Microspatial genetic structure in the land snail Helix aspersa (Gastropoda: Helicidae)

The microspatial genetic structure of allele frequencies at seven isozyme l oci was examined for 15 populations of the land snail Helix aspersa sa samp led in a village from Brittany (north-western France). Spatial heterogeneit y of allele frequencies was highly significant (P < 0.001). Fixation indice s reflected nonrandom mating within neighbourhoods and a slight but consist ent differentiation between colonies (F-ST = 0.044; P < 0.01). Analyses of gene flow or genetic distances failed to reveal a significant relationship with geographical distance, probably because of the complexity of environme ntal heterogeneity. However, matrix comparisons between genetic distances a nd connectivity networks among adjacent colonies (Gabriel-connected graph) yielded a significant correlation in every case, indicating a 'step-by-step ' relationship between neighbouring localities. Moreover, most of the alloz ymes were spatially structured and showed (i) a gradual isolation of coloni es with increasing geographical distances, and (ii), for some correlogram p rofiles, a circular gradient illustrating a multidirectional colonization o f the village. The probable existence of disperser individuals allowed us t o suggest a metapopulation model which would explain the maintenance of suc h animals in fragmented habitats where anthropogenic disturbances and extin ction/recolonization events are commonly observed.