ABSTRACT
This chapter examines the advances in the analysis of the Ribosomal Rna Gene (rDNA) locus, with a particular emphasis on the spacer region, and how the studies are modifying our views on the evolution of such repeated genes. It provides a framework for evolutionary studies on the rDNA system so that patterns of evolution in crossing-over gene system can be related eventually to organismal evolution. The chromosomal and extrachromosomal rDNA have the same basic molecular structure. Both heteroduplex mapping and restriction enzyme analyses of cloned rDNA indicate that the length variation is the result of the insertion or deletion of units of a repeating DNA sequence found in the spacer. Spacer variation within the Drosophila melanogaster species group has also been evaluated by E. S. Coen et al and the patterns of variation as interpreted by these authors suggest that each species has primarily fixed characteristic variants resulting from length variation in the spacer.
