Department of Biology, University of Louisiana-Lafayette, Lafayette, LA, 70504-2451, USA.
J Plant Res. 2010 Jul;123(4):589-605. doi: 10.1007/s10265-009-0286-9. Epub 2009 Dec 29.
As the earliest divergent land plants, bryophytes (mosses, hornworts, and liverworts) provide insight into the evolution of the unique plant process of sporogenesis by which meiosis results in heavy walled spores. New immunohistochemical data on microtubules and gamma-tubulin in four genera of complex thalloid liverworts combined with previously published data on another four genera demonstrate grades in the evolution of spindle organization in meiosis. We have discovered that all recognized forms of microtubule organizing centers (MTOCs) in plant cells (plastid MTOCs, spheroid cytoplasmic MTOCs, polar organizers, and nuclear envelope MTOCs) occur in organization of the meiotic spindle of complex thalloid liverworts. In addition, all aspects of pre-meiotic preparation for quadripartitioning of the sporocyte into a tetrad of spores occur, with the exception of pre-meiotic wall precursors found in certain simple thalloids. The preparation includes morphogenetic plastid migration, cortical bands of microtubules that mark future cytokinetic planes in pre-meiosis, quadrilobing of the cytoplasm during meiotic prophase, and quadripolar microtubule systems that are transformed into functionally bipolar metaphase I spindles. Quadripolar spindle origin is typical of bryophyte sporogenesis even though the MTOCs involved may differ. However, in certain crown taxa of complex thalloids the spindle develops with no traces of quadripolarity and placement of intersporal walls is determined after meiosis, as is typical of higher plants.
作为最早分化的陆生植物，苔藓植物（苔藓、角苔和地钱）为植物减数分裂形成厚壁孢子的孢子发生过程的演化提供了线索。在四个复杂的叶状体地钱属中对微管和γ-微管的新免疫组织化学数据，结合之前对另外四个属的发表数据，证明了减数分裂纺锤体组织进化的阶段。我们发现，植物细胞中所有公认的微管组织中心（MTOC）形式（质体 MTOC、球形细胞质 MTOC、极性组织器和核膜 MTOC）都存在于复杂叶状体地钱减数分裂纺锤体的组织中。此外，除了在某些简单叶状体中发现的减数分裂前细胞壁前体之外，孢子体四分体分裂为四个孢子的所有减数分裂前准备都发生了。准备包括形态发生的质体迁移、在减数分裂前标记未来细胞分裂平面的皮质微管带、减数分裂前期细胞质的四叶状化，以及转化为功能双极有丝分裂 I 纺锤体的四极微管系统。尽管涉及的 MTOC 可能不同，但四极纺锤体的起源是苔藓植物孢子发生的典型特征。然而，在某些复杂叶状体的冠群类群中，纺锤体的发育没有四极性的痕迹，并且在减数分裂后确定孢子间壁的位置，这是高等植物的典型特征。
