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.
Highlights
The success of sexual reproduction relies on the ability of diploid germ cells to generate haploid gametes through the specialized cell division program of meiosis
In order to test whether pairing centers (PCs)–SUN-1/ZYG-12-mediated movement of chromosomes is required for chromosome elongation, we evaluated territory elongation in mutants defective for chk-2, him-3, and syp-1
While it was clear from prior analysis that homologs achieve de novo alignment during C. elegans meiosis [29], the possibility remained that homologous territories might exhibit preferential proximity that could facilitate chromosome sorting
Summary
The success of sexual reproduction relies on the ability of diploid germ cells to generate haploid gametes through the specialized cell division program of meiosis. The common feature of these large-scale spatial reorganization mechanisms involves tethering of one or two specified site(s) on a chromosome to conserved nuclear envelope (NE)- spanning protein complexes, thereby coupling the chromosomes to the cytoskeletal motility apparatus that can transmit forces to drive chromosome movement. Such forces and movements have been proposed to enhance the efficiency of chromosome sorting both by providing opportunities for homology assessment and by destabilizing interactions between incorrect partners. Our understanding remains limited regarding how localized chromosome tethering sites might mediate recognition and bring about colocalization along the entire lengths of chromosomes
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