1. ¹Laboratoire Populations, Génétique et Evolution, Centre National de la Recherche Scientifique, Gif-sur-Yvette Cedex, France
2. ²Institut Universitaire de Formation des Maîtres, Paris, France
3. ³Laboratoire d'Evolution et Systématique, Université Paris-Sud XI, Centre National de la Recherche Scientifique, Orsay Cedex, France

Correspondence: L Monti-Dedieu, Laboratoire Populations, Génétique et Evolution, Centre National de la Recherche Scientifique, Avenue de la Terrasse, 91198 Gif-sur-Yvette Cedex, France. E-mail: monti@paris.iufm.fr

⁴These authors contributed equally to the work.

Received 22 May 2003; Accepted 30 July 2004; Published online 23 February 2005.

Abstract

The evolution of the chromosomal location of ribosomal RNA gene clusters and the organization of heterochromatin in the Drosophila melanogaster group were investigated using fluorescence in situ hybridization and DAPI staining to mitotic chromosomes. The investigation of 18 species (11 of which were being examined for the first time) belonging to the melanogaster and ananassae subgroups suggests that the ancestral configuration consists of one nucleolus organizer (NOR) on each sex chromosome. This pattern, which is conserved throughout the melanogaster subgroup, except in D. simulans and D. sechellia, was observed only in the ercepeae complex within the ananassae subgroup. Both sex-linked NORs must have been lost in the lineage leading to D. varians and in the ananassae and bipectinata complexes, whereas new sites, characterized by intra-species variation in hybridization signal size, appeared on the fourth chromosome related to heterochromatic rearrangements. Nucleolar material is thought to be required for sex chromosome pairing and disjunction in a variety of organisms including Drosophila. Thus, either remnant sequences, possibly intergenic spacer repeats, are still present in the sex chromosomes which have lost their NORs (as observed in D. simulans and D. sechellia), or an alternative mechanism has evolved.