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.     

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     

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.     

Preprophasic microtubule systems and development of the mitotic spindle in hornworts (Bryophyta)

Summary Studies of monoplastidic mitosis in hornworts (Bryophyta) using transmission electron microscopy and indirect immunofluorescence staining of microtubules have revealed that two mutually perpendicular microtubule systems predict division polarity in preprophase. Events of cytoplasmic reorganization in preparation for division occur in the following order: migration of the single plastid to a position perpendicular to the division site, constriction of the plastid where its midpoint intersects the division site, development of an axial system of microtubules parallel to the elongating plastid isthmus, and appearance of an atypical preprophase band of microtubules (PPB). The PPB is asymmetrical with a tight band of microtubules on the side over the plastid isthmus and a broad band of widely spaced microtubules over the nucleus. The axial system contributes directly to development of the spindle. In prometaphase, the axial system separates at the equator and additional microtubule bundles project from polar regions, creating two opposing halfspindles. The PPB is still present during asymmetrical organization of the spindle and microtubules extending from the broad portion of the PPB to poles appear to be incorporated into the developing spindle. Dynamic changes in the microtubular cytoskeleton demonstrate (1) intimate relationship of plastid and nuclear division, (2) contribution of preprophase/prophase microtubule systems to spindle development in monoplastidic cells, and (3) dynamic reorientation of microtubules from one system to another.     

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)     

ULTRASTRUCTURE OF SPOROGENESIS IN THE MOSS,AMBLYSTEGIUM RIPARIUM I. MEIOSIS AND CYTOKINESIS

Emphasis is placed on three aspects of meiosis in the moss Amblystegium riparium (Hedw.) BSG: 1***) nature of the sporogenous layer; 2) prophasic microtubules and polarity; and 3) cleavage pattern. Spore tetrads develop while still encased by archesporial cell walls. The cellular nature of the sporogenous layer differs from the more usual occurrence of free sporocytes released into a common spore sac. Two important events mark the establishment of sporocyte polarity during meiotic prophase: 1) migration of the four plastids to the distal tetrad poles (telophase II poles); and 2) ingrowth of the sporocyte wall in eventual cleavage planes between the tetrad poles. An extensive, plastid-based microtubule system is associated with organelle migration during the establishment of sporocyte polarity in meiotic prophase. Disruption of the nuclear envelope in prometaphase I occurs at sites opposite the four plastids where microtubules extend from plastid envelope to nuclear envelope. Formation of a cell plate following the first meiotic division results in a dyad, whereas in many mosses meiosis is completed in the undivided sporocyte and is followed by simultaneous cleavage into a spore tetrad. Spore cleavage is accomplished by vesicular coalescence resulting in septa that coincide with the prophasic wall ingrowths.     

Gamma-tubulin in basal land plants: characterization, localization, and implication in the evolution of acentriolar microtubule organizing centers

Although seed plants have gamma-tubulin, a ubiquitous component of centrosomes associated with microtubule nucleation in algal and animal cells, they do not have discrete microtubule organizing centers (MTOCs) comparable to animal centrosomes, and the organization of microtubule arrays in plants has remained enigmatic. Spindle development in basal land plants has revealed a surprising variety of MTOCs that may represent milestones in the evolution of the typical diffuse acentrosomal plant spindle. We have isolated and characterized the gamma-tubulin gene from a liverwort, one of the extant basal land plants. Sequence similarity to the gamma-tubulin gene of higher plants suggests that the gamma-tubulin gene is highly conserved in land plants. The G9 antibody to fission yeast gamma-tubulin recognized a single band of 55 kD in immunoblots from bryophytes. Immunohistochemistry with the G9 antibody clearly documented the association of gamma-tubulin with various MTOC sites in basal land plants (e.g., discrete centrosomes with and without centrioles and the plastid surface in monoplastidic meiosis of bryophytes). Changes in the distribution of gamma-tubulin occur in a cell cycle-specific manner during monoplastidic meiosis in the liverwort Dumortiera hirsuta. gamma-Tubulin changes its localization from the plastid surface in prophase I to the spindle, from the spindle to phragmoplasts and the nuclear envelope in telophase I, and back to the plastid surfaces in prophase II. In vitro experiments show that gamma-tubulin is detectable on the surface of isolated plastids and nuclei of D. hirsuta, and microtubules can be repolymerized from the isolated plastids. gamma-Tubulin localization patterns on plastid and nuclear surfaces are not affected by the destruction of microtubules by oryzalin. We conclude that gamma-tubulin is a highly conserved protein associated with microtubule nucleation in basal land plants and that it has a cell cycle-dependent distribution essential for the orderly succession of microtubule arrays.     

Preprophasic establishment of division polarity in monoplastidic mitosis of hornworts

Summary Preprophase in the monoplastidic mitotic cells ofPhaeoceros andNotothylas is characterized by the establishment of a division site in the absence of a typical preprophase band. The future cytokinetic plane is predicted by plastid orientation and development of an elaborate preprophasic microtubule system perpendicular to the division plane. Division of the single plastid is initiated early in preprophase and the constricting plastid migrates to a position perpendicular to the future plane of division. Plastid orientation assures that division of the plastid by mid-constriction will result in distribution of a plastid to each daughter cell. Microtubules parallel the long axis of the plastid and are most numerous adjacent to the nucleus which becomes elongated in the future spindle axis. We conclude that the division site is a fundamental component of the cytokinetic apparatus involved in the determination of cleavage plane prior to nuclear division.     

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.     

Monoplastidic mitosis in the mossTimmiella barbuloides (Bryophyta)

Summary Mitotic cell division of monoplastidic sporogones was investigated in the mossTimmiella barbuloides (Brid.) Moenk. (Pottiales, Bryophyta) by TEM. Division polarity of sporogones is established by the interphase position of the single oblong cup-shaped plastid, which is orientated with its long axis parallel to one of the cell walls. In preprophase the plastid elongates and its extremities bend at right angles. Plastid growth is directed by microtubules and accompanied by plastid tubules. The plastid begins the process of duplication by constricting centrally in the plane of the future cytokinetic septum. There is no preprophase band of microtubules at the division site. The large central nucleus becomes fusiform and aligned parallel to the main plastid axis. By the end of prophase the daughter plastids are positioned at the opposite poles of the nucleus where they probably function as nucleating or organizing centres for the spindle microtubules. Metaphase and anaphase spindles contain long sheets of ER. Cytokinesis involves the formation of a well developed phragmoplast.Abbreviations TEM transmission electron microscopy - PPB preprophase band of microtubules - ER endoplasmic reticulum     

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     

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.     

A cytoskeletal system predicts division plane in meiosis ofSelaginella

Summary An ultrastructural investigation of the monoplastidic microsporocytes ofSelaginella arenicola revealed a unique cytoskeletal array that predicts the future division plane before nuclear division takes place. By midprophase of the first meiotic division, the single plastid has divided once and the two plastids lie on opposite sides of the nucleus which is elongated in the plane of the incipient metaphase I spindle. A cytoplasmic structure, the procytokinetic plate (PCP), predicts the division plane of of both plastid and cytoplasm. The PCP consists of a distinct concentration of vesicles lying in the future division plane and an elaborate system of microtubules aligned parallel to the long axis of plastids and nucleus. Microtubules of the axially aligned system appear to terminate in clusters of vesicles in the central zone of the PCP. The PCP with axially aligned microtubules is as predictive of the division plane in these meiotic cells as is the girdling preprophase band of microtubules in mitotic cells.     

γ-Tubulin,microtubule arrays,and quadripolarity during sporogenesis in the hepatic<Emphasis Type="Italic"> Aneura pinguis</Emphasis> (Metzgeriales)

This is the first report on the organization of a quadripolar microtubule system (QMS) in polyplastidic meiosis of a hepatic with polar organizers (POs). Unlike the monoplastidic sporocytes of mosses and hornworts, in which meiotic quadripolarity can be traced to plastid division and migration, sporocytes of Aneura pinguis are polyplastidic and tetrahedrally lobed before the QMS is organized. Whereas the QMS in mosses and hornworts is plastid-based, the QMS of A. pinguis is focused at four POs where gamma tubulin (-tubulin) is concentrated. An aster of microtubules emanates from each PO centered in the four cytoplasmic lobes and the opposing radial microtubules interact to form the QMS that envelops the nucleus. A functionally bipolar spindle is gradually formed as the four poles converge in pairs on either side of opposite cleavage furrows. The resulting spindle remains quadripolar. Although -tubulin is most concentrated in the deeply concave poles straddling cleavage furrows, it also extends into the spindle itself. Telophase groups of chromosomes curve around the polar cleavage furrows and a phragmoplast that originates in the interzonal region guides a cell plate that extends to the equatorial cleavage furrows. Discrete POs are reformed at opposite tips of the elongated dyad nuclei in prophase II and microtubules radiating from them give rise to the spindles of second meiosis. Spindles remain sharply focused and -tubulin extends into distal portions of the spindle. Interzonal phragmoplasts that expand to join with pre-established cleavage furrows mediate cytokinesis resulting in a tetrad of spores. Each young tetrad member has a radial microtubule system emanating from the nucleus.     

Polar organizers and girdling bands of microtubules are associated with γ-tubulin and act in establishment of meiotic quadripolarity in the hepatic Aneura pinguis (Bryophyta)

Summary. Meiosis in Aneura pinguis is preceded by extensive cytoplasmic preparation for quadripartitioning of the diploid sporocyte into a tetrad of haploid spores. In early prophase the four future spore domains are defined by lobing of the cytoplasm and development of a quadripolar prophase spindle focused at polar organizers (POs) centered in the lobes. Cells entering the reproductive phase become isolated and, instead of hooplike cortical microtubules, have endoplasmic microtubule systems centered on POs. These archesporial cells proliferate by mitosis before entering meiosis. In prophase of each mitosis, POs containing a distinct concentration of γ-tubulin appear de novo at tips of nuclei and initiate the bipolar spindle. Cells entering meiosis become transformed into quadrilobed sporocytes with four POs, one in each lobe. This transition is a complex process encompassing assembly of two opposite POs which subsequently disperse into intersecting bands of microtubules that form around the central nucleus. The girdling bands define the future planes of cytokinesis and the cytoplasm protrudes through the restrictive bands becoming quadrilobed. Two large POs reappear in opposite cleavage furrows. Each divides and the resulting POs migrate into the tetrahedral lobes of cytoplasm. Cones of microtubules emanating from the four POs interact to form a quadripolar microtubule system (QMS) that surrounds the nucleus in meiotic prophase. The QMS is subsequently transformed into a functionally bipolar metaphase spindle by migration of poles in pairs to opposite cleavage furrows. These findings contribute to knowledge of microtubule organization and the role of microtubules in spatial regulation of cytokinesis in plants. Correspondence and reprints: Department of Biology, University of Louisiana-Lafayette, Lafayette, LA 70504-2451, U.S.A.     

Microtubule distribution in dv, a maize meiotic mutant defective in the prophase to metaphase transition

Microsporogenesis in Zea mays, the meiotic reduction of diploid sporocytes to haploid microspores, proceeds through a well-defined developmental sequence. The ability to generate mutants that affect the process makes this an ideal system for elucidating the role of the cytoskeleton during plant development. We have used immunofluorescence microscopy to compare microtubule distribution in wild-type and mutant microsporocytes. During normal meiosis the distribution of microtubules follows a specific temporal and spatial pattern that reflects the polar nature of microspore formation. Perinuclear microtubule staining increases and the nucleus elongates in the future spindle axis during late prophase I. Metaphase I spindles with highly focused poles align along the long axis of the anther locule. Cytokinesis occurs perpendicular to the spindle axis. The second division axis shifts 90 degrees with respect to the first division plane, thereby yielding an isobilateral tetrad of microspores. Microtubule distribution patterns during meiosis suggest that a nuclear envelope-associated microtubule organizing center (MTOC) controls the organization of cytoplasmic microtubules and contributes to spindle formation. The meiotic mutant dv is defective in the transition from a prophase microtubule array to a metaphase spindle. Instead of converging to form focused poles, the metaphase spindle poles remain diffuse as in prometaphase. This defect correlates with several abnormalities in subsequent developmental events including the formation of multinucleate daughter cells, multiple microspindles during meiosis II, multiple phragmoplasts, polyads of microspores, and cytoplasmic microtubule foci. These results suggest that dv is a mutation that affects MTOC organization.     

Pattern of the plastid division in spore mother cells of the hornwortAnthoceros punctatus

Relative changes in plastid DNA content in each stage of plastid division were investigated in order to better understand the division cycle of plastids in spore mother cells in the horwortAnthoceros punctatus. Samples of cells stained with DAPI were observed with epifluorescence microscopy and CHIAS. In spore mother cells of this species, plastids duplicated their own DNA prior to the plastidkinesis of the first plastid division, but did not replicate plastid DNA prior to the plastidkinesis of the second plastid division. Therefore, the DNA content of those plastids in which division had been completed was reduced to half its initial value. This indicates that the DNA replication pattern of plastids in spore mother cells corresponds to that of cell nuclei during premeiosis and meiosis inA. punctatus.     

Polar organizers mark division axis prior to preprophase band formation in mitosis of the hepaticReboulia hemisphaerica (Bryophyta)

Summary Changes in the pattern of microtubules during the cell cycle of the hepaticReboulia hemisphaerica (Bryophyta) were studied by indirect immunofluorescence using conventional and confocal laser scanning microscopy (CLSM). The first indication that a cell is preparing for division is fusiform shaping of the nucleus accompanied by the appearance of well-defined polar organizers (POs) at the future spindle poles. Microtubules emanating from the POs ensheath the nucleus and eventually develop into the half-spindles of mitosis. Some of the microtubules from each PO pass tangential to the nucleus and interact in the region of the future mitotic equator. A preprophase band (PPB) forms in this region later in prophase and coexists with the prophase spindle. Thus, the plane of division appears to be determined by interaction of opposing arrays of microtubules emanating from POs. Prometaphase is marked by disappearance of the POs, loss of astral microtubules, and conversion of the fusiform spindle of prophase to a truncated, barrel-shaped spindle more typical of higher plants. Restoration of cortical microtubules in daughter cell occurs on the cell side distal to the new cell plate, but nucleation of microtubules is associated with the nuclear envelope and not with organized POs. At the next division POs appear at opposite poles of preprophase nuclei with no evidence of division and migration that is characteristic of cells with centriolar centrosomes. These data lend additional support for the view that mitosis in hepatics is transitional between green algae and higher plants.Abbreviations AMS axial microtubule system - CLSM confocal laser scanning microscopy - MTOC microtubule organizing center - PO polar organizer - PPB preprophase band of microtubules - QMS quadripolar microtubule system - TEM transmission electron microscopy     

Division polarity,development and configuration of microtubule arrays in bryophyte meiosis

Summary First and second division spindles and the three cell plates of moss meiosis are oriented in accordance with polarity established during meiotic prophase. Plastids are located at the second division poles and cytoplasmic infurrowing marks the planes along which the cytoplasm will cleave into four spores. Anaphase I spindles that terminate in two focal points of microtubules straddling opposite cleavage furrows reflect the unusual tetrahedral origin of the functionally bipolar spindle. The organelles (except for the plastids which remain in the four cytoplasmic lobes) are polarized in the first division equatorial region at the time of phragmoplast microtubule assembly and remain in a distinct band after microtubule disassembly. Prophasic spindles appear to be directly transformed into metaphase II spindles in the predetermined axes between mutually perpendicular pairs of plastids. Cell plates form by vesicle coalescence in the equatorial regions of the two sets of second division phragmoplasts at approximately the same time as a cell plate belatedly forms in the organelle band. The cytoplasmic markers (plastid migration, cytoplasmic lobing and infurrowing) that predict poles and cleavage planes in free cells lacking a preprophase band strongly strengthens the concept that division sites are capable of preserving preprogrammed signals that can be triggered later in the process of cell division.