Unequal mitotic sister chromatin exchange as the mechanism of ribosomal RNA gene magnification.

It is hypothesized that magnification of the gene coding for ribosomal RNA occurs by unequal mitotic sister chromatid exchange on the basis of five different lines of evidence. These are: (1) rDNA magnification occurs in mitotically active germ cells; (2) decreases in rDNA redundancy can be genetically produced, a phenomenon termed reduction; (3) magnification and reduction events are reversible and reciprocal; (4) it is possible to generate bb(+) and bb somatic bristle mosaics (bb mutants are partially deficient for rRNA genes); and (5) magnification of bb in a ring X chromosome is reduced. Implications of these results and the unequal sister exchange (USE) hypothesis are discussed.