Polytenization of the Ribosomal Genes on the X and Y Chromosomes of Drosophila Melanogaster

It has previously been shown (Endow and Glover 1979), that polytenization of the ribosomal genes in D. melanogaster Ore-R X/Y cells and in hybrid X/X cells (Endow 1980) involves replication of genes predominantly from one of the cell's two nucleolus organizers. This analysis takes advantage of strain-specific differences in X and Y chromosome rDNA hybridization patterns detected using the Southern blotting technique. In this report, I extend the previous observations by examining polytene rDNA patterns in wild-type and hybrid X/Y cells. A dominance hierarchy for the X and Y chromosomes from three strains of D. melanogaster is presented and possible mechanisms of replicative dominance are discussed.