Gene regulation in Drosophila mulleri, D. arizonensis, and their hybrids: The nucleolar organizer

Typically, Drosophila have nucleolar organizer regions (NORs) confined to the sex chromosomes. Salivary gland cells of hybrids between Drosophila mulleri females and D. arizonensis males exhibit features in nucleolar organizer regulation that differentiate the species on one hand, and which show an interplay between the X and the microchromosome on the other hand.In the hybrid females only the X chromosome from D. arizonensis appears to be attached to the nucleolus. In the hybrid males the X chromosome, from D. mulleri, also does not seem to contain a functional NOR. However, in hybrid males the microchromosome from D. arizonensis increases greatly in size and appears to be associated with the nucleolus. The increase in size of the microchromosome involves a 4-fold increase in DNA content. In D. arizonensis and in hybrid females the NOR of the microchromosome appears to be suppressed. In the absence of an arizonensis X chromosome, the NOR of the microchromosome typically is active, while the NOR on the mulleri X chromosome remains suppressed. Therefore, the ribosomal cistrons and interchromosomal regulator element appears to be duplicated on both the X chromosome and microchromosome of D. arizonensis, but with epistatic suppression of the microchromosomal NOR by the arizonensis X-linked NOR. Either arizonensis NOR, X linked or microchromosomal, suppresses the mulleri NOR.