Mechanisms of nucleolar dominance in animals and plants.

In interspecific hybrids, it is often observed that the ribosomal genes of one species are transcriptionally dominant over the ribosomal genes of the other species. This phenomenon has been called and has been reported in such diverse organisms as frogs (Xenopus), Drosophila, many genera of plants, and mammalian somatic cell hybrids. Recent advances in our knowledge of the structure of ribosomal genes and their transcription machinery have led to proposals that at least two different molecular mechanisms can operate to cause nucleolar dominance and that the relative contribution of each mechanism is different for different types of crosses. One proposed mechanism involves competition between ribosomal genes which possess unequal numbers of elements. This mechanism can be abbreviated as the enhancer imbalance mechanism. The second proposed mechanism involves the fact that the ribosomal gene (RNA polymerase I) transcription machinery evolves more rapidly between species than does the machinery for the other two classes of polymerase. This leads to dominance effects based on the apparent inability of a key transcription factor from one species to recognize the ribosomal gene promoter of the other species. This mechanism will be referred to as the speciesspecific factor mechanism. The purpose of this review is to briefly summarize the evidence for these two molecular mechanisms and then to examine each of the known types of nucleolar dominance to assess how well the proposed mechanisms can account for each case.