The genetics of Drosophila subobscura populations. 3. Inversion polymorphism and climatic factors

Summary The natural population of Drosophila subobscura of Mt. Parnes has been followed for two successive years and the frequency of the gene arrangements determined. Some changes occured in the first year but do not explain the existence of the geographical clines for these frequencies. Within the second year no changes were observed. For chromosomes J and U there is evidence for a greater fitness of the heterozygotes.Experiments on temperature effect on viability of adult male flies, on their temporal sterility, on the viability of flies during all their biological cycle and experiments on the effect of dryness on the viability of adult male flies did not show an important change of gene arrangement frequencies in the resistant individuals. Nor did diapausing females have very different gene arrangements, except for chromosome O. Cage experiments with temperature as a selective agent did not elucidate the action of this factor. It is concluded that temperature or dryness do not affect very much the polymorphic system, nor explain the existence of these clines.This polymorphic system is much more stable in D. subobscura than in D. pseudoobscura, its nearctic relative. It is also richer. It is concluded that it is historically older and helps the populations to cope with many environmental changes.