Spatial analysis of allozyme and microsatellite DNA polymorphisms in the land snail Helix aspersa (Gastropoda: Helicidae).

The genetic structure of the land snail Helix aspersa was investigated for 21 populations collected along a road located in the polders of the Bay of Mont-Saint-Michel (Brittany, France), following a sampling scheme the area of which did not exceed 900 m in length. A total of 369 individuals were genotyped for five enzymatic markers and seven microsatellite loci. We used sequential hierarchical F-statistics at different spatial scales and spatial autocorrelation statistics to explore recent historical patterns involved in the observed genetic distribution. Whatever the statistics used, congruent levels of spatial genetic substructuring across loci were demonstrated, excepted for one allozyme locus. Overall spatial genetic arrangement matched in a substantial fashion theoretical predictions based on the limited dispersal power of land snails. Positive autocorrelation over short-distance classes may result from the development of genetically distinct patches of individuals organized in family-structured colonies. Therefore, spatial signatures of average I correlograms can be viewed as the expression of a stepping-stone model of population structure, sometimes involving external migrational events. Overall, the revealed pattern of population subdivision on a microgeographical scale was suggestive of a neighbourhood structure. Finally, microsatellite loci are especially suitable for the detection of small genetic clustering, and combining different classes of markers offers the potential to gain further insight into the description of spatial genetic variability over short temporal and geographical scales.