Chromosome Painting Reveals Asynaptic Full Alignment of Homologs and HIM-8–Dependent Remodeling of X Chromosome Territories during Caenorhabditis elegans Meiosis

Abstract

During early meiotic prophase, a nucleus-wide reorganization leads to sorting of chromosomes into homologous pairs and to establishing associations between homologous chromosomes along their entire lengths. Here, we investigate global features of chromosome organization during this process, using a chromosome painting method in whole-mount Caenorhabditis elegans gonads that enables visualization of whole chromosomes along their entire lengths in the context of preserved 3D nuclear architecture. First, we show that neither spatial proximity of premeiotic chromosome territories nor chromosome-specific timing is a major factor driving homolog pairing. Second, we show that synaptonemal complex-independent associations can support full lengthwise juxtaposition of homologous chromosomes. Third, we reveal a prominent elongation of chromosome territories during meiotic prophase that initiates prior to homolog association and alignment. Mutant analysis indicates that chromosome movement mediated by association of chromosome pairing centers (PCs) with mobile patches of the nuclear envelope (NE)–spanning SUN-1/ZYG-12 protein complexes is not the primary driver of territory elongation. Moreover, we identify new roles for the X chromosome PC (X-PC) and X-PC binding protein HIM-8 in promoting elongation of X chromosome territories, separable from their role(s) in mediating local stabilization of pairing and association of X chromosomes with mobile SUN-1/ZYG-12 patches. Further, we present evidence that HIM-8 functions both at and outside of PCs to mediate chromosome territory elongation. These and other data support a model in which synapsis-independent elongation of chromosome territories, driven by PC binding proteins, enables lengthwise juxtaposition of chromosomes, thereby facilitating assessment of their suitability as potential pairing partners.

Manipulation of Karyotype inCaenorhabditis elegansReveals Multiple Inputs Driving Pairwise Chromosome Synapsis During Meiosis

“…Surprisingly, the GFP transgene arrays used as FISH targets for these experiments behaved very differently from each other in the premeiotic region of the germ line. Whereas no pairing between the arrays was detected in premeiotic germ cell nuclei of nT1/nT1/+/+ tetraploids, consistent with the known behavior of endogenous loci in diploids (MacQueen et al 2002;Nabeshima et al 2011), the transgene arrays were very consistently paired in premeiotic nuclei in mIn1/ mIn1/+/+ tetraploids ( Figure 5D, zone 1). This premeiotic pairing may reflect heterochromatin-like associations, as histone modifications characteristic of heterochromatin are known to be enriched on repetitive transgene arrays in C. elegans germ cells (Bessler et al 2010).…”

“…Probe DNAs were subsequently labeled with either Alexa Fluor 488 or 594 using the Ulysis DNA labeling kit (Life Technologies). Gonad dissection, permeabilization, fixation, hybridization, and DAPI counterstaining were performed as described in Nabeshima et al (2011). Figure 2B were obtained as stacks of optical sections acquired at 0.1-mm intervals using the DeltaVision deconvolution microscope system and full projections of dividing spermatocytes were generated using SoftWoRx Suite software.…”

History of research on C. elegans and other free-living nematodes as model organisms

“…Specific protein complexes associate and regulate double-stranded breaks, crossovers, short and long arms of chromosomes, and apposed homologs versus chromatids (Rog and Dernburg, 2013;Lui and Colaiacovo, 2013). The V shape (Y shape in Nabeshima et al, 2005) is transient in early diplotene, when the long arms but not the short arms have separated. When the short arm chromatids separate, it becomes a tetrad (Chi or X shape; Nabeshima et al, 2005).…”

“…The V shape (Y shape in Nabeshima et al, 2005) is transient in early diplotene, when the long arms but not the short arms have separated. When the short arm chromatids separate, it becomes a tetrad (Chi or X shape; Nabeshima et al, 2005). Further condensation, especially of the short arm, results in the Phi shape described above, and finally a bivalent, where the long arms are located outside and will attach to the spindle microtubules.…”

DNA Helicase HIM-6/BLM Both Promotes MutSγ-Dependent Crossovers and Antagonizes MutSγ-Independent Interhomolog Associations During Caenorhabditis elegans Meiosis

“…Chromosome painting: Chromosome painting was conducted as in Nabeshima et al (2011), using the two-color X chromosome probe.…”