Chromosome homologies within the Drosophila obscura group probed by in situ hybridization

Abstract

Probes specific to chromosome elements were used to investigate chromosome homologies between seven species of the Drosophila obscura group by in situ hybridization. Our results were in perfect agreement with the already established chromosome element homologies between D. subobscura, D. pseudoobscura, D. persimilis, and D. miranda. Furthermore, we were able to identify the chromosomal elements of D. obscura, D. ambigua, and D. subsilvestris. Of special interest was the localization of the two D. melanogaster-derived representatives of the tandemly repetitive genes, cDm500 and 12D8. In contrast to the findings with the element-specific probes, the localizations of the repetitive genes varied in the various species. Whereas D. melanogaster, D. subobscura, D. pseudoobscura, D. persimilis, and D. miranda showed only one strong block of label in the cross in situ hybridizations with cDm500, three labeling blocks were found on two elements for both D. ambigua and D. obscura. The two labeling blocks on one element occur in very close proximity, but are clearly separated in both species by cytologically detectable chromosomal material. We used the distribution of the cDm500 labeling sites to postulate a series of chromosomal rearrangements involved in the karyotype evolution of the analyzed species. Our results support the conclusion that the chromosomal elements retain their essential identity and that the observed gross structural rearrangements are due to fusions and paracentric or pericentric inversions. Cytologically obvious translocations were not recorded and are considered by us to be rare. The frequently occurring translocations of the tandemly repeated gene clusters observed in this study are probably due to a different mechanism, which may be an intrinsic property of this category of genes.