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     

Chromosome evolution in the genus Ophioglossum L.

Cytological observations on eleven species of Ophioglossum revealed low gametic ( n ) chromosome numbers of 30, 34 and 60 in populations of O.eliminatum , contrasting with an earlier report of n = 90 in the same species. The rest of the species is based on n =120.Cytologically studied species of Ophioglossum exhibit a range of chromosome numbers from n = 30 in O.eliminatum to n =720 in O.reticulatum. The weighted highest common factor (HGF) from all the reported chromosome numbers in twelve species was found to be 30. This number is proposed as the palaeobasic chromosome number for the genuS. Reported chromosome numbers which are not multiples of 30 were subjected to sequential analysis, yielding three distinct ultimate base numbers, 4, 5 and 6, which can produce n = 30 in seven different ways. The neobasic number, n= 120, appears to have arisen through various combinations and permutations of these, theoretically 2401 routes; only a relatively few of these routes exist today, suggesting that extreme selection has been exerted against the majority, and further suggesting that Ophioglossum represents an evolutionary dead end through repeated cycles of polyploidy and is possibly at the verge of extinction. The stoichiometric model of evolution, which derives the various chromosome numbers possessed by the twelve species from the basic and ultimate basic chromosome numbers, is used to explain chromosomal evolution in the genus.     

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.     

Probing the role of septins in cardiomyocytes

Heart growth in the embryo is achieved by division of differentiated cardiomyocytes. Around birth, cardiomyocytes stop dividing and heart growth occurs only by volume increase of the individual cells. Cardiomyocytes seem to lose their capacity for cytokinesis at this developmental stage. Septins are GTP-binding proteins that have been shown to be involved in cytokinesis from yeast to vertebrates. We wanted to determine whether septin expression patterns can be correlated to the cessation of cytokinesis during heart development. We found significant levels of expression only for SEPT2, SEPT6, SEPT7 and SEPT9 in heart, in a developmentally regulated fashion, with high levels in the embryonic heart, downregulation around birth and no detectable expression in the adult. In dividing embryonic cardiomyocytes, all septins localize to the cleavage furrow. We used drugs to probe for the functional interactions of SEPT2 in dividing embryonic cardiomyocytes. Differences in the effects on subcellular septin localization in cardiomyocytes were observed, depending whether a Rho kinase (ROCK) inhibitor was used or whether actin and myosin were targeted directly. Our data show a tight correlation of high levels of septin expression and the ability to undergo cytokinesis in cardiomyocytes. In addition, we were able to dissect the different contributions of ROCK signaling and the actomyosin cytoskeleton to septin localization to the contractile ring using cardiomyocytes as an experimental system.     

Caffeine inhibition of cytokinesis: effect on the phragmoplast cytoskeleton in livingTradescantia stamen hair cells

Summary The distribution of microtubules and actin microfilaments during caffeine-induced inhibition of cell plate formation has been studied in livingTradescantia stamen hair cells. Previous studies have shown that caffeine allows cell plate initiation but prevents its completion, resulting in binucleate cells. In the present study, confocal microscopy of cells microinjected with fluorescent brain tubulin or phalloidin, and cultured in the presence 5 mM caffeine, revealed that the initiation and early lateral expansion phase of the phragmoplast occur normally. However, caffeine completely inhibits the formation of the cytoskeletal torus which occurs in untreated cells during the late stages of cell plate and phragmoplast expansion. Caffeine further causes the disintegration of the incomplete cell plate. The results allow us to distinguish two phases in cell plate and phragmoplast growth: the initiation and early expansion phase, which is not affected by caffeine, and the late lateral expansion phase, which is completely inhibited in the presence of caffeine. Also in this study, the use of a high phalloidin concentration has revealed structural detail about the actin microfilaments involved in cell plate formation: microfilaments are observed that link the expanding edge of the phragmoplast with the cortical division site. In addition, cortical actin patches are observed within the actin depleted zone that might play a role in guidance of phragmoplast and cell plate expansion.     

Aluminium Effects on Microtubule Organization in Dividing Root-Tip Cells of Triticum turgidum. II. Cytokinetic Cells

Triticum turgidum were examined, using tubulin immunolabeling and electron microscopy. In cells, which at the beginning of the treatment were at a transitional stage between anaphase and telophase, the transformation of the interzonal microtubule (Mt) system into a phagmoplast was delayed. In cells treated at a telophase/ cytokinetic stage, the lateral phragmoplast expansion towards the cell periphery was delayed or inhibited. Besides, in cells entering telophase through an abnormal mitosis, Al inhibited phragmoplast formation and induced the organization of atypical tubulin bundles. The latter formed a network around the reassembling polyploid nucleus. The Al-effects resulted in the disturbance of cytokinesis and the formation of binuclear or polyploid cells, which lacked typical Mts. Instead of them, the post-telophase cells displayed atypical tubulin aggregations. In addition, Al affects cell plate development. Dividing cells, encompassing early interphase daughter nuclei, contained incomplete, atypical cell plates. The latter were quite thick, wavy and perforated, showing large “islands”, which contained electron transparent material. In some cells, the atypical cell plates gave rise to incomplete daughter walls, but in some others they were dismantled. The aberrant cell plates as well as the young daughter cell walls fluoresced intensely after aniline blue staining, an observation suggesting that they contain significant quantities of callose. The above findings combined with those derived from the study of the Al-effects on the mitotic spindle show that Mt cytoskeleton is a target site of Al toxicity in dividing cells. Received 24 October 2000/ Accepted in revised form 19 January 2001     

Entamoeba histolytica: fibrilar aggregates in dividing trophozoites

Entamoeba histolytica trophozoite cytokinesis is dependent upon cytoskeletal elements such as filamentous actin and myosin. Here we present confocal and transmission electron microscopy studies of this process. A sequence in the formation of the contractile ring was shown with rhodamine-phalloidine staining. Ultrastructural analysis revealed the presence of fibrilar aggregates in the cytoplasm of dividing trophozoites. Among them two filaments of different diameter were identified. These aggregates presented repeating assemblies of thin and thick filaments that in cross section revealed a muscle-like appearance. Our results suggest that these aggregates constitute the contractile ring responsible for the separation of daughter cells.     

Henneguya adiposa Minchew (Myxosporida) in the Channel Catfish: Ultrastructure of the Plasmodium Wall and Sporogenesis*

Wall ultrastructure and sporogenesis were studied in plasmodia of Henneguya adiposa Minchew which infects the channel catfish, Ictalurus punctatus (Rafinesque). Plasmodia were located among connective tissue bands of the adipose fin and were always separated from host fibrocytes by collagen fibers. The plasmodium wall consisted of a single unit membrane which was continuous with numerous pinocytic canals extending into the parasite's ectoplasm. The membrane was highly convoluted, producing an irregular parasite surface, and was covered by a fine granular coat of almost uniform thickness. Early sporogenic stages were located in a zone of cytoplasm rich in mitochondria, just interior to the zone of pinocytic canals. Later sporogenic stages, including mature spores, were concentrated in the center of the plasmodia. Sporogenesis began with the envelopment of one generative cell, the sporont, by a 2nd, nondividing, cell—the enveloping cell. The sporont and its progeny proceeded through a series of divisions until 10 cells were present within the enveloping cell. Once divisions were completed, the 10 cells became arranged into 2 identical spore-producing units, each consisting of one binucleate sporoplasm and 2 capsulogenic cells, all surrounded by 2 valvogenic cells. Later stages of spore development indicated that capsulogenesis, valvogenesis and sporoplasm maturation occurred concomitantly.     

Meiosis in allopolyploid Crepis capillaris. II. Autotetraploids.

Meiotic chromosome pairing of autotetraploid Crepis capillaris was analysed by electron microscopy of surface-spread prophase I nuclei and compared with light microscopic observations of metaphase I chromosome configurations. Prophase I quadrivalent frequencies are high in all three tetrasomes. (A, D, and C) and partially dependent on chromosome size. At metaphase I quadrivalent frequencies are much lower and strongly dependent on chromosome size. There is no evidence for multivalent elimination during prophase I in this system, and the reduction in multivalent frequency at metaphase I can be explained by an insufficiency of appropriately placed chiasmata. The high frequencies of prophase I quadrivalents far exceed the two-thirds expected on a simple model with two terminal independent pairing initiation sites per tetrasome, suggesting that multiple pairing initiation occurs. Direct observations reveal relatively high frequencies of pairing partner switches (PPSs) at prophase I, which confirms this suggestion. The numbers of PPSs per tetrasome show a good fit to the Poisson distribution, and their positional distribution along chromosomes is random and nonlocalized. These observations favour a model of pairing initiation based on a large number of evenly distributed autonomous pairing sites each with a uniform and low probability of generating a PPS.     

Collapsin response mediator proteins (CRMPs) are a new class of microtubule-associated protein (MAP) that selectively interacts with assembled microtubules via a taxol-sensitive binding interaction

Collapsin response mediator proteins are ubiquitously expressed from multiple genes (CRMPs 1-5) and play important roles in dividing cells and during semaphorin 3A (Sema3A) signaling. Nonetheless, their mode of action remains opaque. Here we carried out in vivo and in vitro assays that demonstrate that CRMPs are a new class of microtubule-associated protein (MAP). In experiments with CRMP1 or CRMP2 and their derivatives, only the C-terminal region (residues 490-572) mediated microtubule binding. The in vivo microtubule association of CRMPs was abolished by taxol or epothilone B, which is highly unusual. CRMP2-depleted cells exhibited destabilized anaphase astral microtubules and altered spindle position. In a cell-based assay, all CRMPs stabilized interphase microtubules against nocodazole-mediated depolymerization, with CRMP1 being the most potent. Remarkably, a 82-residue C-terminal region of CRMP1 or CRMP2, unrelated to other microtubule binding motifs, is sufficient to stabilize microtubules. In cells, we demonstrate that glycogen synthase kinase-3β (GSK3β) inhibition potentiates this activity. Thus, CRMPs are a new class of MAP that binds through a unique motif, but in common with others such as Tau, is antagonized by GSK3β. This regulation is consistent with such kinases being critical for the Sema3A (collapsin) pathway. These findings have implications for cancer and neurodegeneration.     

Cytoskeletal organization of the micropylar endosperm in Coronopus didymus L. (Brassicaceae)

Summary. The micropylar chamber of the mustard Coronopus didymus is a developmental domain distinct from the contiguous central chamber and the more extreme chalazal chamber. Early in syncytial development the micropylar endosperm surrounding the embryo becomes populated with unusual fusiform to multilobed nuclei. These nuclei are sheathed by unique parallel arrays of microtubules that focus at tips of the nuclei and flare to connect with a reticulate network in the common cytoplasm. F-actin does not closely invest the nuclei but instead forms an extensive but separate cytoplasmic reticulum. When the embryo is in the early heart stage, the cytoskeleton of the endosperm undergoes a remarkable transition in preparation for cellularization. Microtubules become reorganized into radial arrays emanating from the nuclei, which themselves become spherical. Radial microtubule systems (RMSs), which replace both the parallel microtubules and the cytoplasmic reticulum, organize the common cytoplasm into evenly spaced nuclear cytoplasmic domains (NCDs). F-actin gradually becomes coaligned with the RMSs. Phragmoplasts are initiated adventitiously at the interfaces of opposing RMSs in the absence of mitosis. Cell plate deposition, which is initiated at multiple sites, results in a network of walls formed more or less simultaneously around the densely packed NCDs. The walls, which are rich in 1–3-β-glucans, join with one another and with the existing walls of both the central cell and embryo to complete cellularization in the micropylar chamber. In the adjacent central chamber where the syncytium is restricted to a thin peripheral layer by the large central vacuole, basic organization of the syncytium into NCDs is followed by alternating cycles of alveolation and periclinal cell division resulting in cellularization. Received July 19, 2001 Accepted October 16, 2001     

Cytokinesis in plant and animal cells: endosomes 'shut the door'

For many years, cytokinesis in eukaryotic cells was considered to be a process that took a variety of forms. This is rather surprising in the face of an apparently conservative mitosis. Animal cytokinesis was described as a process based on an actomyosin-based contractile ring, assembling, and acting at the cell periphery. In contrast, cytokinesis of plant cells was viewed as the centrifugal generation of a new cell wall by fusion of Golgi apparatus-derived vesicles. However, recent advances in animal and plant cell biology have revealed that many features formerly considered as plant-specific are, in fact, valid also for cytokinetic animal cells. For example, vesicular trafficking has turned out to be important not only for plant but also for animal cytokinesis. Moreover, the terminal phase of animal cytokinesis based on midbody microtubule activity resembles plant cytokinesis in that interdigitating microtubules play a decisive role in the recruitment of cytokinetic vesicles and directing them towards the cytokinetic spaces which need to be plugged by fusing endosomes. Presently, we are approaching another turning point which brings cytokinesis in plant and animal cells even closer. As an unexpected twist, new studies reveal that both plant and animal cytokinesis is driven not so much by Golgi-derived vesicles but rather by homotypically and heterotypically fusing endosomes. These are generated from cytokinetic cortical sites defined by preprophase microtubules and contractile actomyosin ring, which induce local endocytosis of both the plasma membrane and cell wall material. Finally, plant and animal cytokinesis meet together at the physical separation of daughter cells despite obvious differences in their preparatory events.     

楸树大小孢子发生与雌雄配子体发育的研究

运用石蜡切片法和整体透明法对楸树(Catalpa bungei C.A.Meyer)大、小孢子发生及雌、雄配子体发育过程进行了研究.结果表明:楸树可育雄蕊2枚,花药4室,药壁发育属双子叶型,腺质异型绒毡层.小孢子母细胞减数分裂为同时型,四分体后小孢子不分离形成正四面体型四合花粉,偶有左右对称型和十字交叉型.成熟花粉为二细胞型,无萌发孔.子房上位,2室,中轴胎座,胚珠多数,倒生,单珠被,薄珠心,具珠被绒毡层.单孢原直接发育为大孢子母细胞,四分体线形排列,合点端大孢子发育为功能大孢子,胚囊发育为蓼型.雄蕊发育早于雌蕊,花开后雌、雄蕊趋于同熟.研究认为:虽然楸树雌、雄蕊发育过程中均存在一定比例的败育,但其花而不实"并非雄性或雌性不育所致.推测与其授粉受精和胚后发育有关.     

苞叶姜大小孢子发生和雌雄配子体发育

对苞叶姜(Pyrgophyllum yunnanense)大小孢子发生和雌雄配子体发育进行了显微观察, 研究结果表明: 苞叶姜花药2室, 药壁发育属基本型, 绒毡层为分泌型。小孢子母细胞经减数分裂形成左右对称型四分体, 胞质分裂为连续型, 成熟花粉粒为2-细胞型; 子房下位, 3室, 胚珠多数, 倒生, 具双珠被、厚珠心, 中轴胎座。大孢子母细胞经减数分裂形成线型(或T形)四分体, 珠孔端3个大孢子退化, 合点端的1个发育成功能大孢子, 胚囊发育为蓼型。比较苞叶姜、黄花大苞姜(Caulokaempferia coenobialis)和海南三七(Kaempferia rotunda)的早期胚胎学特征, 表明苞叶姜与黄花大苞姜系统学关系较近, 与海南三七系统学关系较远。在分类等级上, 胚胎学证据支持苞叶姜独立成属。     

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.     

彩色马蹄莲大小孢子发生和雌雄配子体发育的研究

选用石蜡切片法观察了彩色马蹄莲品种‘Majestic Red’的大小孢子发生及雌雄配子体发育的过程。研究结果表明：彩色马蹄莲的胚珠为倒生,具双珠被、厚珠心和珠被绒毡层。大孢子母细胞的减数分裂后形成的四分体为直线型或T型排列,合点端的大孢子发育成为功能大孢子,其余3个大孢子则退化,表明胚囊发育方式为单孢子发生的蓼型胚囊。观察到每个雄花花药多数,花粉囊呈蝶形,每侧有2个小孢子囊。花药壁由外到内分别为表皮、药室内壁、中层和绒毡层,其中绒毡层为变形绒毡层类型。在小孢子形成时,胞质分裂属于连续型,小孢子排列成十字形的四分体,成熟花粉则为二胞花粉粒。     

Mitotic Regulation of SEPT9 Protein by Cyclin-dependent Kinase 1 (Cdk1) and Pin1 Protein Is Important for the Completion of Cytokinesis

Precise cell division is essential for multicellular development, and defects in this process have been linked to cancer. Septins are a family of proteins that are required for mammalian cell division, but their function and mode of regulation during this process are poorly understood. Here, we demonstrate that cyclin-dependent kinase 1 (Cdk1) phosphorylates septin 9 (SEPT9) upon mitotic entry, and this phosphorylation controls association with the proline isomerase, Pin1. Both SEPT9 and Pin1 are critical for mediating the final separation of daughter cells. Expression of mutant SEPT9 that is defective in Pin1 binding was unable to rescue cytokinesis defects caused by SEPT9 depletion but rather induced dominant-negative defects in cytokinesis. However, unlike SEPT9 depletion, Pin1 was not required for the accumulation of the exocyst complex at the midbody. These results suggest that SEPT9 plays multiple roles in abscission, one of which is regulated by the action of Cdk1 and Pin1.     

Ultrastructural study on mitosis and cytokinesis in Scytosiphon lomentaria zygotes (Scytosiphonales,Phaeophyceae) by freeze-substitution

Summary. The ultrastructure of mitosis and cytokinesis in Scytosiphon lomentaria (Lyngbye) Link zygotes was studied by freeze fixation and substitution. During mitosis, the nuclear envelope remained mostly intact. Spindle microtubules (MTs) from the centrosome passed through the gaps of the nuclear envelope and entered the nucleoplasm. In anaphase and telophase, two daughter chromosome masses were partially surrounded with endoplasmic reticulum. After telophase, the nuclear envelope was reconstructed and two daughter nuclei formed. Then, several large vacuoles occupied the space between the daughter nuclei. MTs from the centrosomes extended toward the mid-plane between two daughter nuclei, among the vacuoles. At that time, Golgi bodies near the centrosome actively produced many vesicles. Midway between the daughter nuclei, small globular vesicles and tubular cisternae accumulated. These vesicles derived from Golgi bodies were transported from the centrosome to the future division plane. Cytokinesis then proceeded by fusion of these vesicles, but not by a furrowing of the plasma membrane. After completion of the continuity with the plasma membrane, cell wall material was deposited between the plasma membranes. The tubular cisternae were still observed at the periphery of the newly formed septum. Microfilaments could not be observed by this procedure. We conclude that cytokinesis in the brown algae proceeds by fusion of Golgi vesicles and tubular cisternae, not by a furrowing of the plasma membrane. Received September 12, 2001 Accepted November 12, 2001     

Chromosomes of lemuriformes. II. Chromosome polymorphism in Lemur fulvus collaris (E. Geoffroy 1812).

Two previously unreported diploid numbers, 2N = 50 and 2N = 51, from five individual Lemur fulvus collaris are described. In both chromosome complements, the nombre fondamental is 64. The 2N = 50 complement is composed of 7 pairs of bi-armed chromosomes, 17 pairs of acrocentric chromosomes with small short arms, and acrocentric sex chromosomes. The 2N = 51 complement is identical with these exceptions: Only one member equivalent to bi-armed pair 6 of L.f. collaris (2N = 50) is present, and two extra acrocentrics are found in the 2N = 51 complement. G-banding analyses suggest that these chromosomes are a heteromorphic pair of the Robertsonian type. This conculsion is supported by evidence from studies of meiotic pairing relationships of the three chromosomes and the complements of hybrids resulting from interspecific matings. Comparison of the 2N = 50 and 2N = 51 complements with a published 2N = 48 complement suggests that these new karyotypes do not provide a lineal link between the 2N = 52 and 2N = 48 karyotypes (Rumpler, Y., and R. Albignac 1969 C.R. Soc. Biol., 163: 1989-1992).