Preferential transcription of Xenopus laevis ribosomal RNA in interspecies hybrids between Xenopus laevis and Xenopus mulleri

Ribosomal RNA synthesized by hybrid frogs from the cross between Xenopus laevis and Xenopus mulleri was analyzed by molecular hybridization with purified ribosomal DNA from each species. Although the 18 S and 28 S rRNA sequences are indistinguishable between these two species, the remaining 10% of the 40 S rRNA precursor molecule of each species hybridizes about tenfold more efficiently to homologous rDNA than to heterologous rDNA. Using an assay based upon this fact, we show that in hybrid frogs X. laevis rDNA is transcribed preferentially and X. mulleri rDNA is repressed. X. mulleri rDNA is repressed regardless of which species is the female parent. The repression is nearly complete throughout early embryogenesis until the swimming tadpole stage, after which a low level of X. mulleri rRNA synthesis is detectable in the total embryo population. Some adult frogs made no detectable X. mulleri rRNA, whereas others were found that synthesized substantial amounts. Transcription of X. mulleri rDNA is repressed in embryos from the cross of a X. laevis female, heterozygous for the rDNA deletion mutation, and a wild type X. mulleri male. Half of these embryos contain only X. mulleri rDNA. The X. mulleri rDNA is transcribed eventually in these embryos but the onset of rRNA synthesis is much later than in wild type X. mulleri embryos. In the reverse cross (female X. mulleri × male X. laevis heterozygote) turn-on of X. mulleri rRNA synthesis was not delayed. The results of these four types of crosses indicate that either X. laevis rDNA or X. laevis maternal cytoplasm can each repress expression of X. mulleri rDNA in hybrid embryos. In the presence of X. laevis rDNA the repression can be permanent. The repression by X. laevis cytoplasm is transient and usually reversible.