Telomere Attachment, Meiotic Chromosome Condensation, Pairing, and Bouquet Stage Duration Are Modified in Spermatocytes Lacking Axial Elements

Abstract

During the extended prophase to the meiosis I division, chromosomes assemble axial elements (AE) along replicated sister chromatids whose ends attach to the inner nuclear membrane (NM) via a specialized conical thickening. Here, we show at the EM level that in Sycp3 ؊/؊ spermatocyte chromosomes lack the AE and the conical end thickening, but still they attach their telomeres to the inner NM with an electron-dense plate that contains T 2 AG 3 repeats. Immunofluorescence detected telomere proteins, SCP2, and the meiosis-specific cohesin STAG3 at the Sycp3 ؊/؊ telomere. Bouquet stage spermatocytes were approximately threefold enriched, and the number of telomere but not centromere signals was reduced to the haploid in advanced Sycp3 ؊/؊ spermatocytes, which indicates a special mode of homolog pairing at the mammalian telomere. Fluorescence in situ hybridization with mouse chromosome 8-and 12-specific subsatellite probes uncovered reduced levels of regional homolog pairing, whereas painting of chromosomes 13 revealed partial or complete juxtapositioning of homologs; however, condensation of Sycp3 ؊/؊ bivalents was defective. Electron microscopic analysis of AE-deficient spermatocytes revealed that transverse filaments formed short structures reminiscent of the synaptonemal complex central region, which likely mediate stable homolog pairing. It appears that the AE is required for chromosome condensation, rapid exit from the bouquet stage, and fine-tuning of homolog pairing. INTRODUCTIONMeiosis is essential for sexual reproduction in that it enables the independent assortment of homologous chromosomes (homologs) and the reduction of chromosome number to the haploid. To this end, homologs pair and establish stable connections during an extended prophase that precedes the reductional meiosis I division (MI). A subsequent second division (MII) separates sister chromatids, leading to formation of haploid gametes or spores. Before pairing of homologs at prophase I, these extend and attach their ends (telomeres) to the nuclear envelope. Concomitantly with this change in nuclear architecture, protein axes (axial elements; AEs) are formed along replicated sister chromatids (leptotene). Subsequently, chromosomes move and initiate stable pairing between them (zygotene) by assembly of proteinaceous transverse filaments between AEs (now called lateral elements; LEs), which forms the synaptonemal complex (SC) that connects all homologs at pachytene (von Wettstein et al., 1984;Hunter, 2003).AEs contain SCP3 and SCP2 proteins that assemble on cohesin cores that are laid down during premeiotic S phase (Prieto et al., 2001;Eijpe et al., 2003). Resolution of sister chromatid cohesion, first between chromatids during MI and subsequently between sister centromeres during the meiosis II division, allows chromosome reduction (Buonomo et al., 2000; for reviews, see Lee and Orr-Weaver, 2001;Petronczki et al., 2003). These processes have evolved by addition of meiosis-specific components to the cohesin complex. In mammals, for instance, STAG3...

Evolutionary history of the mammalian synaptonemal complex

“…Though a chromosomal cohesin core is established in the absence of an intact AE in Sycp3 -/-spermatocytes, the respective proteins, e.g. SMC1, SMC3, STAG3, localize more discontinuously and the chromosomal axes appear significantly longer than in wild-type (Yuan et al 2000;Pelttari et al 2001;Liebe et al 2004). Conversely, depletion of meiotic cohesion subunits, e.g.…”

“…Sycp3 -/-spermatocytes, which do not form AEs, SYCP1 can still bind to the chromosomal axes and form stretches of the CR although these are much shorter than in wild-type (Liebe et al 2004). It is possible that, in this case, the cohesin core is sufficient to allow the binding of the TFs (Pelttari et al 2001).…”

“…In mouse meiocytes, SYCP3 is essential for AE assembly. The AE formation is abolished in the absence of SYCP3 (Liebe et al 2004) and the second axial component SYCP2 cannot be recruited to the chromosomes anymore (Yuan et al 2000). Expression of a C-terminally truncated SYCP2 leads to a failure of AE formation in mouse spermatocytes as well (Yang et al 2006).…”

“…Simultaneously with AE formation, the telomeres of the chromosomes become attached to the inner nuclear membrane. In mammals, this attachment is achieved via specialized conical thickenings of the chromosomal axes (Liebe et al 2004). The telomeres move and cluster in the nuclear membrane close to the cytoplasmic centriole during the transition of leptotene to zygotene (zygos, greek paired).…”

Structural components of the synaptonemal complex, SYCP1 and SYCP3, in the medaka fish Oryzias latipes

“…3A, B). The intensity of signals was diminished as meiosis proceeded and the signals were completely lost in secondary spermatocytes ( The oriented expression of SC components presumably reflects the polarized nuclear organization known as the meiotic bouquet structure [43,44]. Close observations using double-labeling of frozen sections with anti-MeSYCP1 and anti-MeSYCP3 antibodies revealed that some cysts of spermatocytes expressed MeSYCP3 but not MeSYCP1 (indicated by a blue arrow in Fig.…”