Identification of Genes That Promote or Antagonize Somatic Homolog Pairing Using a High-Throughput FISH-Based Screen

Department of Genetics, Harvard Medical School, Boston, Massachusetts, United States of America.
PLoS Genetics (Impact Factor: 7.53). 05/2012; 8(5):e1002667. DOI: 10.1371/journal.pgen.1002667
Source: PubMed


The pairing of homologous chromosomes is a fundamental feature of the meiotic cell. In addition, a number of species exhibit homolog pairing in nonmeiotic, somatic cells as well, with evidence for its impact on both gene regulation and double-strand break (DSB) repair. An extreme example of somatic pairing can be observed in Drosophila melanogaster, where homologous chromosomes remain aligned throughout most of development. However, our understanding of the mechanism of somatic homolog pairing remains unclear, as only a few genes have been implicated in this process. In this study, we introduce a novel high-throughput fluorescent in situ hybridization (FISH) technology that enabled us to conduct a genome-wide RNAi screen for factors involved in the robust somatic pairing observed in Drosophila. We identified both candidate "pairing promoting genes" and candidate "anti-pairing genes," providing evidence that pairing is a dynamic process that can be both enhanced and antagonized. Many of the genes found to be important for promoting pairing are highly enriched for functions associated with mitotic cell division, suggesting a genetic framework for a long-standing link between chromosome dynamics during mitosis and nuclear organization during interphase. In contrast, several of the candidate anti-pairing genes have known interphase functions associated with S-phase progression, DNA replication, and chromatin compaction, including several components of the condensin II complex. In combination with a variety of secondary assays, these results provide insights into the mechanism and dynamics of somatic pairing.

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    • "Mrg-binding motif is required for Cap-H2-mediated homolog unpairing and axial compaction in cultured cells Condensin II promotes unpairing of homologous chromosomes, a process that has been proposed to result from condensin II–mediated axial compaction (Bauer et al. 2012; Joyce et al. 2012). Overexpression of Cap-H2 is sufficient to drive compaction and unpairing in interphase chromosomes, and does so in an Mrg15-dependent manner (Bauer et al. 2012; Smith et al. 2013). "
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    • "In this organism, homologous chromosomes are paired in diploid somatic cells, leading to a specialized gene regulatory event, known as transvection, in which a gene is transcriptionally activated or repressed in trans by regulatory elements located on the homologous chromosomes. Remarkably, condensin II subunits have been shown to play a role in antagonizing transvection (Hartl et al. 2008), as well as somatic homolog pairing (Joyce et al. 2012). Moreover, condensin II subunits apparently contribute to chromosome territory formation in polyploid nurse cells (Bauer et al. 2012). "
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    • "Brain cell preparations from 72 to 96-hour third-instar larvae with empty guts showed mitotic pro-metaphases, metaphases and anaphases. Somatic pairing was observed in metaphase plates as reported for other dipterans (Stevens 1908, Metz 1916, Joyce et al. 2012). We distinguished two types of chromosome numbers within the studied samples: 2n=10 and 2n=11. "
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