GEOGRAPHICAL PATTERNS OF GENETIC DIFFERENTIATION IN THE LANDSNAIL HELIX-ASPERSA MULLER (GASTROPODA, PULMONATA)

In order to investigate the initial processes responsible for the geog raphical distribution of the Mediterranean landsnail Helix aspersa, po pulations from 64 locations in Maghreb and in Europe were studied at 1 7 enzyme loci by both starch and polytacrylamide gel electrophoresis. Allele frequencies and genotype distributions were analysed to provide information on population structure and genetic differentiation among populations. Values of Nei's unbiased (D) and Rogers (R) genetic dist ances led to the formation of two main groups, namely Europe-west Alge ria and east Maghreb (average D = 0.182). Such splitting supported the pattern of anatomical variation recently described. Two samples from Morocco were excluded from these groups because of an average D = 0.22 4. Within this Moroccan entity, genetic divergences did not distinguis h the 'giant' snails H. a. maxima from the brownsnails H. a. aspersa. Such biochemical similarities among morphological heterogeneity throws the subspecific status of H. a. maxima into question. Values of fixat ion indices indicated that the greatest differentiation between sample s occurred on the Maghreb territory (theta = 0.393). The patterns of p opulation differentiation are discussed in relation to past geological and historical events. Thus, three scenarios, not mutually exclusive, are considered: the disjunction of populations by plate tectonics dur ing the Tertiary, the formation of geographically isolated populations during the Pleistocene glacial/interglacial cycle and the possibility of human introduction as early as the Neolithic period.