The fine structure of autotrophic and heterotrophic cells of Chlorella vulgaris (Emerson strain)

Cells of the Emerson strain of Chlorella vulgaris become verymuch enlarged when cultured heterotrophically. The ultrastructureof these giant cells has been examined and compared with thatof cells cultured autotrophically. The chief area of differenceis within the chloroplast where there is a massive accumulationof starch and little or no synthesis of lamellae under heterotrophicconditions. Synthesis of cell wall material is apparently possibleunder heterotrophic conditions but the production of the variouscellular organelles does not keep pace with the increase incell volume. (Received August 12, 1968; )     

THE UROPHYSIS AND THE CAUDAL NEUROSECRETORY SYSTEM OF FISHES

1. The caudal neurosecretory system is defined in teleosts as a complex of secretory neurones (Dahlgren cells) in the caudal spinal cord leading by a tract to neurohaemal tissue organized as a typical neurosecretory storage-release organ: the urophysis. 2. The teleost urophysis is generally a distinct, easily recognizable, lobate structure of variable external form. Significant morphological variations lie in the organization of the neurosecretory fibres in relation to the vascular bed and in the degree of penetration of the meninx by the neurosecretory fibres to form an organ external to the spinal cord proper. 3. The elasmobranch caudal system is composed of large cells with short axons projecting to a diffuse vascular bed; there is no organized urophysis. 4. The caudal neurosecretory system and its urophysis appear late in post larval development by comparison with the hypothalamic neurosecretory system. The Dahlgren cells originate from the ependyma in development and also during regeneration of the caudal system in adult life. 5.The elasmobranch system may represent the more primitive condition, and stages in the evolution of the advanced urophysial types can be visualized. The particular histology shown by the caudal system appears to have taxonomic significance. 6.The cytology of the Dahlgren cell and its neurosecretory material is described. The proteinaceous neurosecretory material has an affinity for acid stains but not for the Gomori stains or reagents demonstrating SH/SS groups. The inclusions visible at the light-microscope level are aggregates of elementary neurosecretory granules, 800–2500A diameter, which originate from Golgi centres. The possible participation of preterminal axonal regions–and tubular systems evident therein—in the formation of neurosecretory material is considered. 7.The structure of the axon terminals raises questions about the way in which neurohormone may be released into the blood. Small vesicles have been variously interpreted as cholinergic synaptic vesicles and as products of the fragmentation of membranes of elementary neurosecretory granules. Evidence for the release of ‘neuro-secretion centripetally’ into the cerebrospinal fluid also exists. 8.Functional analysis of the caudal neurosecretory system has proven most difficult, The bulk of earlier data and more recent information indicate a role in ionic regulation. Increased sodium uptake by the gills of goldfish has been reported, as a result of administration of urophysial extract, and electrophysiological studies indicate a responsiveness of the system to variations in blood sodium ion concentration. The urophysis also has a definite pressor effect in eels and will stimulate water retention in anurans. The early claim of Enami that the system was involved in buoyancy regulation has never been substantiated. It must be admitted that the function of this system, virtually ubiquitous in teleost and elasmobranch fishes at least, has been anything but established and still represents a major challenge to comparative physiologists.     

Trypanosoma thecadactyli sp. n. from Forest Geckoes in Panama,and its Development in the Sandfly Lutzomyia trinidadensis (Newstead) (Diptera,Psychodidae)*

SYNOPSIS. Trypanosoma thecadactyli sp. n. is described from the forest gecko Thecadactylus rapicaudus in Panama. Blood smears from 18 of 25 (72%) geckoes sampled were positive for the new trypanosome. The flagellate, ～22 μm long, is ovoid or roughly triangular in shape and has a posterior cytoplasmic projection. Tree buttresses, rock crevices, and caves are microhabitats shared by T. rapicaudus and the phlebotomine sandfly Lutzomyia trinidadensis. Approximately 20% of the wild-caught and dissected L. trinidadensis were infected with flagellates. Twenty-four of 45 (53%) laboratory-reared L. trinidadensis allowed to feed on parasitized geckoes developed heavy infections of the mid- and hindgut which persisted up to 2 weeks, the longest postprandial period before dissection. Transmission probably occurs as a result of geckoes feeding on infected sandflies in nature. The distribution of L. trinidadensis, from Mexico to Brazil, approximates that of the gecko. Laboratory results and ecologic observations implicate L. trinidadensis as a potential vector of T. thecadactyli.     

Host-Integrated Development in the Amoebidiales*

SYNOPSIS. Induction of the resistant stage in the Amoebidiales appears to be dependent upon a morphogenetic factor derived from the host animal. This triggering substance is released either at death or ecdysis of the host arthropod. Growth of the epizoon, Amoebidium parasiticum, in both complex and defined media, follows a sporangiospore sporangium pattern. The amoeba-cyst phase, however, was only noted after crushed host material (Cladocera) was added to the axenically grown thalli. Subsequent tests, evaluated with a quantitative assay, indicate that amoebagenesis is influenced by pH, temperature, condition of the protist, and concentration of the active principle. Commercial “dried daphnia” has consistently yielded an active extract. A large number of complex and simple materials has been screened for activity, and only extracts of ground crab show promise as an alternate source. The amoebagenic factor is heat stable, relatively small, and not soluble in ether, acetone or chloroform. Paper chromatography with polar solvent systems yields a single active band, and this is the best evidence that amoebagenesis is induced by a single, rather than a multiple factor. Paramoebidium, the second genus in this obscure order, undergoes amoebagenesis at ecdysis of the host. The possibility that Amoebidium and Paramoebidium are simply different growth forms of the same protist is discussed.     

Class II major histocompatibility complex genes of the sheep

The class II genes of the sheep major histocompatibility complex (MHC) have been cloned from two unrelated heterozygous sheep into cosmid vectors. By restriction mapping and hybridization with a number of class II probes of human and mouse origin, the cloned genetic material has been assigned to seven distinct alpha genes, 10 distinct beta genes and 14 beta-related sequences. It was difficult to identify homologues of specific HLA class II genes because of a tendency for the ovine genes to cross-hybridize between HLA probes representing different loci. Such cross-hybridization was especially marked among the beta genes. While DQ and DR homologues have been tentatively identified by several criteria, no genes corresponding to DP have been identified. Cosmids containing class II alpha and beta genes have been transfected into mouse LTK- cells, and surface expression of a sheep class II molecule has been obtained.     

The Effect of the Lipogenic Inhibitor Cerulenin, on Macrostomal Cell Formation in Tetrahymena vorax

ABSTRACT. Macrostomal cell formation is blocked by the antibiotic cerulenin at levels of 15 μg/ml or higher. Inhibition can be reversed up to 4 h following cerulenin addition by washing and resuspending cells in new, noncerulenin-treated transforming principle. In these latter cases, additional time equal to the time spent in the inhibitor, is needed for cells to reach control values of transformation. Neither the addition of saturated or unsaturated fatty acids, cholesterol added alone or in combination with stearic acid, nor a mixture of lipids extracted from Tetrahymena vorax reversed the cerulenin effect. Radioisotope incorporation data showed while protein synthesis was reduced by the end of 1 h and tetrahymanol synthesis by the end of 2 h, little or no effect of this inhibitor occurred on RNA or fatty acid synthesis during these times. One interpretation of these results is that cerulenin, by preventing first protein synthesis and later tetrahymanol synthesis, interferes with synthesis and formation of membranes required for the microstome to macrostome transition.     

Seasonal progressions of tissue and cell water relations parameters in evergreen and deciduous perennials

Abstract. Tissue and cell water relations parameters were followed for Heteromeles arbutifolia, Cercis occidentalis and Aesculus californica , in an environment exhibiting seasonally increasing drought. The extensive seasonal osmotic adjustment of evergreen H. arbutifolia and the moderate adjustment in C. occidentalis closely matched their respective seasonal decreases in minimum daily water potential. Summer deciduous A. californica exhibited only small drops in osmotic potential and water potential. Experiments with irrigated plants indicated that drought was not required for the osmotic adjustment of H. arbutifolia and C. occidentalis. However, in H. arbutifolia drought treatment enhanced osmotic adjustment. In irrigated H. arbutifolia , osmotic adjustment was mainly the result of an accumulation of osmotica. In drought-stressed plants, the same change in osmotic potential resulted from a combination of accumulation of osmotica and a decrease in symplast volume.     

Effect of elevated CO2 on rhizosphere carbon flow and soil microbial processes

Direct effects of increased above-ground CO₂ concentration on soil microbial processes are unlikely, due to the high pCO₂ of the soil atmosphere in most terrestrial ecosystems. However, below- ground microbial processes are likely to be affected through altered plant inputs at elevated CO₂. A major component of plant input is derived from litter fall and root turnover. Inputs also derive from rhizodeposition (loss of C-compounds from active root systems) which may account for up to 40% of photoassimilate. This input fuels the activity of complex microbial communities around roots. These communities are centrally important not only to plant–microbe interactions and consequent effects on plant growth, but also, through their high relative activity and abundance, to microbially mediated processes in soil generally. This review focuses on approaches to measure C-flow from roots, in particular, as affected by increased atmospheric CO₂ concentration. The available evidence for impacts on microbial communities inhabiting this niche, which constitutes an interface for possible perturbations on terrestrial ecosystems through the influence of environmental change, will also be discussed. While methodologies for measuring effects of increased CO₂ concentration on plant growth, physiology and C-partitioning are abundant and widely reported, there is relatively little information on plant-mediated effects on soil microbial communities and processes. Importantly, many studies have also neglected to recognize that any secondary effects on microbial communities may have profound effects on plant parameters measured in relation to environmental change. We critically review approaches which have been used to measure rhizodeposition under conditions of increased atmospheric CO₂ concentration, and then consider evidence for changes in microbial communities and processes, and the methodologies which have been recently developed, and are appropriate to study such changes.     

Plant Responses to Water Stress

This Special Issue comprises a series of papers that developsthe theme of plant responses to water stress, encompassing recentdevelopments at the molecular level, through responses of photosynthesisand metabolism, to their application in crop selection and yield.The consideration of water deficits is particularly timely,given the huge developments in this area in the past decade.This issue specifically sets out to place molecular and physiologicalprocesses and their agronomic applications in an environmentalcontext.     

Hitchhiking and the removal of microbial contaminants by the leaf‐cutting ant Atta colombica

1. The ecologically dominant leaf‐cutting ants exhibit one of the most complex forms of morphological caste‐based division of labour in order to efficiently conduct tasks, ranging from harvesting fresh leaf material to caring for the vulnerable fungal crop they farm as food. While much of their division of labour is well known, the role of the smallest workers on foraging trails is puzzling. Frequently these minim workers hitchhike on leaf fragments and it has been suggested that they may act to reduce the microbial contamination of leaf material before they enter the nest. Here we investigated this potentially important role of minims with field colonies of Atta colombica. 2. We experimentally increased the microbial load of leaf fragments and found that this resulted in minims hitchhiking on leaf fragments for longer. Furthermore, we show that leaves naturally have a significant microbial load and that the presence of hitchhikers reduces the microbial load of both experimentally manipulated and natural leaf fragments. 3. Intriguingly, the microbial load of leaves high in the canopy where ants were foraging was much lower than closer to the ground where the ants avoided cutting leaves. This suggests that the often perplexing foraging patterns of leaf‐cutting ants may in part be explained by the ants avoiding leaves that are more heavily contaminated with microbes. 4. The removal of microbial contaminants is therefore an important role of hitchhiking minim workers in natural colonies of Atta leaf‐cutting ants, although other tasks such as trail maintenance and defence also explain their occurrence on trails.