About 120 strains derived from natural populations of D. melanogaster collected between 1983 and 1986 in the Languedoc-Roussillon area (France) were tested for their characteristics in the P-M system, enzymatic polymorphism and infection by the sigma virus. Over these four years, a clear geographical pattern was found for the P-M system over a distance of 250 km, with weak P populations in the northern zone of the area studied, Q populations in the centre and M' populations in the south. This pattern was stable for P sterility potential but a decrease of the M cytotype level was observed in the southernmost zone. During the same period, the frequency of flies infected by the sigma virus also showed a regular geographical pattern with the highest values in the central zone. The frequency of infected flies increased steadily during this period. In the enzymatic study, neither geographical nor temporal trends were detected for the loci analyzed. Correlations among the different systems are discussed.
Summary — Seventeen French natural populations of Drosophila melanogaster were analyzed to detect linkage disequilibrium between pairs of 6 polymorphic allozyme loci. The estimates of linkage disequilibrium were made from azygotic frequencies using both Burrows' and Hills's methods. No difference between these 2 methods was found. The amount of significant linkage disequilibrium detected was small and similar to those in other natural populations of D. melanogaster. Out of the 15 combinations examined, only 2 pairs, Adh-a-Gpdh and Est-C-Est-6, showed a consistent significant linkage disequilibrium in the populations studied. However, for the first pair, the result was probably due to an association between the loci and the inversion (2 L) t of the second chromosome. For the Est-C-Est-6 pair, the disequilibrium detected might result from an interaction effect between the 2 genes inter se. These results again show the difficulties in detecting linkage disequilibrium due to epistasis between allozyme genes in natural populations.
L'évolution dans le temps du polymorphisme de 5 locus enzymatiques a été suivie pour 12 populations naturelles françaises de Drosophila melanogaster. La structure génétique de ces populations ne varie pas, ou peu, d'un automne à l'autre. La bonne résistance au froid des imagos et le fait que la période hivernale suive celle de l'explosion démographique de l'espèce en France, indiquent que les populations analysées ne doivent pas subir d'importantes réductions de leurs effectifs durant l'hiver, susceptibles d'entraîner une forte dérive génétique. Dans ces conditions, le maintien de la même structure d'une année à l'autre peut s'expliquer par le fait que les populations sont panmictiques.
Summary
Temporal variation at five allozyme loci in 12 French natural populations of Drosophila melanogaster
Temporal patterns of allozyme variation over time have been examined for five polymorphic enzyme loci in 12 french natural populations of Drosophila melanogaster. Taken in their totality, the data show only slight changes in the genetical structure of these populations between years. The good viability of the imagoe at low temperatures and the occurrence of winter immediately after the demographic burst of the species in France involve that the population sizes during winter are probably sufficient to eliminate the genetic drift. Therefore, the stability in the genetical composition from one autumn to another could be explained by the way that the populations analyzed are panmictic.
SummaryWe analyzed allozyme variation of 6 gene loci in 14 populations of Drosophila melanogaster originating from different regions of France and captured from fruits of the localities studied (cellars excluded). With respect to these 6 loci the populations are not genetically homogeneous. Allelic
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