Academic journal article Genetics

A Genomewide Survey Argues That Every Zygotic Gene Product Is Dispensable for the Initiation of Somatic Homolog Pairing in Drosophila

Academic journal article Genetics

A Genomewide Survey Argues That Every Zygotic Gene Product Is Dispensable for the Initiation of Somatic Homolog Pairing in Drosophila

Article excerpt

ABSTRACT

Studies from diverse organisms show that distinct interchromosomal interactions are associated with many developmental events. Despite recent advances in uncovering such phenomena, our understanding of how interchromosomal interactions are initiated and regulated is incomplete. During the maternal-to-zygotic transition (MZT) of Drosophila embryogenesis, stable interchromosomal contacts form between maternal and paternal homologous chromosomes, a phenomenon known as somatic homolog pairing. To better understand the events that initiate pairing, we performed a genomewide assessment of the zygotic contribution to this process. Specifically, we took advantage of the segregational properties of compound chromosomes to generate embryos lacking entire chromosome arms and, thus, all zygotic gene products derived from those arms. Using DNA fluorescence in situ hybridization (FISH) to assess the initiation of pairing at five separate loci, this approach allowed us to survey the entire zygotic genome using just a handful of crosses. Remarkably, we found no defect in pairing in embryos lacking any chromosome arm, indicating that no zygotic gene product is essential for pairing to initiate. From these data, we conclude that the initiation of pairing can occur independently of zygotic control and may therefore be part of the developmental program encoded by the maternal genome.

AS the primary storehouse of the cell's genetic material, the nucleus must package the millions of bases that compose the genome into a miniscule volume, while simultaneously permitting the dynamic interplay between nuclear proteins and DNA elements that is required for faithful gene expression. The organization of chromosomes in three-dimensional space is an important factor in both of these functions (reviewed in Branco and Pombo 2007; Lanctôt et al. 2007; Misteli 2007). In general, chromosomes occupy discrete territories within the nucleus (reviewed in Heard and Bickmore 2007), with individual loci undergoing limited movement to reach a suitable environment for gene regulation. Furthermore, recent studies have shown that specific interactions can occur in trans between different chromosomes. For example, application of chromatin conformation capture (3C) technology to naive T-helper cells of mice has shown that the T^sub H^2 locus control region on chromosome 10 physically interacts with the promoter region of the interferon-g (Ifng) gene on chromosome 11 and that this interaction is important for robust Ifng expression (Spilianakis et al. 2005). Similar analyses have shown that the X-inactivation centers (XICs) of maternally and paternally derived X chromosomes become juxtaposed during random X inactivation in female murine embryonic stem cells (Bacher et al. 2006; Xu et al. 2006, 2007). In addition, several groups have used modified versions of 3C technology to survey the genome for sequences that associate with a specific chromosomal region, with multiple analyses reporting reproducible interactions between unlinked genomic segments (Ling et al. 2006; Lomvardas et al. 2006; Wurtele and Chartrand 2006; Zhao et al. 2006; Nunez et al. 2008). The ability of a genetic element on one chromosome to interact with a region on another chromosome raises the important question of specificity: How does a genetic element find and interact with another chromosomal locus in trans?

Although interchromosomal interactions have gained considerable interest in the recent literature, communication between chromosomes was first postulated over a century ago in Drosophila and other dipteran insects (Stevens 1908). Here, homologous chromosomes are intimately associated from end to end in virtually all cells of the organism, a phenomenon known as somatic homolog pairing (reviewed in McKee 2004). As originally suggested by Nettie Stevens (Stevens 1908) and demonstrated by Ed Lewis (Lewis 1954), somatic pairing in Drosophila can have a profound influence on gene expression; subsequent analyses have shown that this influence can bemediated by interhomolog enhancer-promoter interactions and by trans interactions between pairing-sensitive regulatory elements (reviewed in Morris et al. …

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