Automictic parthenogenesis of a geographically parthenogenetic mayfly,Ephoron shigae (Insecta: Ephemeroptera,Polymitarcyidae)

In the geographically parthenogenetic mayfly, Ephoron shigae, egg maturation and counts of chromosome number of unfertilized, parthenogenetic eggs were studied, in comparison with fertilized eggs from a bisexual population. The primary oocyte becomes mature through two successive maturation divisions. The first maturation division (meiotic division) takes place in the primary oocyte to produce a secondary oocyte and a first polar body. The second maturation division soon occurs in the secondary oocyte, in which the nucleus is divided into a mature egg nucleus (female pronucleus) and second polar body nuclei. The first polar body, in some cases, was successively divided into two polar bodies; in other instances, it was not divided. After the successive maturation division, the egg nucleus and the sister second polar body nucleus drew near to fuse into the zygote nucleus. The chromosome number was doubled in the zygote, and this conjugation initiates subsequent embryonic development. This suggests that, in E. shigae, the process of parthenogenetic recovery of diploidy is the automictic type categorized as the ‘terminal fusion’ pattern. © 2009 The Linnean Society of London, Biological Journal of the Linnean Society, 2010, 99 , 335–343.     

Premature cytokinesis in pollen mother cells of transgenic tobacco plants Nicotiana tabacum L

In majority species of dicotyledonous plants cytokinesis in PMCs occurs once after completion of two caryokinesis cycles, that is a simultaneous type. This paper represents cytological picture and frequency characteristics of abnormality which resulted in cytokinesis triggering after first meiotic division in a part of transgenic tobacco PMCs. It was shown that the main process of cytoskeleton reorganization typical for simultaneous cytokinesis remained without any alterations in such cells. However, in most cases premature cell division occurred with abnormalities such as membrane "tunnel" or "gash" formation. It was ascertained that initiation of additional round ofcytokinesis did not block nuclear cycle and cytokinesis after second meiotic division. Thus, transition of cell division from simultaneous type to successive one occurs under this abnormality.     

Life Cycle of the pseudocolonial dinoflagellate Polykrikos kofoidii (Gymnodiniales,Dinoflagellata)

The athecate, pseudocolonial polykrikoid dinoflag‐ellates show a greater morphological complexity than many other dinoflagellate cells and contain not only elaborate extrusomes but sulci, cinguli, flagellar pairs, and nuclei in multiple copies. Among polykrikoids, Polykrikos kofoidii is a common species that plays an important role as a grazer of toxic planktonic algae but whose life cycle is poorly known. In this study, the main life cycle stages of P. kofoidii were examined and documented for the first time. The formation of gametes, 2‐zooid‐1‐nucleus stages very different from vegetative cells, was observed and the process of gamete fusion, isogamy, was recorded. Karyogamy followed shortly after completed plasmogamy. A complex reorganization of furrows (cinguli and sulci) and flagella followed zygote formation, resulting in a 4‐zooid zygote with one nucleus. The fate of zygotes under different nutritional conditions was also investigated; well‐fed zygotes were able to reenter the vegetative cycle via meiotic divisions as indicated by nuclear cyclosis. However, nuclear cyclosis was preceded by a presumably mitotic division of the primary zygote nucleus which by definition would imply that P. kofoidii has a diplohaplontic life cycle. Nuclear cyclosis in germlings hatched from spiny resting cysts indicate that these cysts are of zygote origin (hypnozygotes). Hypnozygote formation, cyst hatching, the morphology of the germling (a 1‐zooid cell), and its development into a normal pseudocolony are documented here for the first time. There is evidence that P. kofoidii has a system of complex heterothallism.     

Effects of several meiotic mutations on female meiosis in maize

A modified enzyme digestion technique of ovary isolation followed by staining and squash preparation has allowed us to observe female meiosis in normal maize meiotically dividing megaspore mother cells (MMCs). The first meiotic division in megasporogenesis of maize is not distinguishable from that in mi-crosporogenesis. The second female meiotic division is characterized as follows: (1) the two products of the first meiotic division do not simultaneously enter into the second meiotic division; as a rule, the chalazal-most cell enters division earlier than the micropylar one, (2) often the second of the two products does not proceed with meiosis, but degenerates, and (3) only a single haploid meiotic product of the tetrad remains alive, and this cell proceeds with three rounds of mitoses without any intervening cell wall formation to produce the eight-nucleate embryo sac. This technique has allowed us to study the effects of five meiotic mutations (aml, aml-pral, afdl, dsy *-9101, and dvl) on female meiosis in maize. The effects of the two alleles of the aml gene (aml and aml-pral) and of the afdl and dsy *-9101mutations are the same in both male and female meiosis. The aml allele prevents the entrance of MMCs into meiosis and meiosis is replaced by mitosis; the aml-pral permits MMCs to enter into meiosis, but their progress is stopped at early prophase I stages. The afdl gene is responsible for substitution of the first meiotic (reductional) division by an equational division including the segregation of sister chromatid centromeres at anaphase I. The dsy * -9101 gene exhibits abnormal chromosome pairing; paired homologous chromosomes are visible at pachytene, but only univalents are observed at diakinesis and metaphase I stages. These mutation specific patterns of abnormal meiosis are responsible for the bisexual sterility of these meiotic mutants. The abnormal divergent shape of the spindle apparatus and the resulting abnormal segregation of homologous chromosomes observed in micro-sporogenesis in plants homozygous for the dv1 mutation have not been found in meiosis of megasporogenesis. Only male sterility is induced by the dv1 gene in the homozygous condition. © 1993 Wiley-Liss, Inc.     

SEXUAL REPRODUCTION OF PERIDINIUM WILLEI (DINOPHYCEAE)1

Sexual reproduction was induced in the dinoflagellate Peridinium willei Huitfeld-Kass when exponentially growing cells were inoculated into nitrogen deficient medium. Small, naked vegetative cells produced by division of thecate cells acted as gametes. The zygote remained motile 13–14 days, during which time it enlarged and the theca formed became warty. Fourteen to 15 days following plasmogamy the zygote was nonmotile with the protoplast contracted. A large red oil droplet appeared and the wall thickened, becoming chitinized. Hypnozygotes with 4 nuclei were observed 7–8 wk following formation. Meiosis was inferred. The hypnozygote germinated, within 8 wk producing one post-zygotic cell retaining the red oil droplet. This cell divided within 24 h into 2 daughter cells each with a prominent red oil droplet. These daughter cells divided after 2 to 3 days into ordinary vegetative cells. Attempts to induce sexual reproduction by inoculation of cells into media deficient in a number of basic elements were unsuccessful.     

SEXUAL REPRODUCTION OF PERIDINUM CINCTUM F. OVOPLANUM (DINOPHYCEAE)1,2

Sexual reproduction is induced in the dinoflagelate Peridinium cinctum f. ovoplanum Lindemann when exponentially growing cells are inoculated into nitrogen deficient medium. Small, naked vegetative cells, produced by division of the thecate cells, then act as gametes. The zygote remains motile for 12–13 days during which time it enlarges and the theca which it forms becomes warty. Thirteen to 14 day s following plasmogamy the zygote is nonmotile, the protoplast contracts, a large red oil droplet appears, the wall thickens and becomes chitinized. This hypnozygote germinates within 7–8 weeks at 20 c producing 1 post-zygotic cell retaining the large red oil droplet. The presence of 4 nuclei in these post-zygotic cells may be demonstrated by staining them with acetocarmine. Two of these nuclei are smaller than the other two and probably abort. One may infer that meiosis occurs immediately prior to or at the germinartion of the hypnozygote. This post-zygotic cell divides within 24 h into 2 daughter cells each with a promment red oil droplet. These daughter cells divide after 2–3 days into ordinary vegetative cells. Attempts to induce sexual reproduction by changes in temperature or light and by inoculation of cells into media deficient in a number of basic elements were unsuccessful.     

Cytostatic Activity Develops during Meiosis I in Oocytes of LT/Sv Mice

Oocytes of wild-type mice are ovulated as the secondary oocytes arrested at metaphase of the second meiotic division. Their fertilization or parthenogenetic activation triggers the completion of the second meiotic division followed by the first embryonic interphase. Oocytes of the LT/Sv strain of mice are ovulated either at the first meiotic metaphase (M I) as primary oocytes or in the second meiotic metaphase (M II) as secondary oocytes. We show here that duringin vitromaturation a high proportion of LT/Sv oocytes progresses normally only until metaphase I. In these oocytes MAP kinase activates shortly after histone H1 kinase (MPF) activation and germinal vesicle breakdown. However, MAP kinase activation is slightly earlier than in oocytes from wild-type F1 (CBA/H × C57Bl/10) mice. The first meiotic spindle of these oocytes forms similarly to wild-type oocytes. During aging, however, it increases in size and finally degenerates. In those oocytes which do not remain in metaphase I the extrusion of first polar bodies is highly delayed and starts about 15 h after germinal vesicle breakdown. Most of the oocytes enter interphase directly after first polar body extrusion. Fusion between metaphase I LT/Sv oocytes and wild-type mitotic one-cell embryos results in prolonged M-phase arrest of hybrids in a proportion similar to control LT/Sv oocytes and control hybrids made by fusion of two M I LT/Sv oocytes. This indicates that LT/Sv oocytes develop cytostatic factor during metaphase I. Eventually, anaphase occurs spontaneously and the hybrids extrude the polar body and form pronuclei in a proportion similar as in controls. In hybrids between LT/Sv metaphase I oocytes and wild-type metaphase II oocytes (which contain cytostatic factor) anaphase I proceeds at the time observed in control LT/Sv oocytes and hybrids between two M I LT/Sv oocytes, and is followed by the parthenogenetic activation and formation of interphase nuclei. Also the great majority of hybrids between M I and M II wild-type oocytes undergoes the anaphase but further arrests in a subsequent M-phase. These observations suggest that an internally triggered anaphase I occurs despite the presence of the cytostatic activity both in LT/Sv and wild-type M I oocytes. Anaphase I triggering mechanism must therefore either inactivate or override the CSF activity. The comparison between spontaneous and induced activation of metaphase I LT/Sv oocytes shows that mechanisms involved in anaphase I triggering are altered in these oocytes. Thus, the prolongation of metaphase I in LT/Sv oocytes seems to be determined by delayed anaphase I triggering and not provoked directly by the cytostatic activity.     

Theca cell monolayers that inhibit maturation of bovine oocytes show differences in their protein secretion pattern

A previous study reported that coculturing bovine cumulus-oocyte complexes (COCs) with theca cell monolayers maintained oocytes in meiotic arrest. The present study evaluated whether the protein secretion pattern in this system is different between theca cells and granulosa cells and whether the presence of COCs influences their pattern of secretion. Follicular cells were isolated from 2- to 5-mm follicles and cultured in TCM-199 supplemented with 10% fetal calf serum. Theca cell monolayers maintained COCs but not denuded oocytes (DOs) in meiotic arrest. Monolayers were incubated for 6 hr in medium supplemented with radioactive L-[³⁵S]methionine. The patterns of protein secreted in the medium were analyzed by electrophoresis SDS-PAGE 10%. These results showed that theca cell monolayers secreted two major proteins. This pattern was different from the protein pattern secreted by granulosa cell monolayers. The molecular weights of these proteins were estimated to be 214 and 190 kDa. Coculturing COCs with theca cell monolayers during the labeling revealed that COCs modulated the secretion of theca cell monolayers. When theca cells were grown on collagen-coated wells, the monolayers did not maintain the oocytes at the germinal vesicle (GV) stage. The secretion of the 214-kDa protein also decreased. Then, when theca cell monolayers are effective to maintain oocytes in meiotic arrest, the cells especially secreted the 214-kDa protein. In conclusion, the 214-kDa protein secreted by theca cell monolayers may play a role in the process of maintaining oocytes in meiotic arrest. Mol. Reprod. Dev. 50:200–206, 1998. © 1998 Wiley-Liss, Inc.     

Dynamic behaviour of stationary pronuclei during their positioning in Paramecium caudatum

During conjugation of Paramecium caudatum, there are two well-known stages when nuclear migration occurs. What happens to the nuclei is closely related to their localisations in cells. The first of these stages is the entrance of one meiotic product into the paroral region. This nucleus survives, while the remaining three outside this area degenerate. The second stage is the antero-posterior localisation of eight synkaryon division products. Four posterior nuclei are differentiated into macronuclear anlagen, whereas four anterior nuclei remain as the presumptive micronuclei. In this experiment, the process of the third prezygotic division of P. caudatum was studied with the help of protargol staining. Here, a third nuclear migration was discovered. By two spindle turnings and two spindle elongations, stationary pronuclei were positioned near migratory pronuclei. This positioning of stationary pronuclei could shorten the distance for transferred migratory pronuclei to recognise and reach the stationary pronuclei. This fosters the synkaryon formation of P. caudatum.     

Untersuchung der ersten Entwicklungsphasen von Embryo und Endosperm der Art Jasione montana L. an lebendem Material

Summary Growth of the zygote and the first phases of the endosperm development of Jasione montana L. in isolated intact ovules was studied. The zygote begins to grow simultaneously with the first division of the primary endosperm nucleus. It forms a long outgrowth into the embryo sac. A distinct oil droplet occurs in the basic part of the zygote, which disappears after the development of the embryo is advanced.The nucleus of the zygote shifts to the top of the outgrowth of the zygote before the prolongated growth of the zygote is completed. The first mitosis in the embryo takes plase in this position at the time when there are 8–16 cells in the endosperm.The endosperm division as it can be seen in the living material is described.     

Two novel meiotic restitution mechanisms in haploid maize (<Emphasis Type="Italic">Zea mays</Emphasis> L.)

Two original mechanisms of nuclear restitution related to different processes of meiotic division of pollen mother cells (PMCs) have been found in male meiosis of the lines of maize haploids no. 2903 and no. 2904. The first mechanism, which is characteristic of haploid no. 2903, consists in spindle deformation (bend) in the conventional metaphase-anaphase I. This leads to asymmetric incomplete cytokinesis with daughter cell membranes in the form of incisions on the mother cell membrane. As a result, the chromosomes of the daughter nuclei are combined into a common spindle during the second meiotic division, and a dyad of haploid microspores is formed at the tetrad stage. The frequency of this abnormality is about 50%. The second restitution mechanism, which has been observed in PMCs of haploid no. 2904, results from disturbance of the fusion of membrane vesicles (plastosomes) at the moment of formation of daughter cell membranes and completion of cytokinesis in the first meiotic division. This type of cell division yields a binuclear monad. In the second meiotic division, the chromosomes of the daughter nuclei form a common spindle, and meiosis results in a dyad of haploid microspores. The frequency of this abnormality is as high as 15%. As a result, haploid lines no. 2903 and no. 2904 partly restore fertility.     

Egg maturation and parthenogenetic recovery of diploidy in the scorpion Liocheles australasiae (Fabricius) (Scorpions, ischnuridae)

In the scorpion Liocheles australasiae, egg maturation and parthenogenetic recoveries of chromosome number and nuclear DNA content were examined by histological, karyological observations and quantitative measurements of DNA. The primary oocyte becomes mature through two successive maturation divisions. The first maturation division takes place in the primary oocyte to produce a secondary oocyte and a first polar body. The second maturation division soon occurs in the secondary oocyte, in which the nucleus is divided into a mature egg nucleus and a second polar body nucleus, not followed by cytoplasmic fission. The first polar body, in one case, was successively divided into two second polar bodies; in the other case it was not divided. In either case, these polar bodies remained attached to the early embryo. The fate of these polar bodies during further embryogenesis were studied. In the karyological analysis, the chromosome number was divided into two groups, one from 27-32, the other was 54-64. The former was presumably the metaphase chromosome number at the meiotic division; the latter was presumably the metaphase chromosome number at the mitotic division. DNA content in the diploid nucleus of the primary oocyte, doubled before the maturation divisions, was reduced through the maturation divisions by one-half in the nuclei of the secondary oocyte and the first polar body and by one-fourth in the nuclei of the egg and the second polar bodies. The first reduction of DNA content corresponded to halving the number of the chromosomes in the first maturation division and the second to the nuclear division in the secondary oocyte. These reductions represent a common process of egg maturation, except the final production of the mature egg with two haploid nuclei, an egg nucleus, and a second polar body nucleus. These two nuclei, which were formed apart in the mature egg, drew near to fuse into a zygote nucleus. The chromosome number and nuclear DNA content were doubled in the zygote and each blastomere in embryos, supporting the hypothesis that the egg nucleus fuses with the second polar body nucleus and this conjugation initiates subsequent embryonic development.     

Orderly formation of the double ring structures for plastid and mitochondrial division in the unicellular red alga Cyanidioschyzon merolae

The formation of the plastid-dividing ring (PD ring) and mitochondrion-dividing ring (MD ring) was studied in a highly synchronous culture of the unicellular red alga Cyanidioschyzon merolae. The timing and the order of formation of the MD and PD rings were determined by observing organelles around the onset of their division, using transmission electron microscopy. In  C. merolae, there is one chloroplast and one mitochondrion per cell, and the shape of the chloroplast changes sequentially from acorn-like, to round, to trapezoidal, to peanut-shaped, in that order, during the early stage of chloroplast division. None of the cells with acorn-shaped or round chloroplasts contained organelles with PD rings or MD rings, while all of the cells with peanut-shaped chloroplasts contained organelles with both PD rings and MD rings. In cells with peanut-shaped chloroplasts, the PD and MD rings were double ring structures, with an outer ring located on the cytoplasmic face of the outer membrane of the organelle, and an inner ring located in the matrix beneath the inner membrane. These results suggested that the double ring structures of the PD ring and the MD ring form when chloroplasts are trapezoidal in shape. Detailed three-dimensional observation of cells with trapezoidal chloroplasts revealed the following steps in the formation of the double ring structures of the PD and MD rings: (i) the inner ring of the PD ring forms first, followed by the outer ring; (ii) then the MD ring forms and becomes visible; (iii) when the double ring structures of the two rings have formed, the microbody then moves from its remote location to the plane of division of the mitochondrion and contraction of the PD and MD rings commences. These steps were also confirmed by computer-aided three-dimensional reconstruction of the images from serial thin sections. This study reveals the order of formation of the double ring structures of the PD and MD rings, and the behavior of the microbody around the onset of division of plastids and mitochondria. The results also provide the first evidence that the inner PD ring is not a tension element formed by the contractile pressure but a definite structure, independent of the outer ring. Received: 31 March 1998 / Accepted: 14 May 1998     

Temporal expression of cell cycle-related proteins during spermatogenesis: establishing a timeline for onset of the meiotic divisions

During spermatogenesis, the complex events of the first meiotic prophase and division phase bring about dramatic changes in nuclear organization. One factor frustrating mechanistic dissection of these events is lack of knowledge about precisely what events occur, in what order they occur, and how they may be interrelated by temporal sequence; in other words, a precise timeline is lacking. This temporal ordering problem can be tackled by following expression and localization in mouse spermatocytes of proteins critical to events of the meiotic cell division process. These include ones that are primarily chromosomal and related to pairing and recombination, as well as kinases and substrates that mediate the cell cycle transition. Distinct and protein-specific patterns occur with respect to expression and localization throughout meiotic prophase and division and dramatic relocalization of proteins occurs as spermatocytes enter the meiotic division phase. This information provides a foundation for a meiotic timeline that can be augmented to provide, eventually, a complete catalog of meiotic events and their temporal sequence. Such a framework can clarify mechanisms of normal meiosis as well as mutant phenotypes and aberrations of the meiotic process that lead to aneuploidy.     

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)     

MITOSIS, CELL WALL AND FLAGELLA SYNTHESIS IN SYNCHRONIZED CULTURES OF THE PRASINOPHYTE, SCHERFFELIA DUBIA

Cell division occurs within the parental cell wall, yielding two progeny cells. Since Scherffelia dubia sheds all four flagella prior to cell division, the maturing progeny cells must regenerate new cell walls and flagella during and/or after cytokinesis. To better understand these processes, we have synchronized cell division in cultures of S. dubia and observed all stages of mitosis, cytokinesis, and progeny cell maturation, including flagella and cell wall formation, via DAPI staining of fixed cells, DIC microscopy of live cells embedded in agarose and standard TEM. Microscopical observations revealed the following sequence of events: 1) Golgi stacks divide during late interphase and immediately begin producing theca scales; 2) deflagellation and release of the parental cell wall from the plasma membrane occurs during early prophase; 3) synthesis of theca and flagella scales within the Golgi and/or scale reticulum continues throughout mitosis; 4) during cytokinesis, a coalescence of vesicles containing theca scales at the posterior end of the cell results in a cleavage furrow slightly diagonal to the cells' longitudinal axis (40 min); 5) post-mitotic nascent basal body formation and flagella elongation at the inherited basal bodies (and later at the mature nascent basal bodies) occurs concurrently with continued cell wall synthesis; 6) the cleavage furrow rotates into a transverse position (35 min); 7) reorientation of the nuclei results in a "head to tail" orientation of the maturing progeny cells; and 8) matured progeny cells emerge from the posterior end of the parental theca not before 8 hrs after the onset of mitosis.     

Chromatin remodeling in mammalian zygotes.

With sperm-egg fusion at the time of fertilization the gamete nuclei are remodeled from genetically quiescent structures into pronuclei capable of DNA synthesis. Features of this process that are critical to insure the genetic integrity of the zygote and the success of subsequent embryonic development include: oocyte responses that prevent polyspermy; completion of the 2nd meiotic division by the oocyte; exchange of proteins in the sperm nucleus; and, remodelling of the oocyte chromosomes and sperm nucleus into functional pronuclei. Elucidation of the biological and molecular mechanisms underlying zygote formation and chromatin remodeling should enhance our understanding of the potential vulnerability of the zygote to toxicant-induced damage.     

The dynamic behaviour of mitochondria in living zygotes during maturation and meiosis in Chlamydomonas reinhardtii

To understand fully the function of mitochondria during the development of cells and organs, it is important to elucidate the dynamics of their morphology. However, the detailed morphology of mitochondria during meiosis has not yet been studied in algae. We examined the mitochondrial morphology of Chlamydomonas reinhardtii and classified zygotes into seven types by mitochondrial morphology in order to analyse the morphological change in mature and meiotic zygotes. We also investigated the oxygen consumption of living zygotes and the effects of tubulin and actin polymerization inhibitors on mitochondria, using fluorescence microscopy and oxygen electrodes. During zygote maturation, mitochondria fragmented into small particles, with a large decrease in oxygen consumption. When mature zygotes were exposed to light, mitochondria became tubular and formed a network, and oxygen consumption gradually recovered. At the same time, particle-like mitochondrial nucleoids became stringy and produced new nucleoid particles. Tubular mitochondria accumulated around the cell nucleus and then spread throughout the cell. Cell division followed (first and second rounds), and the resultant daughter cells had tubular mitochondria in a mesh-like arrangement. An inhibitor of tubulin polymerization, demecolcine, inhibited the assembly of mitochondria around the cell nucleus, whereas an inhibitor of actin polymerization, latrunculin B, inhibited the formation of tubular mitochondria. These results suggest that microtubules are probably involved in mitochondrial accumulation around the cell nucleus, whereas microfilaments may maintain the tubular network of mitochondria.     

Electron microscopy of sexual reproduction in Nephroselmis olivacea (Prasinophyceae, Chlorophyta)

The electron microscopy of zygote formation and the early stages of zygote germination in Nephroselmis olivacea Stein are presented. Although the gametes differ behaviorally during the early stages of gamete fusion, the alga is isogamous. The minus gamete settled on the substrate, and attached with its left side. The plus gamete swam to the minus gamete, attached ventral to the right side of the minus gamete, while slightly on its left side, and plasmogamy started. No specialized organelle for gamete fusion was seen using either scanning or transmission electron microscopy. Gametic fusion was uniform; the right side of the minus gamete always fused with the ventral, slightly left side of the plus gamete, which suggests the participation of the d‐rootlets of the flagellar apparatus of the two gametes. Body scales were retained throughout the entire sexual process. Before karyogamy, a network of endoplasmic reticulum developed between the nuclei. This position corresponded to the contractile vacuole of the plus gamete. Fusion proceeded as the minus gamete was drawn to the plus gamete and resulted in a hemispherical zygote. Fibrous material appeared on the cell surface, embedding the body scales to form a layer that thickened and contributed to the strong adhesion of the zygote to the substrate. During this stage, karyogamy was completed. A thick zygotic wall composed of two layers, an electron‐dense outer layer and a straticulate electron‐lucent inner layer developed beneath the layer of fibrous material and scales. Zygote germination was induced. After the first meiotic division, the layer of fibrous material and scales ruptured and the inner layer of the zygotic wall thinned, allowing the emergence of two germ cells. They had newly formed scales and two starch grains, but no typical pyrenoid.     

Two novel meiotic restitution mechanisms in haploid maize (Zea mays L.)

Two original mechanisms of nuclear restitution related to different processes of meiotic division of pollen mother cells (PMCs) have been found in male meiosis of the lines of maize haploids no. 2903 and no. 2904. The first mechanism, which is characteristic of haploid no. 2903, consists in spindle deformation (bend) in the conventional metaphase-anaphase I. This leads to asymmetric incomplete cytokinesis with daughter cell membranes in the form of incisions on the mother cell membrane. As a result, the chromosomes of the daughter nuclei are combined into a common spindle during the second meiotic division, and a dyad of haploid microspores is formed at the tetrad stage. The frequency of this abnormality is about 50%. The second restitution mechanism, which has been observed in PMCs of haploid no. 2904, results from disturbance of the fusion of membrane vesicles (plastosomes) at the moment of formation of daughter cell membranes and completion of cytokinesis in the first meiotic division. This type of cell division yields a binuclear monad. In the second meiotic division, the chromosomes of the daughter nuclei form a common spindle, and meiosis results in a dyad of haploid microspores. The frequency of this abnormality is as high as 15%. As a result, haploid lines no. 2903 and no. 2904 partly restore fertility.