Amplification of rDNA and type I sequences in Drosophila males deficient in rDNA.

rDNA magnification is a heritable change in rDNA content that occurs in D. melanogaster males when chromosomes deficient in rDNA are placed together for several generations. We have examined the restriction endonuclease cleavage pattern of the rDNA from an X chromosome undergoing magnification, and find no evidence for the selective amplification of either uninterrupted rDNA units or those containing insertion sequences. In addition, we observe an amplification of rDNA in the first generation of extremely bobbed male progeny to a level exceeding that of wild-type flies, but that reduces to the wild-type level in subsequent generations. The type I rDNA insertion elements also occur as tandem arrays, independently of rDNA. Southern hybridizations indicate that the majority of these sequences are located in the heterochromatin surrounding the nucleolus organizer on the X chromosome, and we find that they, too, amplify transiently in the first generation of magnifying males.