Nucleolar dominance is a dramatic disruption in the formation of nucleoli and the expression of ribosomal RNA (rRNA) genes, characteristic of some plant and animal hybrids. Here, we report that F1 hybrids produced from reciprocal crosses between 2 sister species of Xenopus clawed frogs, X. muelleri and X. borealis, undergo nucleolar dominance somewhat distinct from a pattern previously reported in hybrids between phylogenetically more distant Xenopus species. Patterns of nucleolar development, 45S rRNA expression, and gene copy inheritance were investigated using a combination of immunostaining, pyrosequencing, droplet digital PCR, flow cytometry, and epigenetic inhibition. In X. muelleri × X. borealis hybrids, typically only 1 nucleolus is formed, and 45S rRNA genes are predominantly expressed from 1 progenitor's alleles, X. muelleri, regardless of the cross-direction. These changes are accompanied by an extensive (∼80%) loss of rRNA gene copies in the hybrids relative to their parents, with the transcriptionally underdominant variant (X. borealis) being preferentially lost. Chemical treatment of hybrid larvae with a histone deacetylase inhibitor resulted in a partial derepression of the underdominant variant. Together, these observations shed light on the genetic and epigenetic basis of nucleolar dominance as an underappreciated manifestation of genetic conflicts within a hybrid genome.
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