Long-term changes in the chromosomal inversion polymorphism of Drosophila subobscura. I. Mediterranean populations from southwestern Europe.

The chromosomal polymorphism of seven Mediterranean populations of Drosophila subobscura has been compared with that of the same populations collected 26 to 35 years ago. Significant latitudinal clines for the frequencies of A(ST), E(ST), O(ST). and U(ST) chromosomal arrangements have been detected in the old and new samples. Standard gene arrangements are frequent in the north and decrease in frequency towards the south. Significant negative regression coefficients between latitude and transformed frequency have also been observed for the more frequent nonstandard gene arrangements. The pattern of the clines is practically the same in the old and new collections. Furthermore, the frequencies of gene arrangements of all chromosomes have changed significantly during this period in a systematic way: an increase in the frequency of those arrangements typical of southern latitudes and a decrease for those more common in northern latitudes is observed in all populations. These changes could be due to climatic factors that are correlated with latitude, making the chromosomal composition of this species more "southern.''