Gene amplification proceeds by a chromosome copy mechanism

Abstract

Two general mechanisms which could account for gene amplification of ribosomal DNA in the oocytes of amphibians have been tested. In one mechanism at least the first extra copy of ribosomal DNA would be derived from the chromosomal ribosomal DNA and would predict a nuclear inheritance for the amplified ribosomal DNA. In an alternative mechanism, the amplified ribosomal DNA would be derived from one or more of the amplified ribosomal DNA copies which had been released from the oocyte nucleus and transmitted as an autonomous “episome” through the germ line into the oocytes of the progeny. The latter mechanism would require a maternal inheritance for the amplified ribosomal DNA. It was possible to decide between these two mechanisms by the use of two species of Xenopus, X. laevis and X. mulleri, the ribosomal DNA's of which differ in nucleotide sequence, and yet these two species form viable hybrid progeny. Although all hybrid animals contained chromosomal ribosomal DNA derived from both parents, each female hybrid progeny amplified predominantly if not exclusively the ribosomal DNA of X. laevis in the nuclei of its oocytes. The significance of X. laevis “dominance” is not yet understood. The fact that the female progeny of an X. laevis male and an X. mulleri female amplify the paternal ribosomal DNA rules out maternal inheritance of amplified ribosomal DNA and therefore the episome mechanism.