Nucleolar dominance and maternal control of 45S rDNA expression. Xenopus

Using a system of interspecies hybrids, trihybrids, and recombinants with varying proportions of genomes from three distinct Xenopus species, we provide evidence for de novo epigenetic silencing of paternal 45S rRNA genes and their species-dependent expression dominance that escapes transcriptional inactivation after homologous recombination. The same pattern of imprinting is maintained in offspring from mothers being genetic males (ZZ) sex-reversed to females, indicating that maternal control of rDNA expression is not sex chromosome-linked. Nucleolar dominance (nucleolus underdevelopment) in Xenopus hybrids turns out to be associated with a major non-Mendelian reduction in the number of 45S rDNA gene copies rather than a specific pattern of their expression. The loss of rRNA gene copies in F1 hybrids was non-random with respect to parental species, with the transcriptionally dominant variant preferentially removed from hybrid zygotes. This dramatic disruption in the structure and function of 45S rDNA impacts transcriptome patterns of small nucleolar RNAs and messenger RNAs, with genes from the ribosome and oxidative stress pathways being among the most affected. Unorthodoxies of rDNA inheritance and expression may be interpreted as hallmarks of genetic conflicts between parental genomes, as well as defensive epigenetic mechanisms employed to restore genome integrity.