26S and 18S rRNA synthesis in bobbed mutants of Drosophila melanogaster.

For the most part, bobbed mutations of Drosophila melanogaster consist of deletions of 26S and 18S rDNA located on the X and Y chromosomes. Studies on the synthesis of rRNA of third instar larvae and one day old adult females of three severe bobbed genotypes, indicate that no decrease can be detected, compared ot wild type strains. One of the bobbed mutants studied was a rather unusual type: these flies possess a quantity of rDNA that should confer upon them a near wild type phenotype whereas they actually show an extreme bobbed phenotype. The two other bobbed mutants are of a classical type: their severe bobbed phenotype corresponds to large deletions of rDNA. Two hypotheses can be proposed to explain the extreme bobbed phenotype of the flies, in spite of the fact that rRNA synthesis occurs normally. A regulatory phenomenon may interfere at the stages studied, but in earlier stages a net decrease in rRNA synthesis may have occurred producing an irreversible effect in the tissues affected by bobbed mutations (abdominal cuticle, bristles). The second hypothesis is that the rRNA produced may not be functional, perhaps because it is specific of earlier stages.