CRUCIFORM NUCLEAR DIVISION IN SOROSPHAERA VERONICAE

Sorosphaera veronicae Schroet. is an endobiotic, holocarpic, obligately parasitic fungus presently classified in the Plasmodiophoromycetes. The ultrastructure of nuclear envelope formation in somatic nuclear division in cystosoral plasmodia was studied. The inner membrane of the nuclear envelope during prophase appears to invaginate and blebb off intranuclear membranous vesicles. The intranuclear membranous vesicles become associated with the surface of the separating chromatin in anaphase and eventually are involved in the formation of daughter nuclear envelopes within the original nuclear envelope. The sequence of nuclear envelope breakdown and reformation in S. veronicae is noteworthy because it emphasizes alternate methods of nuclear envelope formation other than the generally considered “typical” formation described in Allium cepa L.     

THE ULTRASTRUCTURE OF CRUCIFORM NUCLEAR DIVISION IN SOROSPHAERA VERONICAE (PLASMODIOPHOROMYCETE)

Vegetative nuclear divisions in cystosoral Plasmodia from the shoot system of Sorosphaera veronicae Schroeter were studied with standard transmission electron microscopy. Each metaphase nucleus forms a cruciform configuration as the persistent nucleolus elongates perpendicularly to chromatin aligned on the equatorial plate. The nuclear envelope remains intact during metaphase and anaphase. Each spindle pole consists of a fenestrated nuclear envelope with an exteriorly situated centriole and closely associated endoplasmic reticulum. Intranuclear membranous vesicles occur within metaphase and anaphase nuclei and are closely associated with chromatin and nuclear envelope. Microtubules pass from centrioles into the nucleus and are also attached to chromatin at kinetochores.     

NUCLEAR DIVISION IN ALTERNARIA TENUIS

A study of the hyphal and conidial nuclei of Alternaria tenuis with the Feulgen nuclear reaction indicates that division is mitotic. Five chromosomes and a spindle apparatus are formed during nuclear division.     

VEGETATIVE NUCLEAR DIVISION IN NEUROSPORA

A re-examination of the mode of vegetative nuclear division in Neurospora crassa was facilitated by the availability of the mutant “clock” which produces definite growth bands. Since the growth rhythm is correlated with nuclear divisions, stained mycelial mats of this mutant prepared at intervals from the beginning of a growth period provided a sequence of stages of division. In a 28-hour period the following broad features of nuclear behavior were observed: In the early part of the period during rapid mycelial growth, dividing elongated nuclei predominated. At the end of the period the mycelium contained mostly rounded resting nuclei. In the middle of a growth period nuclear forms of various degrees of annularity occurred along with elongated and rounded nuclei. Elongated and rounded nuclei completed division cycles without change in form, although the corresponding stages of the two types were similar. Elongated nuclei assumed a spiral form at the beginning of division. As division proceeded, relaxation of the nuclear gyres was accompanied by a visible duplication of the chromatin thread and the appearance of chromomere-like bodies on the daughter threads. One of the chromomere-like bodies became displaced and was interpreted to be a chromosome or a segment of a chromosome that acts as a mitotic center. All the chromosomes were found to be interconnected and to act as a unit throughout the division cycle. Only after the separation of the daughter chromatin threads could seven chromosomes be counted. Electron microscopic studies complemented the observations with the light microscope. On the basis of the evidence it was concluded that the vegetative nuclear division in Neurospora differs from the classical mitotic pattern in the following respects: (1) absence of visible centrioles, (2) the presence of interconnected chromosomes, (3) the comparatively late appearance of countable chromosomes, and (4) the frequent presence of interzonal connections between separating chromatin threads.     

NUCLEAR DIVISION IN THE DEVELOPING PYCNOSPORES OF MACROPHOMINA PHASEOLI

Nuclear behavior was studied in Giemsa-stained pycnidial material of Macrophomina phaseoli. The young pycnospore was at first uninucleate but repeated nuclear division occurred as it enlarged, and the mature spore was always multinucleate. The developing spore, therefore, provided a favorable site for observing mitosis. Noteworthy features were: (1) a metaphase characterized by a loss in stainability of the nucleolus and by the appearance of centrioles, spindle fibers and countable chromosomes; (2) a short early anaphase; (3) a long late anaphase with a broad band of interzonal fibers separating the daughter nuclei; (4) the occurrence of irregularities such as aneuploidy, chromosome bridge formation and chromosome lagging. The basic chromosome number was six. Following a study of Giemsa-stained cellophane cultures of the fungus it was concluded that the method of nuclear division in the vegetative hyphae is similar to that observed in the developing spores. Some comparisons are made with observations on nuclear division in the vegetative hyphae of other fungi.     

COORDINATNE NUCLEAR AND CHLOROPLAST DIVISION IN UNILOCULAR SPORANGIA OF LAMINARIA ANGUSTATA (LAMINARIALES,PHAEOPHYCEAE)1

Changes in the number of nuclei and chloroplasts were examined during the process of unispore formation in unilocular sporangia of Laminaria angustata. Just before meiosis, eight chloroplasts were always present in unilocular sporangial mother cells. The number of chloroplasts remained constant through meiosis. After the resulting four nuclei divided again (third nuclear division), a close association between a nucleus and a chloroplast developed among each of the eight nuclei and eight chloroplasts. The eight chloroplasts divided ahost synchronously before the synchronous division of the eight nuclei. Following the 16 nucleate stage with 16 chloroplasts and the final 32 nucleate stage with 32 chloroplasts, 32 unispores, each with a nucleus and a chloroplast, were fomd in unilocular sporangza of L. angustata. Immunofluorescence microscopy using an anti-centrin antibody showed that two anticentrin-stained structures (as future mitotic poles) occurred adjacent to each of the premitotic four nuclei, and each spot was located near a chloroplast. Therefore, after the third division, each of the eight nuclei established close contact with a chloroplast presumably mediated by the centrosomes.     

NUCLEAR STRUCTURE AND DIVISION IN THE VEGETATIVE MYCELIUM OF THE SAPROLEGNIACEAE

Bakerspigel , Alexander . (The New Mount Sinai Hospital, Toronto.) Nuclear structure and division in the vegetative mycelium of the Saprolegniaceae. Amer. Jour. Bot. 47(2): 94—100. Illus. 1960.–Saprolegnia parasitica, S. ferax and Achlya racemosa are additional examples of fungi in whose vegetative mycelium the nuclei do not divide in a manner directly comparable to ordinary mitosis. During division the entire nucleus, which consists of a crescent, ring or cap of chromatin and the nucleolus, becomes angular and elongates. The chromatin separates into 2 portions which are situated at opposite ends of the elongating nucleus. As division proceeds the elongated nucleus constricts at the midregion and the sister nuclei separate. Each sister nucleus consists of a crescent of chromatin and a portion of the divided nucleolus. Spindles, chromosomal filaments or metaphase plates could not be demonstrated during division.     

CHROMOSOME NUMBERS IN THE HYPOCREALES 1. NUCLEAR DIVISION IN THE ASCUS OF NECTRIA PEZIZA

El -Ani , Arif S. (Columbia U., New York, N. Y.) Chromosome numbers in the Hypocreales. 1. Nuclear division in the ascus of Nectria peziza. Amer. Jour. Bot. 46(6): 412–417. Illus. 1959.—The 4 nuclear divisions in the ascus of Nectria peziza were studied with the aid of acetoorcein and aceto-carmine techniques. The ascus was found to arise by crozier formation. Synapsis takes place while the chromosomes are still contracted and elongation of chromosomes continues throughout the pachytene phase. The haploid complement was found to consist of 5 chromosomes, the second of which is the nucleolus-organizing chromosome. This chromosome number which had never been reported in the Hypocreales was also found in several other species of this group of fungi. Chromosome numbers in the Hypocreales are discussed with regard to the species concept in the imperfect genus Fusarium.     

THE CRUCIFORM MUSCLE AND ITS ASSOCIATED SENSE ORGAN IN SCROBICULARIA PLANA (DA COSTA)

Long dendritic processes from neurones of the sense organ ganglionpenetrate the epithelial lining of the narrow and short ductsituated between the intramuscular slit and a large posteriorcavity which opens to the siphonal space through a narrow canalin the sense organ papilla. Motor nerve fibres innervate theanterior ends of the cruciform muscle strands. The latter becomesstretched during opening of animal and contracted during closure.The slit is widened during opening and narrowed during closure.The sense organ responds to foul water suggesting that it isa chemoreceptor for this stimulus. (Received 10 July 1977;     

TRANSIENT LOCALIZATION OF γ-TUBULIN AROUND THE CENTRIOLES IN THE NUCLEAR DIVISION OF BOERGESENIA FORBESII (SIPHONOCLADALES, CHLOROPHYTA)

Mitosis in Boergesenia forbesii (Harvey) Feldman was studied by immunofluorescence microscopy using anti-β–tubulin, anti-γ–tubulin, and anti-centrin antibodies. In the interphase nucleus, one, two, or rarely three anti-centrin staining spots were located around the nucleus, indicating the existence of centrioles. Microtubules (MTs) elongated randomly from the circumference of the nuclear envelope, but distinct microtubule organizing centers could not be observed. In prophase, MTs located around the interphase nuclei became fragmented and eventually disappeared. Instead, numerous MTs elongated along the nuclear envelope from the discrete anti-centrin staining spots. Anti-centrin staining spots duplicated and migrated to the two mitotic poles. γ–Tubulin was not detected at the centrioles during interphase but began to localize there from prophase onward. The mitotic spindle in B. forbesii was a typical closed type, the nuclear envelope remaining intact during nuclear division. From late prophase, accompanying the chromosome condensation, spindle MTs could be observed within the nuclear envelope. A bipolar mitotic spindle was formed at metaphase, when the most intense staining of γ-tubulin around the centrioles could also be seen. Both spindle MT poles were formed inside the nuclear envelope, independent of the position of the centrioles outside. In early anaphase, MTs between separating daughter chromosomes were not detected. Afterward, characteristic interzonal spindle MTs developed and separated both sets of the daughter chromosomes. From late anaphase to telophase, γ-tubulin could not be detected around the centrioles and MT radiation from the centrioles became diminished at both poles. γ-Tubulin was not detected at the ends of the interzonal spindle fibers. When MTs were depolymerized with amiprophos methyl during mitosis, γ-tubulin localization around the centrioles was clearly confirmed. Moreover, an influx of tubulin molecules into the nucleus for the mitotic spindle occurred at chromosome condensation in mitosis.     

NUCLEAR DNA CONTENT VARIATION IN HELIANTHUS (ASTERACEAE)

Nuclear 2C DNA content was microspectrophotometrically determined for 19 diploid (2n=34) species of Helianthus L. DNA amount varied over a fourfold range. Helianthus neglectus had the lowest (53.40 ± 0.51 Feulgen absorbancy units [FAU]) and H. agrestis the highest (216.30 ± 2.30 FAU) DNA content. Chromosome size ranged from 1.1–1.7 μm long in H. neglectus to 3.2-5.0 μm long in H. agrestis. When the highly divergent H. agrestis was excluded from analyses, the mean of the annual species (70.05 FAU) was significantly different from the mean of the perennials (97.70 FAU). No correlations of DNA content with habitat, soil type, annual precipitation, or geographical location were apparent among species.