Ribosomal RNA cistrons of X chromosomes clonally derived from D. melanogaster laboratory populations: redundancy, organization and stability.

To determine whether heterogeneity exists in the organization or redundancy of the rRNA cistrons of inbred populations of Drosophila melanogaster, we have derived a number of sublines from the strains Oregon R and Canton S. These two strains were chosen because our previous studies have demonstrated a difference in the competence of these strains to exhibit a "compensatory response" of the rDNA. In each subline, the X chromosomes are descended from a single maternal X, that is, each line is homozygous for a particular nucleolus organizer (NO). These derivative lines have been characterized in terms of rDNA content and organization, using quantitative liquid hybridizations and Southern blot analyses, respectively. Our studies reveal that both of the highly inbred parent populations contained a heterogeneous array of X chromosomal rDNA contents. Once isogenized, the rDNA redundancy of a given X chromosomal NO can be shown to remain stable for at least 20 generations in culture. We detect no restriction pattern heterogeneity among X chromosomes isolated from a given strain, despite relatively large differences in their rDNA contents. This leads us to suggest that there is no significant clustering of intervening sequence-bearing (ivs+) genes within the rDNA loci of chromosomes from the populations examined. Furthermore, we conclude that apparent alterations in rDNA redundancy known as the compensatory response are not related to the heterogeneity of rDNA content within a population.