Regulation of Nucleolar Dominance Within the Species Drosophila Melanogaster and Within Interspecies Drosophila Hybrids

Abstract

In eukaryotes, ribosomal RNA (rRNA) genes are tandemly repeated, forming ribosomal DNA (rDNA) loci that contain hundreds of rRNA genes to meet the demand of ribosome biogenesis. Individual rDNA loci can be entirely silenced or transcribed (termed nucleolar dominance) to control rRNA dosage. The mechanisms through which rDNA silencing is regulated has been extensively studied, however how or why particular loci are chosen to be silenced/transcribed remain elusive. This dissertation aims to understand how the large-scale silencing of nucleolar dominance is determined and how this “choice” might affect a cell or organism. This dissertation utilizes RNA in situ hybridization techniques and chromosome inversions/translocations to characterize nucleolar dominance within the species D. melanogaster. We show that nucleolar dominance is a developmentally-regulated program in D. melanogaster X/Y males, where the Y rDNA dominates in expression over the X rDNA locus over developmental time and in a tissue-specific manner. We expand on the observation that nucleolar dominance does not typically occur between two X rDNA loci in females across development, and that this difference between male and female nucleolar dominance is not due to cellular sex. We further show that the short arm of the Y chromosome, including its rDNA locus, likely contains elements that dictate its dominant expression over the X rDNA locus. This study on nucleolar dominance in D. melanogaster adds insights into how the “choice” of rDNA locus silencing/expression is made.

Nucleolar dominance was originally discovered to occur in interspecies hybrids where one species rDNA dominates over another. Whether or not nucleolar dominance is similar within a species and in interspecies hybrids has been extensively studied in Arabidopsis, but has been limited in studies of Drosophila. By utilizing RNA in situ hybridization probes that distinguish rRNA from the closely related species D. melanogaster and D. simulans, we expand on the characterization of nucleolar dominance in Drosophila interspecies hybrids to different stages of development. We find that both rDNA loci in interspecies hybrid Drosophila are expressed in early embryogenesis and that nucleolar dominance (D. melanogaster rDNA dominance and D. simulans rDNA silencing) is established across development, similar to that seen in “pure species” Drosophila and Arabidopsis.

When D. simulans females and D. melanogaster males form hybrids, the female progeny die during early embryonic divisions as a result of chromosome bridge formation at the large X-chromosome specific heterochromatic block of 359-bp satellite DNA, a satellite DNA specific to D. melanogaster. The D. melanogaster mutant Zhr1, which lacks 359-bp satellite DNA on the X chromosome, is known to rescue this female lethality. This dissertation reveals for the first time that Zhr1-rescued interspecies hybrid females have reversed nucleolar dominance where D. simulans rDNA is expressed and D. melanogaster rDNA is silenced. This dissertation suggests that rDNA and Zhr (359-bp) have a relationship that may connect nucleolar dominance with female lethality seen in these D. melanogaster – D. simulans hybrids. This dissertation lays the groundwork to further understand how the organization and function of highly repetitive regions (rDNA) and proximal species-specific satellite DNAs (359-bp) may have relationships that act as barriers between species. In summary, this dissertation begins to uncover the mechanisms underlying nucleolar dominance within the species D. melanogaster and aims to understand how the regulation and divergence of nucleolar dominance and satellite DNAs may have consequences that manifest in barriers between species.