Similarities and differences in the structure of X and Y chromosome rRNA genes of Drosophila.

In Drosophila melanogaster, the rRNA genes (rDNA) are clustered at single sites on two non-homologous chromosomes, the X and Y. Examination of the structure of X and Y rDNAs with restriction endonucleases reveals that the X rDNA contains repeating units not present in the Y. Such observations, as well as genetic evidence, illustrate difficulties with the hypothesis that recombination is the predominant mechanism preserving similarities between these two clusters maintained on different chromosomes. This raises the possibility that selection pressure has a significant role in maintaining the parallel evolution of these two separate but homologous redundant gene clusters.