GENETIC-RELATIONSHIPS AMONG SUSPECTED CONTACT ZONE POPULATIONS OF HELIX-ASPERSA (GASTROPODA, PULMONATA) IN ALGERIA

In Maghreb, the land snail Helix aspersa consists of two anatomically and biochemically divergent groups of populations between which the se paration occurs roughly to the east of the Lesser Kabylia. To document the patterns of migration across the suspected contact zone between t hese two geographical entities, we have analysed changes in allele fre quencies at 13 polymorphic allozyme loci in five intermediate Algerian populations ranging from Azeffoun (Great Kabylia) to El Hedaick (Skik da), thus including three western and two eastern colonies. Multivaria te analyses and exact tests for population differentiation were used t o show the genetic divergence between samples; deviations from panmixi a were calculated to account for the effect of migration. Whereas some loci showed abrupt changes in allelic composition across the hybrid z one, some others were introgressed, especially into eastern population s in the central colony of Djemila. It is tempting to ascribe this all elic distribution to gene exchanges between populations on the east an d west. However, the large number of private alleles found in adjoinin g populations apparently does not support this explanation, although s uch an increase in rare alleles has been observed in other hybrid zone surveys. In addition, it may be that this discrepancy between the pat terns of common and private allele variation may result from the time since contact occurred. Whatever the explanation of this finding, the genetic structure in Djemila, and particularly the large linkage diseq uilibria observed, indicates population mixing. Although not ruling ou t the possibilities of tectonic events and of human transport, the mos t likely hypothesis for contacts between eastern and western populatio ns are that these result from climatic phenomena during the Quaternary .