The location of the nucleolus organizer regions in Drosophila hydei.

The positions of the nucleolus organizer regions in metaphase chromosomes of Drosophila hydei were detected by in situ hybridization experiments. In agreement with earlier conclusions the nucleolus of the X chromosome was found to originate in a terminal region of the heterochromatic arm. The Y chromosome contains two nucleolus organizers, one in a terminal postion of the long arm, and the other in the short arm. The implications with respect to the evolution of the Y chromosome are discussed.