Ribosomal RNA Gene Silencing in Inter-population Hybrids of Tigriopus californicus: Nucleolar Dominance in the Absence of Inter-genic Spacer Subrepeats

A common feature of inter-specific animal and plant hybrids is the uniparental silencing of ribosomal RNA gene transcription, or nucleolar dominance. A leading explanation for the genetic basis of nucleolar dominance in animal hybrids is the enhancer-imbalance model. The model proposes that limiting transcription factors are titrated by a greater number of enhancer-bearing subrepeat elements in the inter-genic spacer (IGS) of the dominant cluster of genes. The importance of subrepeats for nucleolar dominance has repeatedly been supported in competition assays between between Xenopus laevis and X. borealis minigene constructs injected into oocytes. However, a more general test of the importance of IGS subrepeats for nuclear dominance in vivo has not been conducted. In this report, rRNA gene expression was examined in inter-population hybrids of the marine copepod T. californicus. This species offers a rare opportunity to test the role of IGS subrepeats in nucleolar dominance because the internal subrepeat structure, found in the IGS of virtually all animal and plant species, is absent in T. californicus. Our results clearly establish that nucleolar dominance occurs in F1 and F2 inter-population hybrids of this species. In the F2 generation, nucleolar dominance appears to break down in some hybrids in a fashion that is inconsistent with a transcription factor titration model. These results are significant because they indicate that nucleolar dominance can be established and maintained without enhancer-bearing repeat elements in the IGS. This challenges the generality of the enhancer-imbalance model for nucleolar dominance and suggests that dominance in rRNA transcription in animals may be determined by epigenetic factors as has been established in plants.

Key Words: IGS, RNA polymerase I, Xenopus, enhancer-imbalance, rRNA

Online ISS 1091-6490
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