Transformations of the Pleiomorphic Plant MTOC during Sporogenesis in the Hepatic Marchantia polymorpha

Sporogenesis in the hepatic Marchantia polymorpha L. provides an outstanding example of the pleiomorphic nature of the plant microtubule organizing center （MTOC）. Microtubules are nucleated from γ-tubuUn in MTOCs that change form during mitosis and meiosis. Following entry of cells into the reproductive pathway of sporogenesis, successive rounds of mitosis give rise to packets of 4-16 sporocytes. Mitotic spindles are organized at discrete polar organizers （POs）, a type of MTOC that is unique to this group of early divergent land plants. An abrupt and radical transformation in microtubule organization occurs when sporocytes enter meiosis： POs are lost and γ-tubulin is closely associated with surfaces of two large elongated plastids that subsequently divide into four. Migration of the four plastid MTOCs into a tetrahedral arrangement establishes the future spore domains and the division polarity of meiosis. As is typical of many bryophytes, cones of microtubules from the four plastid MTOCs initiate a quadripolar microtubule system （QMS） in meiotic prophase. At this point a transformation in the organization of the MTOCs occurs. The γ-tubulin detaches from plastids and forms a diffuse spheroidal pole in each of the spore domains. The plastids, which are no longer MTOCs, continue to divide. The diffuse MTOCs continue to nucleate cones of microtubules during transformation of the QMS to a bipolar spindle. Following meiosis I, γ-tubulin is associated with nuclear envelopes, and the spindles of meiosis II are organized from diffuse MTOCs at the tetrad poles. At simultaneous cytokinesis, radial microtubule systems are organized at nuclear envelope MTOCs in each of the tetrad members.     

Morphogenetic plastid migration and microtubule organization during megasporogenesis inIsoetes

Summary The large megasporocytes ofIsoetes provide an exceptional system for studying microtubule dynamics in monoplastidic meiosis where plastid polarity assures coordination of plastid and nuclear division by the intimate association of MTOCs with plastids. Division and migration of the plastid in prophase establishes the tetrahedrally arranged cytoplasmic domains of the future spore tetrad and the four plastid-MTOCs serve as focal points of a unique quadripolar microtubule system (QMS). The QMS is a dynamic structure which functions in plastid deployment and contributes directly to development of both first and second division spindles. The nucleation of microtubules at discrete plastid-MTOCs is compared with centrosomal nucleation of microtubules in animal cells where growth of microtubules involves dynamic instability.Abbreviations AMS axial microtubule system - MTOC microtubule organizing center - N nucleus - QMS quadripolar microtubule system - P plastid - PPB preprophase band of microtubules     

Spindle Dynamics and the Role of γ-Tubulin in Early Caenorhabditis elegans Embryos

gamma-Tubulin is a ubiquitous and highly conserved component of centrosomes in eukaryotic cells. Genetic and biochemical studies have demonstrated that gamma-tubulin functions as part of a complex to nucleate microtubule polymerization from centrosomes. We show that, as in other organisms, Caenorhabditis elegans gamma-tubulin is concentrated in centrosomes. To study centrosome dynamics in embryos, we generated transgenic worms that express GFP::gamma-tubulin or GFP::beta-tubulin in the maternal germ line and early embryos. Multiphoton microscopy of embryos produced by these worms revealed the time course of daughter centrosome appearance and growth and the differential behavior of centrosomes destined for germ line and somatic blastomeres. To study the role of gamma-tubulin in nucleation and organization of spindle microtubules, we used RNA interference (RNAi) to deplete C. elegans embryos of gamma-tubulin. gamma-Tubulin (RNAi) embryos failed in chromosome segregation, but surprisingly, they contained extensive microtubule arrays. Moderately affected embryos contained bipolar spindles with dense and long astral microtubule arrays but with poorly organized kinetochore and interpolar microtubules. Severely affected embryos contained collapsed spindles with numerous long astral microtubules. Our results suggest that gamma-tubulin is not absolutely required for microtubule nucleation in C. elegans but is required for the normal organization and function of kinetochore and interpolar microtubules.     

Diversity in meiotic spindle origin and determination of cytokinetic planes in sporogenesis of complex thalloid liverworts (Marchantiopsida)

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 γ-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.     

MONOPLASTIDIC CELL DIVISION IN LOWER LAND PLANTS

In many bryophytes and vascular cryptogams mitosis and/or meiosis takes place in cells containing a single plastid. In monoplastidic cell division plastid polarity assures that nuclear and plastid division are infallibly coordinated. The two major components of plastid polarity are morphogenetic plastid migration and microtubule organization at the plastids. Before nuclear division the plastid migrates to a position intersecting the future division plane. This morphogenetic migration is a reliable marker of division polarity in cells with and without a preprophase band of microtubules (PPB). The PPB, which predicts the future division plane before mitosis, is a characteristic feature of land plants and its insertion into the cytokinetic apparatus marks the evolution of a cortical microtubule system and a commitment to meristematic growth. Microtubule systems associated with plastid division, the axial microtubule system (AMS) in mitosis and the quadripolar microtubule system (QMS) in meiosis, contribute to predictive positioning of plastids and participate directly in spindle ontogeny. Division polarity in monoplastidic sporocytes is remarkable in that division sites are selected prior to the two successive nuclear divisions of meiosis. Plastid arrangement prior to meiosis determines the future spore domains in monoplastidic sporocytes, whereas in polyplastidic sporocytes the spore nuclei play a major role in claiming cytoplasmic domains. It is hypothesized that predivision microtubule systems associated with monoplastidic cell division are early forming components of the mitotic apparatus that serve to orient the spindle and insure equal apportionment of nucleus and plastids. “Can it be supposed that cytoplasm would be intrusted with so important a task as the preparation of a chloroplast for each of the four nuclei that are later to preside over the spores before there is any indication that such nuclear division is to take place?” Bradley Moore Davis, 1899     

Morphogenetic plastid migration and microtubule arrays in mitosis and cytokinesis in the green alga Coleochaete orbicularis

This study provides data on cell division in Coleochaete orbicularis, an important taxon in evolutionary theories deriving land plants from green algae. Vegetative growth in discoid species of Coleochaete results from marginal cell division in two planes—radial and circumferential. Like many algae and certain of the simple land plants, Coleochaete is monoplastidic. Prior to mitosis, the single plastid migrates to a position where it will divide and be distributed into the daughter cells. Unlike monoplastidic cell division in hornworts, mosses, and lycopsids; microtubule nucleation is not intimately associated with the plastids. Instead, microtubule organization is associated with centriolar centrosomes throughout the cell cycle, as is common in algae. The cytokinetic apparatus lacks preprophase bands of microtubules, but includes typical phragmoplasts consisting of brushlike arrays of microtubules on either side of a dark zone. However, the origin and role of phragmoplasts is unusual. Phragmoplasts appear to develop among microtubules that emanate from the polar centrosomes rather than from nuclear envelopes and/or plastids. The function of phragmoplasts in Coleochaete is unclear, as the process of cytokinesis is not strictly centrifugal. Some infurrowing occurs in radial division, and cytokinesis appears to be entirely centripetal by infurrowing in circumferential division. The cortical arrays of microtubules differ from those typical of land plants in that they develop as a network in association with centrosomes after mitosis.     

Plastid polarity and meiotic spindle development in microsporogenesis ofSelaginella

Summary Microsporogenesis inSelaginella was studied by fluorescence light microscopy and transmission electron microscopy. As in other examples of monoplastidic meiosis the plastids are involved in determination of division polarity and organization of microtubules. However, there are important differences: (1) the meiotic spindle develops from a unique prophase microtubule system associated with two plastids rather than from a typical quadripolar microtubule system associated with four plastids; (2) the division axes for first and second meiotic division are established sequentially, whereas as in all other cases the poles of second division are established before those of first division; and (3) the plastids remain in close contact with the nucleus throughout meiotic prophase and provide clues to the early determination of spindle orientation. In early prophase the single plastid divides in the plane of the future division and the two daughter plastids rotate apart until they lie on opposite sides of the nucleus. The procytokinetic plate (PCP) forms in association with the two slender plastids; it consists of two spindle-shaped microtubule arrays focused on the plastid tips with a plate of vesicles at the equatorial region and a picket row of microtubules around one side of the nucleus. Second plastid division occurs just before metaphase and the daughter plastids remain together at the spindle poles during first meiotic division. The meiotic spindle develops from merger of the component arrays of the PCP and additional microtubules emanating from the pair of plastid tips located at the poles. After inframeiotic interphase the plastids migrate to tetrahedral arrangement where they serve as poles of second division.Abbreviations AMS axial microtubule system - FITC fluorescein isothiocyanate - MTOC microtubule organizing center - PCP procytokinetic plate - QMS quadripolar microtubule system - TEM transmission electron microscope (microscopy)     

Behavior of delta-tubulin during spindle formation in Xenopus oocytes: requirement of cytoplasmic dynein-dependent translocation

Vertebrate oocytes do not contain centrosomes and therefore form an acentrosomal spindle during oocyte maturation. gamma-Tubulin is known to be essential for nucleation of microtubules at centrosomes, but little is known about the behaviour and role of gamma-tubulin during spindle formation in oocytes. We first observed sequential localization of gamma-tubulin during spindle formation in Xenopus oocytes. gamma-Tubulin assembled in the basal regions of the germinal vesicle (GV) at the onset of germinal vesicle breakdown (GVBD) and remained on the microtubule-organizing centre (MTOC) until a complex of the MTOC and transient-microtubule array (TMA) reached the oocyte surface. Prior to bipolar spindle formation, oocytes formed an aggregation of microtubules and gamma-tubulin was concentrated at the centre of the aggregation. At the late stage of bipolar spindle formation, gamma-tubulin accumulated at each pole. Anti-dynein antibody disrupted the localization of gamma-tubulin, indicating that the translocation described above is dependent on dynein activity. We finally revealed that XMAP215, a microtubule-associated protein cooperating with gamma-tubulin for the assembly of microtubules, but not gamma-tubulin, was phosphorylated during oocyte maturation. These results suggest that gamma-tubulin is translocated by dynein to regulate microtubule organization leading to spindle formation and that modification of the molecules that cooperate with gamma-tubulin, but not gamma-tubulin itself, is important for microtubule reorganization.     

The quadripolar microtubule system in lower land plants

The quadripolar microtubule system (QMS) is a complex array that is associated with predivision establishment of quadripolarity in sporocytes of lower plants (bryophytes and lycopsids). The QMS unerringly predicts the polarity of the two meiotic divisions and plays a central role in development of both the mitotic apparatus (MA) and cytokinetic apparatus (CA) which together accomplish quadripartitioning of the sporocyte into four haploid spores. The QMS is typically, but not exclusively, associated with monoplastidy and precocious quadrilobing of the cytoplasm. In early meiotic prophase the single plastid divides and the resultant plastids migrate so that either the tips of two plastids or the four plastids resulting from a second division are located in the future spore domains. Microtubules that emanate from the plastid tips or from individual plastids in the spore domains interact in the future planes of cytokinesis and give rise to the QMS. The QMS, which encages the prophase nucleus, consists of at least four and usually six (when spore domains are in tetrahedral arrangement) bipolar spindle-like arrays of microtubules presumably with minus ends at plastids in spore domains and plus ends interacting in the future plane of cytokinesis. Each of the six arrays is essentially like the single axial microtubule system (AMS) that intersects the division site and is transformed into the spindle in monoplastidic mitosis in hornworts. As comparative data accumulate, it appears that the AMS is not unique to monoplastidic cell division but instead represents a basic microtubule arrangement that survives as spindle and phragmoplast in cell division of higher plants.     

THE QUADRIPOLAR MICROTUBULE SYSTEM AND MEIOTIC SPINDLE ONTOGENY IN HORNWORTS (BRYOPHYTA: ANTHOCEROTAE)

Ontogeny of the meiotic spindle in hornworts was studied by light microscopy of live materials, transmission electron microscopy, and indirect immunofluorescence microscopy. As in monoplastidic meiosis of mosses and Isoetes, the single plastid divides twice, and the four resultant plastids migrate into the future spore domains where they organize a quadripolar microtubule system (QMS). Additionally, a unique axial microtubule system (AMS) was found to parallel the plastid isthmus at each division in meiosis, much as in the single plastid division of mitosis. This finding is used to make a novel comparison of mitotic and meiotic spindle development. The AMS contributes directly to development of the mitotic spindle, whereas ontogeny of the meiotic spindle is more complex. Nuclear division in meiosis is delayed until after the second plastid division; the first AMS disappears without spindle formation, and the two AMSs of the second plastid division contribute to development of the QMS. Proliferation of microtubules at each plastid results in the QMS consisting of four cones of microtubules interconnecting the plastids and surrounding the nucleus. The QMS contributes to the development of a functionally bipolar spindle. The meiotic spindle is comparable to a merger of two mitotic spindles. However, the first division spindle does not terminate in what would be the poles of mitosis; instead the poles converge to orient the spindle axis midway between pairs of non-sister plastids.     

Microtubule patterning during meiotic maturation in mouse oocytes is determined by cell cycle-specific sorting and redistribution of gamma-tubulin.

The topography of microtubule assembly events during meiotic maturation of animal oocytes demands tight spatial control and temporal precision. To better understand what regulates the timing and location of microtubule assembly, synchronously maturing mouse oocytes were evaluated with respect to gamma-tubulin, pericentrin, and total tubulin polymer fractions at specific stages of meiotic progression. gamma-Tubulin remained associated with cytoplasmic centrosomes through diakinesis of meiosis-1. Following chromatin condensation and perinuclear centrosome aggregation, gamma-tubulin relocated to a nuclear lamina-bounded compartment in which meiosis-1 spindle assembly occurred. gamma-Tubulin was stably associated with the meiotic spindle from prometaphase-1 through to anaphase-2, but also exhibited cell cycle-specific relocalization to cytoplasmic centrosomes. Specifically, anaphase onset of both meiosis-1 and -2 was characterized by the concomitant appearance of gamma-tubulin and microtubule nucleation in subcortical centrosomes. Brief pulses of taxol applied at specific cell cycle stages enhanced detection of gamma-tubulin compartmentalization, consistent with a gamma-tubulin localization-dependent spatial restriction of microtubule assembly during meiotic progression. In addition, a taxol pulse during meiotic resumption impaired subsequent gamma-tubulin sorting, resulting in monopolar spindle formation and cell cycle arrest in meiosis-1; despite cell cycle arrest, polar body extrusion occurred roughly on schedule. Therefore, sorting of gamma-tubulin is involved in both the timing of location of meiotic spindle assembly as well as the coordination of karyokinesis and cytokinesis in mouse oocytes.     

The Pleiomorphic Plant MTOC： An Evolutionary Perspective

γ-Tubulin is an essential component of the microtubule organizing center （MTOC） responsible for nucleating microtubules in both plants and animals. Whereas γ-tubulin is tightly associated with centrosomes that are inheritable organelles in cells of animals and most algae, it appears at different times and places to organize the myriad specialized microtubule systems that characterize plant cells. We have traced the distribution of γ-tubulin through the cell cycle in representative land plants （embryophytes） and herein present data that have led to a concept of the pleiomorphic and migratory MTOC. The many forms of the plant MTOC at spindle organization constitute pleiomorphism, and stage-specific “migration” is suggested by the consistent pattern of redistribution of γ-tubulin during mitosis. Mitotic spindles may be organized at centriolar centrosomes （only in final divisions of spermatogenesis）, polar organizers （POs）, plastid MTOCs, or nuclear envelope MTOCs （NE-MTOCs）. In all cases, with the possible exception of centrosomes in spermatogenesis, the γ-tubulin migrates to broad polar regions and along the spindle fibers, even when it is initially a discrete polar entity. At anaphase it moves poleward, and subsequently migrates from polar regions （distal nuclear surfaces） into the interzone （proximal nuclear surfaces） where interzonal microtubule arrays and phragmoplasts are organized. Following cytokinesis, γ-tubulin becomes associated with nuclear envelopes and organizes radial microtubule systems （RMSs）. These may exist only briefly, before establishment of hoop-like cortical arrays in vegetative tissues, or they may be characteristic of interphase in syncytial systems where they serve to organize the common cytoplasm into nuclear cytoplasmic domains （NCDs）.     

How do Plants Organize Microtubules Without a Centrosome？

A microtubule nucleates from a γ-tubuUn complex, which consists of γ-tubulin, proteins from the SPC971SPC98 family, and the WD40 motif protein GCP-WD. We analyzed the phylogenetic relationships of the genes encoding these proteins and found that the components of this complex are widely conserved among land plants and other eukaryotes. By contrast, the interphase and mitotic arrays of microtubules in land plants differ from those in other eukaryotes. In the interphase cortical array, the majority of microtubules nucleate on existing microtubules in the absence of conspicuous microtubule organizing centers （MTOCs）, such as a centrosome. During mitosis, the spindle also forms in the absence of conspicuous MTOCs. Both poles of the spindle are broad, and branched structures of microtubules called microtubule converging centers form at the poles. In this review, we hypothesize that the microtubule converging centers form via microtubule-dependent microtubule nucleation, as in the case of the interphase arrays. The evolutionary insights arising from the molecular basis of the diversity in microtubule organization are discussed.     

gamma-tubulin complexes: binding to the centrosome, regulation and microtubule nucleation

Microtubule assembly is initiated in vivo by gamma-tubulin complexes. Cytoplasmic gamma-tubulin complexes are targeted to centrosomes or to other microtubule organizing centers (MTOCs) via a set of so called gamma-tubulin complex binding proteins (GTBPs) that probably interact with the conserved Spc97p/Spc98p protein family of gamma-tubulin complexes. In other cell types, gamma-tubulin complexes may initiate microtubule formation near chromosomes in a MTOC-independent manner. Recently, major advances have been achieved through the finding that gamma-tubulin, Spc97p and Spc98p form a conserved core that is probably responsible for microtubule nucleation, and by the discovery that a yeast GTBP is regulated in a cell-cycle-dependent manner and in response to an external signal. Furthermore, it was found that the small GTPase Ran in its GDP-bound state may promote spindle assembly. In addition, an essential function of gamma-tubulin in basal body duplication has been demonstrated in Paramecium.     

Plant gamma-tubulin interacts with alphabeta-tubulin dimers and forms membrane-associated complexes

gamma-Tubulin is assumed to participate in microtubule nucleation in acentrosomal plant cells, but the underlying molecular mechanisms are still unknown. Here, we show that gamma-tubulin is present in protein complexes of various sizes and different subcellular locations in Arabidopsis and fava bean. Immunoprecipitation experiments revealed an association of gamma-tubulin with alphabeta-tubulin dimers. gamma-Tubulin cosedimented with microtubules polymerized in vitro and localized along their whole length. Large gamma-tubulin complexes resistant to salt treatment were found to be associated with a high-speed microsomal fraction. Blue native electrophoresis of detergent-solubilized microsomes showed that the molecular mass of the complexes was >1 MD. Large gamma-tubulin complexes were active in microtubule nucleation, but nucleation activity was not observed for the smaller complexes. Punctate gamma-tubulin staining was associated with microtubule arrays, accumulated with short kinetochore microtubules interacting in polar regions with membranes, and localized in the vicinity of nuclei and in the area of cell plate formation. Our results indicate that the association of gamma-tubulin complexes with dynamic membranes might ensure the flexibility of noncentrosomal microtubule nucleation. Moreover, the presence of other molecular forms of gamma-tubulin suggests additional roles for this protein species in microtubule organization.     

Sporogenesis in Eusporangiate Ferns: I. Monoplastidic Meiosis in Angiopteris (Marattiales)

Angiopteris (Marattiales) undergoes the more primitive form of monoplastidic meiosis, while other ferns have evolved the polyplastidic type typical of seed plants. In monoplastidic cell division, the single plastid divides and serves as site of the microtubule organizing center (MTOC) for spindle formation resulting in coordinated division of plastid, nucleus, and cytoplasm. In plants with polyplastidic cell division, the MTOC is diffuse and generally perinuclear. Monoplastidic cell division is seen as a plesiomorphic feature that was inherited from algal ancestors containing a single plastid and modified through evolution. Monoplastidic meiosis occurs in all groups of bryophytes (although in only a few hepatics), Isoetes, Selaginella, certain generic segregates of Lycopodium, and in members of the Marattiales. It is not known to occur in psilophytes, Equisetum, leptosporangiate ferns, or seed plants. Received 30 January 2001/ Accepted in revised form 24 April 2001     

Gamma-tubulin is essential for microtubule organization and development in Arabidopsis

The process of microtubule nucleation in plant cells is still a major question in plant cell biology. gamma-Tubulin is known as one of the key molecular players for microtubule nucleation in animal and fungal cells. Here, we provide genetic evidence that in Arabidopsis thaliana, gamma-tubulin is required for the formation of spindle, phragmoplast, and cortical microtubule arrays. We used a reverse genetics approach to investigate the role of the two Arabidopsis gamma-tubulin genes in plant development and in the formation of microtubule arrays. Isolation of mutants in each gene and analysis of two combinations of gamma-tubulin double mutants showed that the two genes have redundant functions. The first combination is lethal at the gametophytic stage. Disruption of both gamma-tubulin genes causes aberrant spindle and phragmoplast structures and alters nuclear division in gametophytes. The second combination of gamma-tubulin alleles affects late seedling development, ultimately leading to lethality 3 weeks after germination. This partially viable mutant combination enabled us to follow dynamically the effects of gamma-tubulin depletion on microtubule arrays in dividing cells using a green fluorescent protein marker. These results establish the central role of gamma-tubulin in the formation and organization of microtubule arrays in Arabidopsis.     

Microtubule nucleation at non-spindle pole body microtubule-organizing centers requires fission yeast centrosomin-related protein mod20p

BACKGROUND: Many types of differentiated eukaryotic cells display microtubule distributions consistent with nucleation from noncentrosomal intracellular microtubule organizing centers (MTOCs), although such structures remain poorly characterized. In fission yeast, two types of MTOCs exist in addition to the spindle pole body, the yeast centrosome equivalent. These are the equatorial MTOC, which nucleates microtubules from the cell division site at the end of mitosis, and interphase MTOCs, which nucleate microtubules from multiple sites near the cell nucleus during interphase. RESULTS: From an insertional mutagenesis screen we identified a novel gene, mod20+, which is required for microtubule nucleation from non-spindle pole body MTOCs in fission yeast. Mod20p is not required for intranuclear mitotic spindle assembly, although it is required for cytoplasmic astral microtubule growth during mitosis. Mod20p localizes to MTOCs throughout the cell cycle and is also dynamically distributed along microtubules themselves. We find that mod20p is required for the localization of components of the gamma-tubulin complex to non-spindle pole body MTOCs and physically interacts with the gamma-tubulin complex in vivo. Database searches reveal a family of eukaryotic proteins distantly related to mod20p; these are found in organisms ranging from fungi to mammals and include Drosophila centrosomin. CONCLUSIONS: Mod20p appears to act by recruiting components of the gamma-tubulin complex to non-spindle pole body MTOCs. The identification of mod20p-related proteins in higher eukaryotes suggests that this may represent a general mechanism for the organization of noncentrosomal MTOCs in eukaryotic cells.     

Sporogenesis in Bryophytes: Patterns and Diversity in Meiosis

Meiosis in bryophytes retains unusual features that provide clues to the innovation of sporogenesis in early land plants. Sporocytes are typically quadrilobed before nuclear division and the meiotic spindle is quadripolar with poles in the four future spore domains. Whereas seed plants consistently have anastral spindles arising from γ-tubulin in the perinuclear area, bryophytes have spindles organized at POs, plastids, or nuclear envelope. All of these MTOCs are significantly different from centrosomes of the algal ancestors. Mosses and hornworts have quadrilobed sporocytes with meiotic spindles organized at plastids. Meiosis in liverworts is extremely varied. Sporocytes of Jungermanniopsida are deeply quadrilobed and have microtubule bands marking division planes prior to cytoplasmic shaping. Spindles are organized at POs or nuclear envelope. Sporocytes of Marchantiopsida are quadrilobed to apolar with spindles organized at plastids, POs, or nuclear envelope. Pre-meiotic bands have been reported in only one marchantiod, the early divergent Blasia. An atlas of cytological data on 13 liverworts, 3 mosses and 2 hornworts is presented and analyzed.