A two-stage model for the control of rDNA magnification.

Males of the genotype bb/Ybb- have been shown to produce both magnified (bbm+) and, less frequently, reduced (bbrl) X chromosomes. An analysis of the progeny of single magnifying bb/Ybb- males reveals that bbm+ revertants may be recovered either as rare single events or, more frequently, in large clusters. To analyze the role of the bb phenotype in the induction of rDNA magnification we have constructed a series of bb and bb+ derivatives of Ybb-. Males carrying an X chromosomal bb allele and one of these derivatives (bb/bbYbb- or bb/bb+Ybb-) produce small numbers (one to two) of bbm+ progeny at a frequency similar to that observed for bb/Ybb- males but do not produce large clusters of bbm+ revertants. In addition, bb/bb+Ybb- males produce essentially equal numbers of magnified (bbm+) and reduced (bbrl) X chromosomes. These data, together with a consideration of the growth properties of the male germline in Drosophila, suggest that magnification/reduction may occur at two different times during development. Those events that give rise to large clusters, and, thus, necessarily arise early in germ cell development, appear to be dependent on the bb phenotype. However, those events that give rise to single bbm+ chromosomes arise late in spermatogenesis, probably at meiosis, and are independent of the bb phenotype.