Chromosome behavior in the primary nucleus ofAcetabularia calyculus as revealed by epifluorescent microscopy

Summary Chromosome behavior preceding secondary nuclei formation within a giant primary nucleus (50–100 m in diameter) inAcetabularia calyculus was observed by the fluorescence emitted from 4-6-diamidino-2-phenylindole (DAPI)-stained DNA.Throughout the period when the large nucleolus was present in the primary nucleus, thin chromonemata were observed twining around the nucleolus. Nuclear division was initiated by degeneration of the sausage-shaped nucleolus into a number of spherical subunits soon after the initiation of cap formation. On the fourth day of cap development, the chromonemata became thicker and chromomeres appeared. They accumulated adjacent to the single spherical nucleolus. The lump of chromosomes became loosened and thick chromosomes were scattered in the nucleus. The peculiar shapes of chromosomes which suggest the existence of chiasmata were frequently observed until the chromosome segregation started. This sequence of chromosome behavior seems to be the prophase of meiotic division. Chromosome segregation, the first meiotic division, occurred on the seventh day of cap development, probably being accompanied by the second meiotic division. Immediately after nuclear division of the primary nucleus, secondary nuclei were formed and cyst formation started 24 hours after repeated mitoses of the secondary nuclei.     

The life cycle ofAcetabularia (Dasycladales,Chlorophyceae): A compilation of evidence for meiosis in the primary nucleus

Summary The life cycle ofAcetabularia is described with special reference to nuclear divisions. Recent arguments, derived from the fields of cytology, genetics and systematics are in favour of the hypothesis, that meiosis occurs during the division of the primary nucleus. This hypothesis is summarized in a schematical representation of the whole life cycle.     

Observations on the sterile whorls ofAcetabularia

Summary The sterile whorls ofAcetabularia increase greatly the surface area of the cell. These structures function in the uptake of solutes from the environment. The development of the whorls is controlled by light and also by the availability of nutrients.     

Ultrastructural features of the life cycle ofAcetabularia mediterranea

Summary The portion of the life cycle ofAcetabularia mediterranea from cyst formation to gamete release is described. During maturation, the cyst nucleus undergoes a series of mitoses in which the nuclear membrane remains intact, and the spindle microtubules are confined to the intranuclear space. There is a dark requirement for the completion of cyst maturation at the end of which the nuclei are in groups and centrioles are present. The final migration of the nuclei, cleavage of the cytoplasm into gametes, and the development of flagella is cxtremely rapid, taking six to twelve hours. Cysts which fail to germinate are blocked at a late stage in the maturation process, either before or after the final nuclear division.The observations are related to mitosis in other green algae (Chlorophyceae) and to gametogenesis inBryopsis hypnoides. Changes in the amount and distribution of heterochromatin in the nuclei are discussed in relation to the life cycle ofA. mediterranea.The substance of this paper was first presented orally at the Symposium onAcetabularia held at the Max-Planck-Institut für Zellbiologie, Wilhelmshaven, Federal Republic of Germany, July 12–15th, 1972.Supported in part by Grant GM 06637 from the U.S. Public Health Service.     

Multinuclear stages of the life cycle ofAcetabularia mediterranea

Summary The reproductive stages of the life cycle ofAcetabularia mediterranea have been studied in Feulgen-stained material. Light-microscopical photographs of secondary nuclei, cyst formation, gamete formation, gamete release, zygote formation and early development of the germlings are presented. The time course of cytological events during gamete formation is described. Mitoses are found between 16 and 24 hours after the induction of cyst germination.The author is indebted to Mrs. S.Artelt who helped with the prepration of the specimens and the photographs.     

The role of the cell-apex in elongation ofAcetabularia

Summary Exposure of only the apical portion of anAcetabularia cell to light causes the cell to elongate as much as cells which are totally illuminated. No elongation takes place if only the apical portion of the cell is kept in the dark.In totally illuminated cells, preferential exposure to DCMU of the apical region only also inhibits cell elongation.Local-photosynthesis in the apical region might be essential for elongation. The participation of other photomorphogenetic reactions in the apical region are not excluded.     

Morphology of the nucleo-cytoplasmic interactions during the development ofAcetabularia cells

Protoplasma - An electron microscopic survey of nuclear events and changes in the perinuclear cytoplasm during the generative phase ofAcetabularia is presented with details on late stages in the...     

Division of the generative nucleus in cultured pollen tubes of the Bromeliaceae

Pollen germination, division of the generative nucleus and position of the generative nucleus in the pollen tube during in vitro germination were examined for six bromeliad cultivars. The influence of mixed amino acids (casein hydrolysate) and individual amino acids (Arg, Asn, Asp, Glu, Gly, Met, Phe, Orn, Tyr) were tested. Aechmea fasciata and A. chantinii pollen tubes showed more generative nuclear division in cultured pollen tubes than the other four cultivars tested. Casein hydrolysate did not stimulate generative nuclear division. In general arginine (1 mM) improved division of the Aechmea generative nucleus and to a lesser extent this of Vriesea `Christiane', Guzmania lingulata and Tillandsia cyanea. A concentration of 2 mM arginine reduced pollen tube growth of Aechmea. The vegetative nucleus was ahead of the generative nucleus in approximately 50% of the pollen tubes of all cultivars studied. In about 25% of the pollen tubes, the generative nucleus was ahead and in ±25% pollen tubes the vegetative and generative nuclei were joined together. The distance between the two generative nuclei and the distance from the generative nuclei to the pollen tube tip differed significantly for Aechmea fasciata and A. chantinii. The influence of different amino acids for Aechmea fasciata and A. chantinii varied with respect to pollen germination and generative nuclear division. Arg and Met improved nuclear division of both Aechmea cultivars. Pollen germination and sperm cell production were not linked. This information is important to ameliorate in vitro pollination methods used to overcome fertilization barriers in Bromeliaceae and other higher plants.     

Current-voltage relationships of potassium channels in the plasmalemma ofAcetabularia

Summary The outer membrane of mechanically prepared protoplasmic droplets fromAcetabularia mediterranea has been investigated by patch-clamp techniques. These membranes are shown to consist of physiologically intact plasmalemma. With the Cl^– pump inhibited, microscopic currents through K⁺-selective channels were studied. These currents compare well with macroscopic K⁺ currents as previously determined by standard microelectrode techniques and tracer flux measurements. There is about one K⁺ channel per m² in the plasmalemma. The current-voltage relationship (I–V curve) of the main open channel (channel A) is sigmoid over a voltage range between about –100 and +100 mV with saturation currents of about ±10 pA. A second species (or different state of channel A) of K⁺-selective channels (channel B) differs from channel A by smaller saturation currents (about ±7 pA) and a much smaller open probability. The open probability of channel A increases from almost zero at large negative voltages to about 1/2 at large positive voltages. Taking the closed times into account, the mean steady-stateI–V curve of channel A displays outward rectification about the equilibrium voltage for K⁺ and negative slope conductance at larger negative voltages. The open channelI–V curve of the open channels A and B, the changes of theI–V curve of the open channel A upon variation of the external K⁺ concentration, as well as the mean steady-stateI–V curves of channel A are described by simple reaction kinetic models, the parameters of which are determined to fit the experimental data. The results are discussed with respect to data from other K⁺ channels in plants and with respect to regulation of the cytoplasmic K⁺ concentration inAcetabularia.     

The flagellar root system in the gamete ofAcetabularia mediterranea

Summary Ultrastructural investigation of the flagellar root system ofAcetabularia gametes reveals one type of organization for both male and female gametes. There is a modified cruciate system with four microtubular bands X-2-X-2, with X=4. A prominent distal striated fiber and a small proximal striated fiber connect the flagellar bases. A striated root fiber type I underlies the microtubular root type II, and a short striated root fiber type I underlies the microtubular root type I (terminology ofMelkonian 1980 b). This specific root system has some details in common with theChlamydomonas type, and others with theUlvaphyceae and the siphonalean algaeDerbesia andBryopsis. This might indicate the phylogenetic relationships.     

RNA Synthesis in isolated living and fixed nuclei ofAcetabularia

Summary Isolated, gently fixed nuclei ofAcetabularia mediterranea are capable of incorporating nucleotides into RNA. This has been demonstrated by (a) sensitivity of the synthesized products to RNase, (b) inhibition of RNA synthesis by actinomycin D and (c) incubation of nuclei without CTP and GTP. Experiments with -amanitin show that at least two RNA polymerases are active in isolated fixed nuclei. Also isolated, living nuclei incorporate nucleotides into RNA. RNA synthesis was located autoradiographically and quantified by a TV-analysator.     

Analysis of specificities in the movement of cytoplasmic clusters in the cell ofAcetabularia mediterranea

The movement of cytoplasmic clusters of high density was studied in the stalk ofAcetabularia mediterranea using time-lapse microcinematography during regeneration. The kinetics of expansion and contraction of cytoplasmic clusters is described in acropetal and basipetal directions.     

Spontaneous and induced changes in the membrane potential and resistance ofAcetabularia mediterranea

Summary The normal resting potential across theAcetabularia mediterranea cell membrane is –170 mV and the resistance is about 0.1 k·cm². The time courses of potential and resistance changes have been studied in connection with several slow dynamic properties of the membrane. These effects include spontaneous and stimulated depolarizing spikes, spontaneous repolarization after prolonged maintenance of a quasistable depolarized condition, and a hyperpolarizing response when current is applied to the cell in this depolarized state. These processes show considerable similarities to each other, which suggests that they might all be explained by changes in permeability to Cl^–. In normal conditions, membrane punch-through occurs with a relatively small hyperpolarizing bias.     

Lampbrush-type chromosomes in the primary nucleus of the green alga Acetabularia mediterranea

Structures with a lampbrush-chromosome-like morphology are described in the nucleoplasm of primary nuclei of the green alga, Acetabularia mediterranea, by light and electron microscopy in sections of cells fixed in situ and in spread preparations of isolated nuclear components. These chromosomes reveal typical loops (up to 20 μm long), chromomere-like nodules (1–2 μm in diameter), and 2–4 μm large axial globules. Associations of some of these chromosomes with nucleolar structures and with the nuclear envelope are also recognized. The light microscopically identified loops are correlated with distinct fibrillogranular structures observed in the thin sections and with the very long matrix units seen in the spreadpreparations. The similarity of these structures to the lampbrush chromosomes of various animal cell types, all exclusively stages of meiotic prophase, is discussed as well as the possible relation of the appearance of lampbrush chromosomes to a defined phase of the vegetative growth of this alga.