The molecular through ecological genetics of abnormal abdomen. II. Ribosomal DNA polymorphism is associated with the abnormal abdomen syndrome in Drosophila mercatorum.

Restriction endonuclease cleavage analyses of cloned and genomic DNA samples indicate that the structure of the DNA encoding the large cytoplasmic RNAs (rDNAs) is altered in Drosophila mercatorum lines which exhibit an abnormal abdomen (aa) phenotype. In a majority of the rDNA repeat units from aa flies, the 28S coding sequence is interrupted by a large [5-6 kilobase pairs (kbp)] insert. A subclone containing this inserted DNA (ins 3) hybridizes primarily to rDNA-containing sequences in in situ and genomic blot hybridization experiments. Additionally, genomic nitrocellulose blot hybridization analyses show that ins- containing rDNA repeat units are clustered in a spontaneously arising aa mutant. This rDNA alteration in D. mercatorum flies with the aa phenotype more closely resembles the bobbed (bb) defect of D. hydei than the bb defect of D. melanogaster, which involves alterations in rDNA copy number. By analogy with the other Drosophila systems, we propose that the altered D. mercatorum rDNA repeat units are defective in rRNA production at a critical stage. The lowered levels of rRNA ultimately would limit the concentration of ribosomes needed to produce large quantities of a protein (in these cases, juvenile hormone esterase) needed for normal development.