Anastomoses and nuclear condition in Cephalosporium coccorum petch

Nuclear condition and formation of anastomoses inC. coccorum Petch have been reported. Preliminary study on the nuclear condition shows that the vegetative mycelium and conidia are uninucleate. Anastomoses in mycelium and conidia are discernible. Hyphal fusion through anastomosing bridges and nuclear migration through them show the possibility of heterokaryosis in the organism. Nutrient free medium is suitable for the development of anastomoses both in mycelium as well as in conidia.     

Nuclear and cell migration during pollen development in rice (Oryza sativa L.)

Nuclear and cell migration during pollen development in rice were studied using semi-thin section light microscopy, differential interference contrast microscopy and epifluorescence microscopy. Four migrations of nuclei and cells were observed and described in detail here. The first nuclear migration occurs at the uninucleate microspore stage, when the nucleus of the microspore migrates from the center to the periphery of the cell, and then to the wall opposite the pollen aperture where pollen mitosis I takes place. The second migration occurs at the early bicellular pollen stage, with the vegetative nucleus migrating three-quarters of the circumference of the pollen wall, finally locating at the periphery of the wall where the microspore cell nucleus is positioned. The third migration occurs at the late bicellular pollen stage, with the vegetative nucleus migrating from the periphery of the cell to the central part of the pollen and the generative cell migrating from the opposite side of the aperture to a position between the aperture and the vegetative nucleus where pollen mitosis II takes place. The fourth migration appears at the mature pollen stage when the two sperm cells and the vegetative nucleus migrate to the opposite side of the aperture, finally becoming positioned in the cytoplasm of the vegetative cell distal to the aperture where the male germ unit forms. Cytological observations of pollen abortion resulting from allelic interaction at the S-a, S-b and S-c loci show that abnormalities in the first or second nuclear migration result in the formation of empty abortive pollen, whereas abnormalities in the third or fourth migrations cause production of stainable abortive pollen.     

Density thresholds for Nesidiocoris tenuis (Heteroptera: Miridae) in tomato crops

Nesidiocoris tenuis (Reuter) (Heteroptera: Miridae) is common in vegetable crops of the Mediterranean area, with an increasing worldwide range of geographical distribution. This omnivore is a reputed predator of small arthropod pests, but also produces injuries on vegetative and reproductive plant parts. The aim was to estimate density thresholds based on N. tenuis and whitefly abundance for the management of N. tenuis in tomato crops. The assay was carried out in mesh-walled and plastic greenhouses in southern Spain during 2004 and 2007. The natural population dynamics of N. tenuis and whitefly were monitored, and impact on yield quantified. The economic injury level and intervention threshold were predicted based on the zoophytophagous response of N. tenuis and the yield compensation of tomato plants. The proportion of aborted flowers on the tomato plants was related directly to the abundance of N. tenuis and inversely to the interaction between the number of N. tenuis and the number of whitefly immatures. Over-compensation of fruit weight was predicted for flower abortion rates due to N. tenuis lower than 0.171. No yield reduction is expected for values <0.65 N. tenuis per leaf, independent of the whitefly abundance, nor for up to 5 N. tenuis and >26 whitefly immatures per leaf. For intermediate N. tenuis levels, the outcome depends on the prey density. The probability of N. tenuis producing yield loss in tomato crops increases at N. tenuis:whitefly ratios >0.168. Yield reduction is expected after N. tenuis population peaks, when whitefly numbers have been reduced.     

Alternaria leaf spot of tobacco II. Independent segregation of morphological and virulence traits in conidial populations of Alternaria tenuis nees

Morphologically, conidia ofAlternaria tenuis exhibit dual forms, A and B. Virulent single conidial isolates commonly produce a considerably higher percentage of A-type conidia than B-type conidia. While avirulent isolates produce more B-type than A-type conidia. Genetic factors governing conidial morphology appear to be extranuclear and not genetically linked to virulence. Selection for A-type conidia for six cultural cycles resulted in a rapid and marked segregation of A-type morphology and virulence.The relationship between conidial morphology and virulence commonly observed in fresh isolates appears fortuitous and is apparently due to separate and independent selection pressures (adverse climate and resistant host tissue) operating on the sameA. tenuis population which results in two genetically independent traits (morphology and virulence) in the population appearing linked. The frequent breakdown of this relationship, especially in culture, indicates that conidial morphology is not a reliable criterion for virulence.     

Peculiarities of Exogenous Dormancy of Aspergillus nigerConidia

Aspergillus nigerconidia are characterized by exogenous dormancy: the first stage of their germination is accomplished in twice-distilled water. However, germ tube formation requires the availability of carbon and nitrogen sources. Exogenous dormancy in A. nigerconidia exhibits the following peculiar features: (i) nitrogen-containing substances are active stimulators of germination; (ii) temperature-dependent changes in the lipid bilayer and in the neutral lipid composition of conidia are virtually identical to those occurring in growing mycelium under temperature stress; and (iii) the spore viability threshold does not exceed 45°C; i.e., the spores are more heat-resistant than the mycelium, but they are less heat-resistant than the spores that are in the state of endogenous dormancy. According to the current classification of the types of cell metabolism arrest, the exogenous dormancy of A. nigerconidia resembles the pattern of metabolism characteristic of vegetative cells during the idiophase.     

Karyology and the possible function of the dual conidia ofDissoconium (Hyphomycetes)

The morphological development and nuclear behaviour of two species ofDissoconium are described. The fungi are monokaryotic in all stages of their life cycle. Liberated pairs of conidia may form anastomoses, probably serving survival under hostile environmental conditions. The function of the one-celled conidia inD. dekkeri remains unexplained.     

Growth and sporulation of the fungus Claviceps purpurea in batch fermentation

Summary The sporulation of the fungus Claviceps purpurea is connected with the change in its respiration rate which is effected by deficiency of dissolved oxygen tension. It stops the vegetative growth of the fungus and induces the formation of conidiophores with conidia production until glucose is exhausted. With exhaustion of glucose the conidiophores continue to produce conidia by transforming vegetative cell material into conidia. Therefore final conidial concentration in batch fermentation depends on these two processes which can be regulated by oxygen input.     

FEULGEN MICROSPECTROPHOTOMETRIC STUDIES OF PANDORINA MORUM AND OTHER VOLVOCALES (CHLOROPHYCEAE)12

The nuclear DNA content during normal vegetative growth and division has been examined in three species of Volvocales, Chlamydomonas reinhardtii Dangeard, Pandorina morum Bory, and Volvox carteri f. nagariensis Iyengar. The results are consistent with the nuclear cycle reported in the literature for Eudorina. Nuclear DNA content does not increase during the prolonged cell growth phase. At the time of colony formation, nuclear DNA doubles, the nucleus divides, and this alternation continues until the final 2ⁿ complement of progeny nuclei is formed. The 4- and 8-nucleate stages of dividing gonidia of V. carteri have a nuclear DNA content in the same range as the somatic cells; they are not polyploid or polytene. Four normal clones of Pandorina, having 2, 5 or 12 chromosomes, all had similar amounts of DNA per nucleus, suggesting that the species has a nuclear genome of fairly constant size rather than consisting of many strains representing a polyploid series. One unique clone, a hybrid with double the chromosome number of either its parents, had twice as much DNA as the normal clones. The Feulgen spectrophotometric method is sufficiently sensitive to detect 2-fold differences in DNA content at the level of 2 × 10^?13 g of DNA /nucleus, and its use avoids the complications associated with the presence of organelle DNA.     

CYTOLOGY OF HELMINTHOSPORIUM TURCICUM AND ITS ASCIGEROUS STAGE,TRICHOMETASPHAERIA TURCICA

Knox- Davies , P. S., and J. G. Dickson . (U. Wisconsin, Madison.) Cytology of Helmintho sporium 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.     

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.     

Interactions between population processes in a cyclic species: parasites reduce autumn territorial behaviour of male red grouse

The causes of population cycles fascinate and perplex ecologist. Most work have focused on single processes, whether extrinsic or intrinsic, more rarely on how different processes might interact to cause or mould the unstable population dynamics. In red grouse (Lagopus lagopus scoticus), two causal mechanisms have been supported: territorial behaviour (changes in autumn aggressiveness) and parasites (parasite induced reduction in fecundity). Here, we report on how these two regulatory processes might interact, by testing whether the parasite suspected to cause the grouse cycles, the nematode Trichostrongylus tenuis, reduces male autumn territorial behaviour. We either treated males with an anthelmintic, to remove parasites (dosed or D-males), or challenged them with infective T. tenuis larvae, to increase parasite intensity (challenged or C-males). We first show that dosing was effective in removing T. tenuis parasites, while parasite intensities increased in challenged birds during the autumn. Because old males initially had more parasites than young males, the treatments generated greater differences in parasite intensity in old than in young males. We also show that various aspects of territorial behaviour (increase in testosterone-dependent comb size in autumn, territorial call rate, likelihood of winning territorial interactions and over-winter survival) were significantly higher in dosed than in challenged males, but in old birds only. Our data thus supported the hypothesis that parasites reduce male aggressiveness during the autumn territorial contests, and could thereby influence recruitment. Our results also highlight that the territorial behaviour of young males, which have fewer parasites, is not as limited by parasites as that of old, previously territorial males. We discuss the implications of these findings for our understanding of the processes regulating red grouse populations and causing their complex, unstable population dynamics.     

Developmental cytology of the genus Vaucheria I. Organisation of the vegetative filament

The developmental cytology of the vegetative filament of Vaucheria has been investigated with light and electron microscopes. The vegetative filament consists of three distinct zones: the apical zone, the sub-apical zone and the zone of vacuolation. Organelle distribution and associations in these zones have a primary role in controlling morphogenetic events which influence growth and induce differentiation of sexual and asexual organs. Cell elongation in the polarised vegetative filament occurs by vesicular addition in the apical zone. Two types of active cyclosis are found exclusively in the zone of vacuolation. Nuclear cyclosis involves microtubular bands which associate with persistent centrioles found adjacent to the outer nuclear membrane. A specialised mitochondrial-dictyosome association is transported by a microfilament-like system of anastomosing strands. This latter system is extremely labile to conventional fixatives.     

Light-independent conidiation in Trichoderma spp.: a novel approach to microcycle conidiation

Microcycle conidiation in Trichoderma hamatum and T. harzianum has been achieved in complete darkness for the first time. The time required for mass conidiation without intervening vegetative growth was decreased to 24 h instead of 6 to 7 days. The conidia produced by microcycle conidiation were viable and had pigmentation and antagonistic behaviour similar to those of the parental stock cultures.N. Khurana, R.K. Saxena, R. Gupta and R.C. Kuhad are with the Department of Microbiology, University of Delhi South Campus, Benito Juarez Road, New Delhi-110021, India.     

Evaluation of the potential role of water in spread of conidia of the Neotyphodium endophyte of Poa ampla

Neotyphodium endophytes are asexual, filamentous fungi, mutualistically associated with diverse cool season grasses. Infected seeds and vegetative organs of infected host plants are the only known modes of propagation of the asexual endophytes. In the last decade certain Epichloë and Neotyphodium-infected grass species have been shown to have epiphyllous structures of the endophytes, hyphae, conidiophores, and conidia, growing on leaf blades. The production of epiphyllous conidia suggests the possibility that some of these endophytes may have the ability for plant-to-plant spread. The objective of this study was to determine the possible mechanisms involved in liberation and dispersal of asexual spores of Neotyphodium growing in vitro and epiphyllously on leaves of Poa ampla. Our results indicate that water dispersal is the most likely means of dissemination of conidia of the Neotyphodium sp. Wind generated by dry compressed air does not dislodge the conidia from slide cultures or from P. ampla leaves.     

DNA synthesis during meiosis of eight-spored strains ofChlamydomonas reinhardi

Summary In strain 137F ofChlamydomonas reinhardi, the zygospores undergo one round of nuclear DNA replication followed by three divisions to produce octospores. The third division without replication has been interpreted by Sueoka et al. (1967, 1969) to mean that the gametes and vegetative cells have at least binemic chromosomes. We have repeated their experiments using the same strain. However, the meiotic products were inviable — unable to undergo postmeiotic vegetative growth, DNA replication or division. On the other hand, using a variant of strain 137C which also has three divisions during germination we have shown that meiosis is normal. Zygospores from this strain undergo two rounds of nuclear DNA replication prior to the formation of octospores. These meiotic products are viable and capable of postmeiotic vegetative growth, replication and division. Since the third division without DNA replication subsequent to the two meiotic divisions leads to inviable products, and the strain which has viable products after three divisions does not lack the additional replication, meiosis inChlamydomonas reinhardi provides no evidence of a bineme chromosome structure.     

GENE SEQUENCE DIVERSITY AND THE PHYLOGENETIC POSITION OF ALGAE ASSIGNED TO THE GENERA PHAEOPHILA AND OCHLOCHAETE (ULVOPHYCEAE,CHLOROPHYTA)1

The phylogenetic position of microfilamentous marine green algae assigned to the species Phaeophila dendroides, Entocladia tenuis (Phaeophila tenuis, and Ochlochaete hystrix was examined through phylogenetic analyses of nuclear‐encoded small subunit rDNA and chloroplast‐encoded tufA gene sequences. These analyses placed the P. dendroides strains within the Ulvophyceae, at the base of a clade that contains representatives of the families Ulvaceae, Ulvellaceae, and the species Bolbocoleon piliferum, supporting an earlier hypothesis that P. dendroides constitutes a distinct lineage. Substantial divergence in both nuclear and plastid DNA sequences exists among strains of P. dendroides from different geographic localities, but these isolated strains are morphologically indistinguishable. The lineage may have an accelerated rate of gene sequence evolution relative to other microfilamentous marine green algae. Entocladia tenuis and O. hystrix are placed neither in the P. dendroides clade nor in the Ulvellaceae as previous taxonomic schemes predicted but instead form a new clade or clades at the base of the Ulvaceae. Ruthnielsenia gen. nov. is proposed to accommodate Kylin's species, which cannot be placed in Entocladia (=Acrochaete), Phaeophila, or Ochlochaete. Ruthnielsenia tenuis (Kylin) comb. nov., previously known only from Atlantic coasts, is reported for the first time from the Pacific coast of North America (San Juan Island, WA, USA). Isolates of R. tenuis from the Atlantic and Pacific coasts of North America have identical small subunit rDNA and tufA gene sequences.     

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.     

Characterization of the Heterokaryotic and Vegetative Diploid Phases of MAGNAPORTHE GRISEA

The heterokaryotic and vegetative diploid phases of Magnaporthe grisea, a fungal pathogen of grasses, have been characterized. Prototrophic heterokaryons form when complementary auxotrophs are paired on minimal medium. Hyphal tip cells and conidia (vegetative spores) taken from these heterokaryons are auxotrophs with phenotypes identical to one or the other of the parents. M. grisea heterokaryons thus resemble those of other fungi that have completely septate hyphae with a single nucleus per cell. Heterokaryons have been utilized for complementation and dominance testing of mutations that affect nutritional characteristics of the fungus. Heterokaryons growing on minimal medium spontaneously give rise to fast-growing sectors that have the genetic properties expected of unstable heterozygous diploids. In fast-growing sectors, most hyphal tip cells are unstable prototrophs. The conidia collected from fast-growing sectors include stable and unstable prototrophs, as well as auxotrophs that exhibit a wide range of phenotypes, including many recombinant classes. Genetic linkage in meiosis has been detected between two auxotrophic mutations that recombine in vegetatively growing unstable diploids. The appearance of recombinants suggests that homologous recombination occurs during vegetative growth of M. grisea. No interstrain barriers to heterokaryosis and diploid formation have been detected. The mating type of the strains that are paired does not influence the formation of heterokaryons or diploids.     

Establishment and maintenance of nuclear position during zoospore formation in allomyces macrogynus: roles of the cytoskeleton

The involvement of the microtubule (MT) and actin microfilament (MF) cytoskeletons in establishing nuclear positions during zoosporogenesis in Allomyces macrogynus was assessed using selective cytoskeletal disrupting treatments and documented with light microscopy. These experiments were coupled with low-speed centrifugation studies to determine the degree to which cytoskeletal elements anchor nuclear position. At the onset of zoospore formation, nuclei were positioned only in cortical cytoplasmic regions of the zoosporangia (ZS). Immunofluorescence microscopy revealed that MTs primarily emanated from centrosomal regions into the surrounding cytoplasm at this stage. During delimitation of the cytoplasm into individual uninucleate zoospores, nuclei migrated from cortical regions to become distributed throughout the cytoplasm. Coincident with nuclear migrations, MTs were primarily organized at and emanated from nuclear surfaces, forming extensive perinuclear arrays. Nuclear migrations were suppressed in ZS induced to sporulate in the presence of cytochalasin D, an actin MF inhibiting compound. Disruption of MTs with nocodazole did not block nuclear migrations, although resultant nuclear spacing was irregular. Centrifugation treatments of control and drug-treated ZS demonstrated that nuclear positions were stabilized by perinuclear MT arrays. The results indicate that nuclear motility in ZS of A. macrogynus is the result of an actin-based system while perinuclear MTs arrays function to establish and fix nuclear position during zoospore formation. Copyright 1998 Academic Press.