Colas Isabelle, Darrier Benoit, Arrieta Mikel, Mittmann Sybille U, Ramsay Luke, Sourdille Pierre, Waugh Robbie
Cell and Molecular Sciences, The James Hutton InstituteDundee, United Kingdom.
Institut National de la Recherche Agronomique UMR 1095, Génétique, Diversité & Ecophysiologie des CéréalesClermont-Ferrand, France.
Front Plant Sci. 2017 Jul 13;8:1235. doi: 10.3389/fpls.2017.01235. eCollection 2017.
The production of balanced fertile haploid gametes requires the faithful separation of paired (synapsed) chromosomes toward the end of meiotic prophase I (desynapsis). This involves the timely dissolution of the synaptonemal complex during the pachytene-diplotene transition, a stage traditionally referred to as the "diffuse stage." In species with large genomes such as, barley ( L.) and wheat ( L.) we know most about the early stages of meiotic prophase I. There, synapsis initiates at the telomeric ends of chromosomes and progresses toward the centromeric regions through the ordered assembly of the synaptonemal complex (SC). Synapsis is impacted by recombination (crossing over, CO) which locally modifies the extent of chromatin compaction and extension. CO is uneven along the chromosomes, occurring mainly toward the telomeric regions resulting in a highly skewed distribution of recombination events. However, we know very little about the process of desynapsis which occurs during the "diffuse stage," where the synapsed and recombined chromosomes faithfully desynapse and separate into daughter cells. Here, using 3D-SIM super-resolution immuno-cytology combined with the use of antibodies directed against two crucial SC proteins, ASY1 and ZYP1, we followed the whole of meiosis I (i.e., both synapsis and desynapsis) in both barley and wheat. We showed that synapsis forms a characteristic tri-partite SC structure in zygotene (more clearly seen in barley). Toward the end of meiosis I, as the SC starts to disassemble, we show that extensive chromosome axis remodeling results in the formation of characteristic "tinsel-like" structures in both wheat and barley. By using a mutant () that is severely compromised in polymerization of ZYP1during synapsis, we show that tinsel structure formation during SC dissolution is not dependant on full synapsis and may relate instead to changes in expansion stress. Our observations highlight a potentially new role for ASYNAPSIS1 (ASY1) in desynapsis, in addition to chromosome synapsis and cohesion.
产生平衡可育的单倍体配子需要在减数分裂前期I末期（去联会）将配对（联会）的染色体准确分离。这涉及到在粗线期-双线期转变过程中及时溶解联会复合体，这个阶段传统上被称为“弥散期”。在大麦（Hordeum vulgare L.）和小麦（Triticum aestivum L.）等具有大基因组的物种中，我们对减数分裂前期I的早期阶段了解最多。在那里，联会起始于染色体的端粒末端，并通过联会复合体（SC）的有序组装向着丝粒区域推进。联会受到重组（交叉互换，CO）的影响，重组会局部改变染色质压缩和伸展的程度。CO在染色体上分布不均，主要发生在端粒区域，导致重组事件的高度偏态分布。然而，我们对在“弥散期”发生的去联会过程知之甚少，在这个时期，联会并重组的染色体准确地去联会并分离到子细胞中。在这里，我们使用3D-SIM超分辨率免疫细胞学技术，并结合针对两种关键SC蛋白ASY1和ZYP1的抗体，追踪了大麦和小麦整个减数分裂I过程（即联会和去联会）。我们发现，在偶线期，联会形成了特征性的三分体SC结构（在大麦中更清晰可见）。在减数分裂I末期，随着SC开始解体，我们发现广泛的染色体轴重塑导致小麦和大麦中都形成了特征性的“流苏状”结构。通过使用一个在联会期间ZYP1聚合严重受损的突变体（asy1），我们表明在SC解体过程中流苏结构的形成不依赖于完全联会，而可能与扩张应力的变化有关。我们的观察结果突出了ASY1在去联会过程中除了染色体联会和黏连之外的潜在新作用。
