pref erred glabrous plants and, in particular, A. fasciatus consumed high a mounts of S. dioica. Arianta snails, however, consumed irrespective of morphotype. The selective behaviour was seen both at the seedling sta ge and the adult plant stage. Arion slugs were more choosy when they w ere given a choice of glabrous smithii and densely hairy lapponica pla nts then when they were served smithii and the less hairy variety serp entinicola. This study implies that the Arion slugs are important agen ts of selection on subalpine meadows. The selective grazing seems to c ounteract the flow of the recessive allele that determines glabrousnes s, from serpentine into meadow populations. Gastropods are rare in the dry serpentine habitat. The evolution of a distinct glabrous morphoty pe on serpentine is therefore rather due to relaxation of selection th an to adaptation to serpentine stress.