rDNA magnification in D. melanogaster: state of rDNA copies following the first step.

D. melanogaster males of XYbb/O genetic constitution undergoing rDNA magnification were mated singly to XXbb+/O females, yielding XYbb/O male progeny, and to XNO- w sn bb+ females, yielding XYbb/XNO- females. The male and female offspring were scored for the bb+ phenotype. Results show that there is a higher percentage of bb+ flies in the XYbb/O male progeny than in XYbb/XNO- female progeny, in single crosses as well as in the combined data. rRNA/DNA hybridization experiments agree with this observation, by showing that the rDNA content in the progeny of premagnified flies was higher in the sons than in the daughters. These data indicate that the increase of ribosomal RNA genes is not due to a stable event such as an unequal mitotic sister exchange, whereas they do not contrast with the extracopy model.