Interlock Formation and Coiling of Meiotic Chromosome Axes During Synapsis

Abstract

The meiotic prophase chromosome has a unique architecture. At the onset of leptotene, the replicated sister chromatids are organized along an axial element. During zygotene, as homologous chromosomes pair and synapse, a synaptonemal complex forms via the assembly of a transverse element between the two axial elements. However, due to the limitations of light and electron microscopy, little is known about chromatin organization with respect to the chromosome axes and about the spatial progression of synapsis in three dimensions. Three-dimensional structured illumination microscopy (3D-SIM) is a new method of superresolution optical microscopy that overcomes the 200-nm diffraction limit of conventional light microscopy and reaches a lateral resolution of at least 100 nm. Using 3D-SIM and antibodies against a cohesin protein (AFD1/REC8), we resolved clearly the two axes that form the lateral elements of the synaptonemal complex. The axes are coiled around each other as a left-handed helix, and AFD1 showed a bilaterally symmetrical pattern on the paired axes. Using the immunostaining of the axial element component (ASY1/HOP1) to find unsynapsed regions, entangled chromosomes can be easily detected. At the late zygotene/early pachytene transition, about one-third of the nuclei retained unsynapsed regions and 78% of these unsynapsed axes were associated with interlocks. By late pachytene, no interlocks remain, suggesting that interlock resolution may be an important and rate-limiting step to complete synapsis. Since interlocks are potentially deleterious if left unresolved, possible mechanisms for their resolution are discussed in this article.

Spatiotemporal Asymmetry of the Meiotic Program Underlies the Predominantly Distal Distribution of Meiotic Crossovers in Barley

“…monocots (Mikhailova et al, 2006;Wang et al, 2009), and as Arabidopsis ASY1 shares 72% amino acid sequence identity with barley ASY1 (see Supplemental Figure 1 online), it was anticipated that the anti-At-ASY1 Ab would be an effective marker for axis formation. Following meiotic S-phase, up to 28 telomeric signals can be detected using FISH with a telomeric repeat sequence probe.…”

Live imaging of rapid chromosome movements in meiotic prophase I in maize

“…Chromosome pairing and most steps of meiotic recombination take place in zygonema. By pachynema, pairing is complete, but chromosomes show frequent entanglements (interlocks) that must be removed (28,36). Analyses of prophase I chromosome motility shed light on how chromosome mechanics may aid these processes.…”

Contrasting roles of condensin I and II in mitotic chromosome formation

“…Three-dimensional structured illumination microscopy (3D-SIM) of the distinctive kinking and twisting phenotype displayed in mitotic chromosomes following CAP-D3 (condensin II) depletion revealed apparent cross-overs of sister chromatids reminiscent of meiotic cross-overs seen using 3D-SIM (Wang et al, 2009). We suggest that chromosome compaction is a two-step process, with condensin II mediating long-range DNA interactions and establishing an initial chromatin axis that subsequently allows condensin I to mediate short-range lateral interactions and formation of compact loops of chromatin.…”