Ascospore Delimitation Research Articles

Aberrant nuclear behavior at meiosis and anucleate spore formation by sporulation-deficient ( spo) mutants of Saccharomyces cerevisiae

The fine structural characteristics of wild-type and sporulation-deficient mutants ( spo) of yeast were examined. The results indicate that prospore wall formation, growth and closure, and nuclear budding and separation at meiosis represent parallel and normally coordinated developmental pathways of morphological change whose integration can be disrupted by gene mutation. At the restrictive temperature most cells of spo 1-1/ spo 1-1 diploids terminate prior to the first spindle body duplication. In spo 2-1/ spo 2-1 diploids the nucleus divides precociously both at meiosis I and at meiosis II. This aberrant behavior is followed by the formation of anucleate spores. In spo 3-1/ spo 3-1 diploids development is normal until meiosis II. At this point nuclear segregation becomes retarded relative to ascospore delimitation. As a result much of the nuclear material fails to be incorporated into the ascospores.

The fine structure of ascospore delimitation in the yeast Wickerhamia fluorescens

Photographic records of complete serial sections of asci in different stages of sporulation show that one of the four nuclear lobes produced during meiosis in the ascus of the yeast Wickerhamia fluorescens has a complex spindle-pole body, which is the site from where the presumptive ascospore wall, or prospore wall, develops and eventually surrounds the ascospore nucleus and associated cytoplasm. The three remaining nuclei develop spindle-pole bodies and prospore walls to lesser and varying degrees. With few exceptions, all three degenerate. The outer membrane of the prospore wall forms a fold, or rim, on the outside of the spore. Thickening of the spore wall takes place first in the asymmetric ring, then around the spore body, and finally at the site where the nucleus is associated with the wall. It is shown that ascospore delimitation in W. fluorescens and Saccharomyces cerevisiae are similar to each other, and that it differs from the type observed in a number of Euascomycetes.

Ascospore wall development in Saccharomyces cerevisiae.

Ascospore delimitation in Saccharomyces cerevisiae is initiated by a pair of unit membranes between which the spore wall is subsequently laid down.

A DEVELOPMENTAL MUTANT THAT AFFECTS ASCOSPORE DELIMITATION IN NEUROSPORA TETRASPERMA

Self-fertile mycelia of N. tetrasperma containing normal nuclei and nuclei with a previously undescribed mutation (Gsp-1) yield perithecia bearing asci most of which contain a single giant ascospore. The mutation can be maintained in a self-fertile heterokaryon. All attempts to recover Gsp-1 nuclei from the heterokaryon have failed, although complementary nuclei (that is, those not carrying the Gsp-1 mutation) are easily recovered. Hence, in addition to its effect on ascospore delimitation, Gsp-1 is a recessive lethal. Data are presented to show that Gsp-1 is linked to mating type and probably arose as a spontaneous mutation.

CYTOLOGY OF THE ASCUS IN ACHAETOMIUM GLOBOSUM AND A. LUTEUM

The cytology of the ascus in Achaetomium globosum and A. luteum was studied using BAC (butanol-acetic acid-chromic acid, 3:2:1) fixative and propiono-iron-haematoxylin stain. Ascus initial is the binucleate penultimate cell of the crozier. In A. luteum pachytene chromosomes are very long, easily countable, and the nucleolar organizer is clearly distinct. In general, divisions I and III stages are prominent in both the species studied, while division II stages are less frequent. Division IV starts during the delimitation of ascospores and continues in the mature ascospores. The chromosome number as counted from metaphase III configurations in A. globosum and pachytene configurations in A. luteum is 7 (n=7).

Chromosome Numbers in the Hypocreales. II. Ascus Development in Nectria Cinnabarina

The cytology of ascus development in Nectria cinnabarina was investigated with the orceinsmear technique, from crozier formation to ascospore maturation. At prophase I synapsis occurs while the chromosomes are still contracted, and the nucleus passes through dictyotene, a diffuse stage rarely seen in plants. A haploid complement of five chromosomes has been precisely determined. The first two divisions in the ascus constitute meiosis, and the third (mitotic) is followed by ascospore delimitation. A fourth division takes place in the ascospore, which is subsequently divided by a septum into two uninucleate cells. Of all species of Nectria thus far investigated N. cinnabarina is the only species in which additional nuclear divisions in the ascus do occur, accounting for the multinucleate condition in the ascospore cells. The bearing that this distinctive nuclear condition has on phylogeny and evolution in the Hypocreales is discussed.

CHROMOSOME NUMBERS IN THE HYPOCREALES. II. ASCUS DEVELOPMENT IN NECTRIA CINNABARINA

The cytology of ascus development in Nectria cinnabarina was investigated with the orceinsmear technique, from crozier formation to ascospore maturation. At prophase I synapsis occurs while the chromosomes are still contracted, and the nucleus passes through dictyotene, a diffuse stage rarely seen in plants. A haploid complement of five chromosomes has been precisely determined. The first two divisions in the ascus constitute meiosis, and the third (mitotic) is followed by ascospore delimitation. A fourth division takes place in the ascospore, which is subsequently divided by a septum into two uninucleate cells. Of all species of Nectria thus far investigated N. cinnabarina is the only species in which additional nuclear divisions in the ascus do occur, accounting for the multinucleate condition in the ascospore cells. The bearing that this distinctive nuclear condition has on phylogeny and evolution in the Hypocreales is discussed.

Nuclear Behaviour and Ascospore Delimitation in Xylosphaera polymorpha

SUMMARY: Nuclear behaviour and ascospore delimitation in Xylosphaera polymorpha was studied by light and electron microscopy. Electron-dense bodies occurred near nuclei at all stages from crozier formation to the development of young ascospores. It is suggested that these bodies be termed archontosomes. Asco-spores were delimited by the invagination of an ascus vesicle and the arch-ontosome-astral-ray complex participated in both the delimitation and orientation of the ascospores. The possible role of this complex in determining ascospore form is discussed.

The Fine Structure of Ascospore Formation in Pyronema Domesticum

provided by the light microscope. In this report I attempt to show the relationships between the light- and electron-microscopic data, based on evidence obtained from electron micrographs. Light microscopists have provided us with four strikingly different methods of ascospore delimitation. Gjurasin (1893) reported that the divisions of the nuclei in the ascus were karyokinetic and that asters were associated with the division process. In the 8-nucleate stage of the ascus he described the folding of the rays of the aster around the individual nuclei. Harper (1897, 1899, 1900) expanded these observations of nuclear behavior and described free cell-formation, a unique

The Cytology of Mycosphaerella pinodes*

The behaviour of nuclei during ascus and spore formation and maturation in Mycosphaerella pinodes (Berk. Blox.) Stone was studied by means of the HCl-Giemsa technique. The mechanisms of dikaryotization could not be determined but typical croziers were formed. Nuclear fusion occurred in the young ascus and was followed by four conventional successive divisions. At pachytene the chromosomes could be counted and measured. The haploid chromosome number was found to be 15. Ascospore delimitation could be seen at metaphase IV so that each spore is homokaryotic. Two nuclear divisions occurred in the ascospores so that each cell became quadrinucleate.

ROSELLINIA LIMONIISPORA: NUCLEAR CHANGES IN THE DEVELOPING ASCUS

The nuclear cycle in the developing ascus of Rosellinia limoniispora, as revealed in aceto-orcein and acetocarmine smears, follows the general pattern for the higher Ascomycetes: crozier formation, early synapsis of homologous chromosomes in the fusion nucleus followed by nucleolar fusion, rapid elongation of chromosomes and ascus up to late pachytene of prophase I followed by contraction of the chromosomes and nucleolus, and the formation of eight nuclei as a result of two meiotic and one mitotic divisions. The orientation of nuclei in divisions II and III is haphazard. Ascospore delimitation is by simple cleavage of the cytoplasm; no centriole – astral ray mechanism was observed. A nuclear division occurs in each young ascospore, one daughter nucleus being cut off into a cell and degenerating.

CYTOLOGY OF HELMINTHOSPORIUM TURCICUM AND ITS ASCIGEROUS STAGE, TRICHOMETASPHAERIA TURCICA

Knox‐Davies, P. S., and J. G. Dickson. (U. Wisconsin, Madison.) Cytology of Helminthosporium turcicum and its ascigerous stage, Trichometasphaeria turcica. Amer. Jour. Bot. 47(5) : 328—339. Illus. 1960.–The cells of the vegetative hyphae were generally multinucleate. Interphase nuclei resembled those of higher organisms, with a matrix of thread‐like chromatin material surrounding a spherical nucleolus. “Beaked” nuclei frequently associated with anastomosing hyphae were interpreted as migrating nuclei. Nuclear division in the vegetative hyphae was rapid. Various division stages were distinguished but it was difficult to make accurate chromosome counts. The nucleoli were discarded at prophase or prometaphase and were reorganized in daughter nuclei at telophase. An outstanding feature of nuclear division was that all the nuclei in a cell divided simultaneously. Conidiophores and conidia were occasionally joined by wide cytoplasmic connections. They were multinucleate throughout their development. Mechanisms therefore exist for the perpetuation of heterokaryons through the conidium. Ascus development was studied in a hybrid between a dark and an albino isolate. Crozier formation was typical and nuclear fusion occurred in the young ascus. Four nuclear divisions were completed in the ascus before there was evidence of ascospore delimitation. Further nuclear division took place in the ascospores whose cells were multinucleate. The occurrence of less than 8 ascospores in an ascus appeared to follow degeneration of nuclei rather than the incorporation of a number of division‐Ill nuclei in a single ascopore. Chromosome counts and irregularities in the appearance and behavior of nuclei and chromosomes in the asci indicate that aneuploidy occurs in Trichometasphaeria turcica. It is suggested that aneuploidy is a common phenomenon in the conidial stage of the fungus H. turcicum, and possibly also in other imperfect fungi.

CYTOLOGICAL STUDIES OF ASCUS DEVELOPMENT IN COCHLIOBOLUS SATIVUS

Conjugate divisions occurred in the binucleate crozier of Cochliobolus sativus (Ito & Kurib. ex Kurib.) Drechs. ex Dast. Two cross walls were laid down in such a way that the penultimate cell was binucleate. A lateral bulge developed in this cell and it quickly assumed the clavate shape of the young ascus. The two nuclei in the clavate ascus fused and this was accompanied by a fusion of their nucleoli. In meiosis two divisions occurred with pairing between highly contracted chromosomes early in the first division. A third division (mitotic) followed and resulted in the formation of eight haploid nuclei. Ascospore delimitation began at this stage and each spore was initially uninucleate. Evidence of structural hybridity was seen in all divisions in the ascus. The haploid chromosome complement, as determined from pachytene and metaphase stages, consisted of seven or eight chromosomes.

Hybridization Studies on a Zinc-Induced Variant of Hypomyces Ipomoeae

SUMMARYA striking variant has been produced by cultivation of monoploid hyphae of Hypomyces Ipomoeae on medium containing a sub-lethal concentration of zinc nitrate.Analyses of 5 consecutive single-conidium culture-series, consisting of 25 cultures each, have proved the mutant strain, aborta, to be homocaryotic. Single-conidium and mass-transfer cultures of the aborta strain have shown a high degree of uniformity.The most striking feature associated with the variant was that four, rather than eight, spores were formed in aborta × normal asci, due apparently to inability of nuclei carrying the aborta factors to delimit spores.Evidence strongly indicated that in rare instances ascospores in aborta × normal asci may include more than one of the eight nuclei resulting from maturation of the zygote nucleus. Thus, in two instances, single ascospores isolated from hybrid perithecia yielded mixed cultures containing both normal and aborta hyphae. The two fx aborta strains thus obtained bore opposite sexual-reaction factors. In two other instances, single ascospores from hybrid perithecia yielded cultures containing normal hyphae of both sexual reactions, as was evidenced by the appearance of fertile perithecia in both cultures.The inability of aborta nuclei to delimit ascospores may be attributed to the inactivation or to the deletion of one or more genes determining ascospore delimitation. The ascospore-delimitation factors and the sexual-reaction factors are in different linkage groups.The genie alteration or mutation, although having occurred on medium containing a sub-lethal concentration of zinc, cannot be considered a specific effect of zinc ion without further evidence.

The Mechanics of Sexual Reproduction in Neurospora

(1935). The Mechanics of Sexual Reproduction in Neurospora. Mycologia: Vol. 27, No. 4, pp. 418-438.