The mechanism of nucleolar dominance in Xenopus hybrids.

We have mimicked the phenomena of nucleolar dominance by injecting cloned ribosomal genes from X. laevis and X. borealis into oocytes from either species and into embryos. The results of these experiments suggest that the dominance of the laevis ribosomal genes in hybrid embryos is primarily due to the fact that the laevis spacer has more copies of an enhancer element for the ribosomal gene promoter than does the borealis spacer. A contributing effect is that the laevis promoter functions well with transcription machinery from either species while the borealis promoter interacts poorly with laevis transcription machinery.