Department of Biology, University of Louisiana-Lafayette, Lafayette, Louisiana 70504 USA.
Am J Bot. 2008 Jun;95(6):664-71. doi: 10.3732/ajb.2007388.
Extant liverworts are "living fossils" considered sister to all other plants and as such provide clues to the evolution of the microtubule organizing center (MTOC) in anastral cells. This report is the first on microtubule arrays and their γ-tubulin-nucleating sites during meiosis in a member of the Ricciales, a specialized, species-rich group of complex thalloid (marchantioid) liverworts. In meiotic prophase, γ-tubulin becomes concentrated at several sites adjacent to the nuclear envelope. Microtubules organized at these foci give rise to a multipolar prometaphase spindle. By metaphase I, the spindle has matured into a bipolar structure with truncated poles. In both first and second meiosis, γ-tubulin forms box-like caps at the spindle poles. γ-Tubulin moves from spindle poles to the proximal surfaces of telophase chromosomes where interzonal microtubules are nucleated. Although a phragmoplast is organized, no cell plate is deposited, and second division occurs simultaneously in the undivided sporocyte. γ-Tubulin surrounds each of the tetrad nuclei, and phragmoplasts initiated between both sister and nonsister nuclei direct simultaneous cytokinesis. The overall pattern of meiosis (unlobed polyplastidic sporocytes, nuclear envelope MTOC, multipolar spindle origin, spindles with box-like poles, and simultaneous cytokinesis) more closely resembles that of Conocephalum than other marchantiod liverworts.
现存的苔类植物是被认为与所有其他植物有亲缘关系的“活化石”，因此为无丝分裂细胞的微管组织中心（MTOC）进化提供了线索。本报告是关于 Ricciales 成员减数分裂过程中微管排列及其γ-微管蛋白成核位点的第一个报告，Ricciales 是一个专门的、物种丰富的复杂叶状（新月形）苔类植物群。在减数分裂前期，γ-微管蛋白集中在靠近核膜的几个位点。在这些焦点组织的微管产生了一个多极的前中期纺锤体。到中期 I 时，纺锤体已经成熟为具有截断极的双极结构。在第一次和第二次减数分裂中，γ-微管在纺锤体的两极形成盒状帽。γ-微管从纺锤体两极移动到末期染色体的近端表面，在那里形成间区微管。尽管组织了一个成膜体，但没有沉积细胞板，并且第二次分裂同时发生在未分裂的孢子细胞中。γ-微管围绕着每个四分体核，在姐妹核和非姐妹核之间启动的成膜体指导同时的胞质分裂。减数分裂的整体模式（无裂片的多质体孢子细胞、核膜 MTOC、多极纺锤体起源、具有盒状极的纺锤体和同时的胞质分裂）更类似于 Conocephalum，而不是其他新月形苔类植物。
