Density gradient centrifugation has been utilized to characterize the subcellular distribution of... more Density gradient centrifugation has been utilized to characterize the subcellular distribution of physiologically relevant enzymes in yeasts and filamentous fungi (Leal-Morales
The 2008-2012 Country Program Evaluation for Guyana concludes that the Bank's strategy was al... more The 2008-2012 Country Program Evaluation for Guyana concludes that the Bank's strategy was aligned with the development objectives and economic priorities of the government. Nevertheless, its program has produced mixed results. In infrastructure, progress was achieved by consolidating the primary road network and extending the electrical grid to unserved areas; however, efforts to reduce losses in the electricity sector did not meet their target. In competitiveness, the financial and judicial sectors benefitted from institutional and structural reforms, and the business environment was strengthened through the adoption of streamlined procedures. However, key initiatives to deepen the financial market and increase access to credit have met with less success. Gains have also been made towards the achievement of the Millennium Development Goals; however, the absence of updated data in the social sectors hinders the evaluation of results. The average age of the loan portfolio is 4.56 years, the oldest in the Bank. With the launch of the Low Carbon Development Strategy, Guyana has an opportunity to benefit from global commitments for rainforest conservation and climate change. The Bank should continue to support new initiatives that lie within the LCDS, and should work closely with Guyana to enhance its national capacity to monitor and report the results. Although Guyana has shown the greatest improvement among D-2 countries in terms of its Country Institutional and Policy Evaluation score, FSO resources available to the country under the current program were 32% less than the allocation under the previous program. The sustainability of the Bank?s work and its relevance in Guyana could be affected by a further reduction in future FSO allocations. The CS emphasized the strengthening and use of national systems by Bank projects. Several operations use the Integrated Financial Management System for accounting, and the intention is to use this for all new Bank-funded operations. Given the findings in this CPE, OVE recommends that the Administration: (i) take advantage of the current period of stability and growth to support Guyana on its path toward becoming a green economy; (ii) seek to leverage declining FSO resources; and (iii) continue strengthening Guyana's national systems.
The 2008-2012 Country Program Evaluation (CPE) concludes that the Bank's program was programm... more The 2008-2012 Country Program Evaluation (CPE) concludes that the Bank's program was programmatically aligned with Government's National Plan for Human Development and addressed five areas essential for growth and poverty reduction. The one significant omission in the Bank's program from a development perspective was the lack of attention to governance an area identified as a priority for future strategies by the previous CPE. The Bank played a central role in financing the country's fiscal gap over the CPE period, reflecting increased country demand in the context of the global financial crisis. Nevertheless, time constraints limited the Bank's analytical efforts to identify important policy reforms in the first series of three programmatic policy-based loans, resulting in relatively weak policy content in those loans. Even with significant increases in the level of concessional resources allocated to Nicaragua and greater complexity in programming, the efficiency of program execution improved. In terms of developmental effectiveness, project-level results have generally been poorly documented. Furthermore, the Bank has at times overlooked the issue of recurrent cost financing for services initiated under investment loans, leading to the untimely suspension of programs and undermining the effectiveness of the initial investment. In light of CPE findings, OVE recommends that the Bank: (i) undertake further diagnostic and analytic work to delineate the Bank?s potential role and value-added; (ii) continue to support improvements in the efficiency of public expenditure, including in the electricity sector; (iii) strengthen the measurement and reporting of results at the project level; (iv) identify potential sources of financing for the continued provision of services beyond project completion; and (v) consider allowing D-2 countries access to emergency lending facilities to meet fiscal needs in times of crisis.
La Evaluacion del Programa de Pais de Guyana 2008-2012 concluye que la estrategia del Banco estab... more La Evaluacion del Programa de Pais de Guyana 2008-2012 concluye que la estrategia del Banco estaba alineada con los objetivos economicos y las prioridades economicas del gobierno. Sin embargo, los resultados del programa de dicha estrategia fueron desiguales. Con respecto a la infraestructura, aunque se lograron avances mediante la consolidacion de la red vial primaria y la ampliacion de la red electrica para atender a zonas sin servicio, las medidas adoptadas para reducir las perdidas tecnicas y no tecnicas en el sector de la energia electrica no alcanzaron la meta fijada. En cuanto a la competitividad, los sectores financiero y juridico se beneficiaron de reformas institucionales y estructurales, y la adopcion de procedimientos simplificados ha fortalecido el entorno empresarial del pais. Las iniciativas mas importantes emprendidas para profundizar el mercado financiero e incrementar el acceso al credito, sin embargo, han sido menos fructiferas. Se ha seguido avanzando en la consecucion de los Objetivos de Desarrollo del Milenio; no obstante, la falta de datos actualizados sobre el sector social limita la evaluacion de los resultados. La duracion media de la cartera de prestamos es de 4,56 anos, la mas antigua del Banco. Gracias a la implantacion de la estrategia de desarrollo con baja emision de carbono, el pais tiene la posibilidad de beneficiarse de los compromisos asumidos a nivel mundial de conservar las selvas tropicales y hacer frente al cambio climatico. El Banco debe seguir respaldando iniciativas nuevas que se encuadren dentro de dicha estrategia, y debe colaborar estrechamente con Guyana a fin de incrementar la capacidad del pais para realizar el seguimiento de los resultados y divulgar informacion al respecto. Aunque Guyana ha registrado el avance mas destacado entre los paises del grupo D2, en cuanto a su puntaje en la evaluacion de politicas e instituciones nacionales, el monto de recursos del FOE que tuvo a su disposicion fue de un 32% menos que los asignados en la estrategia anterior. La sostenibilidad de la labor del Banco y su pertinencia en Guyana podrian verse afectadas por una nueva reduccion de la futura asignacion de recursos del FOE. La estrategia de pais hizo hincapie en el fortalecimiento y el uso de los sistemas nacionales en los proyectos del Banco. Varias operaciones se valen del Sistema contable integrado de administracion financiera para realizar la contabilidad, y se tiene la intencion de utilizar este sistema en todas las operaciones nuevas. Dadas las conclusiones de la presente evaluacion del programa de pais, OVE recomienda que la Administracion (i) aproveche el periodo actual de estabilidad y crecimiento para apoyar a Guyana en sus esfuerzos por convertirse en una economia verde; (ii) procure apalancar los recursos del FOE que van en disminucion, y (iii) siga fortaleciendo los sistemas nacionales de Guyana.
The hyphoid equation y = x cot (xV/N) provides a mathematical foundation for the cellular basis f... more The hyphoid equation y = x cot (xV/N) provides a mathematical foundation for the cellular basis for dimorphism. Its parameters, N and V, define two morphogenetically critical factors: the amount of wall-building vesicles produced per unit time and the rate of advance of the VSC (vesicle supply center). The V/N ratio describes the essence of vesicle dynamics of the cell. High values determine a hyphal morphology; low values, a yeast cell morphology. Manipulation of this ratio by computer graphics produces a full spectrum of morphologies seen in dimorphic fungi: from hyphal tubes to budding yeast cells plus other variations including pseudomycelium formation and triangular cell morphogenesis. In this manner, the specific dimorphic features exhibited by fungi of the genera Mucor, Candida, Saccharomyces, and Trigonopsis have been duplicated on the computer screen. The ultimate validity of the present mathematical analysis of dimorphism, which is centered mainly on the rate of displacement of the VSC, depends on the existence of a VSC in living fungal cells. There is good circumstantial evidence to believe that the Spitzenkorper found in the growing hyphal tips of higher fungi functions as a VSC, and to extrapolate that other similar vesicle accumulations seen in regions of wall formation, represent VSCs. Accordingly, one may predict that the key molecular event(s) regulating dimorphism would be those responsible for moving the VSC.
Proceedings of the National Academy of Sciences, 1976
The fine structure of isolated chitin synthetase (UDP-2-acetamido-2-deoxy-D-glucose:chitin 4-beta... more The fine structure of isolated chitin synthetase (UDP-2-acetamido-2-deoxy-D-glucose:chitin 4-beta-acetamido-deoxyglucosyltransferase; EC 2-4-1-16) particles (chitosomes) from Mucor rouxii and the elaboration of chitin microfibrils were studied by electron microscopy. Chitosomes are spheroidal, but often polymorphic, structures, mostly 40-70 nm in diameter. Their appearance after negative staining varies. Some reveal internal granular structure enclosed by a shell measuring 6-12 nm thick; others do not show internal structure but have a pronounced depression of the external surface. In thin sections, isolated chitosomes appear as microvesicular structures with a tripartite shell 6.5-7.0 nm thick. Morphologically similar structures can be seen in intact cells of M. rouxii. Isolated chitosomes undergo a seemingly irreversible series of transformations when substrate and activators are added. The internal structure changes, and a coiled microfibril (fibroid) appears inside the chitosome...
GS-1 (ncu04189) is a protein required for the synthesis of b-1,3-glucan in Neurospora crassa. As ... more GS-1 (ncu04189) is a protein required for the synthesis of b-1,3-glucan in Neurospora crassa. As chitin, b-1,3-glucan is a morphogenetically relevant component of the fungal cell wall. Previously, we showed that chitin synthases are delivered to the growing hyphal tip of N. crassa by secretory microvesicles that follow an unconventional route and accumulate in the core of the Spitzenkörper (Spk). Tagged with the green fluorescent protein (GFP), GS-1 accumulated in the hyphal apex forming a dynamic and pleomorphic ring-like structure ('Spitzenring') that corresponded to the Spk outer macrovesicular stratum and surrounded the inner core of chitin synthase-containing microvesicles. TIRF microscopy revealed that GS-1-GFP reached the hyphal apex as a population of heterogeneous-size particles that moved along defined paths. On sucrose density gradients, GS-1-associated particles mainly sedimented in a high density range 1.1272-1.2124 g ml-1. Clearly, GS-1 and chitin synthases of N. crassa are contained in two different types of secretory vesicles that accumulate in different strata of the Spk, a differentiation presumably related to the spatial control of cell-wall synthesis.
We examined in fine detail growth kinetics and intracellular events during lateral and apical bra... more We examined in fine detail growth kinetics and intracellular events during lateral and apical branching in hyphae of Neurospora crassa. By high-resolution video-enhanced light microscopy, we found remarkable differences in the events preceding lateral vs apical branching. While apical branching involved a significant disturbance in the apical growth of the parental hypha, lateral branching occurred without any detectable alterations in the growth of the parental hypha. Prior to the emergence of a lateral branch, an incipient Spitzenk€ orper was formed about 12-29 lm behind the apex. Lateral branch formation did not interfere with the elongation rate of the primary hypha, the shape of its apex or the behavior of its Spitzenk€ orper. In sharp contrast, apical branching was preceded by marked changes in physiology and morphology of the parental hypha and by a sharp drop in elongation rate. The sequence involved a cytoplasmic contraction, followed by a retraction, dislocation, and disappearance of the Spitzenk€ orper; hyphal elongation decreased sharply and a transient phase of isotropic growth caused the hyphal apex to round up. Growth resumed with the formation of two or more apical branches, each one with a Spitzenk€ orper formed by gradual condensation of phase-dark material (vesicles) around an invisible nucleation site. The observed dissimilarities between lateral and apical branching suggest that these morphogenetic pathways are triggered differently. Whereas apical branching may be traced to a sudden discrete disruption in cytoplasmic organization (cytoplasmic contraction), the trigger of lateral branching probably stems from the subapical accumulation of wall precursors (presumably vesicles) reaching a critical concentration.
Abstract Zoospores ofPhytophthora palmivora, induced to encyst by vigorous agitation, synthesize ... more Abstract Zoospores ofPhytophthora palmivora, induced to encyst by vigorous agitation, synthesize alkali-insoluble β-glucans at a linear rate for ca. 10 min. Cell-free extracts of zoospores and cysts ofP. palmivora contained particulate enzymes capable of synthesizing alkali-insoluble, noncellulosic β-1,3-, β-1,6-glucans from uridine diphosphate glucose. Similar levels of activity were detected in zoospores and cysts. Enzymatic activity was stimulated by Mg2+; Ca3+ and Mn2+ were less effective. Cellobiose and other β-linked glucose disaccharides were strong activators. Evidently, the swimming zoospore is equipped with the enzymatic ability to make insoluble, noncellulosic β-glucan—a major component of the cyst wall; hence, a mechanism must exist which prevents the operation of glucan synthetase(s) until the time of encystment. The cell-free extracts also had the ability to synthesize soluble β-1,3-glucan. Most of this material was characterized as neutral mycolaminaran and mycolaminaran phosphate.
Abstract Mycolaminarans, soluble β-1,3-glucans, are the storage polysaccharides in mycelium, spor... more Abstract Mycolaminarans, soluble β-1,3-glucans, are the storage polysaccharides in mycelium, sporangia, zoospores, cysts, and chlamydospores of diverse species of Phytophthora . Glycogen is absent. Two types of mycolaminaran have been recognized: neutral mycolaminaran and mycolaminaran phosphate. In the mycelium of all six species studied, neutral mycolaminaran is the sole reserve polysaccharide. Mycolaminaran phosphate accompanied by a smaller proportion of neutral mycolaminaran constitutes the reserve polysaccharides of sporangia, zoospores, and cysts. In chlamydospores, the reserve polysaccharide is neutral mycolaminaran with smaller amounts of mycolaminaran phosphate. In contrast to other life-cycle stages, oospores store very little, if any, mycolaminaran; only minute amounts of neutral mycolaminaran and no mycolaminaran phosphate were found in mature sexual structures (oogonium-oospore-antheridium complex). Large quantities of β-glucans are produced and metabolized during morphogenesis in Phytophthora . During the development of vegetative mycelium there is abundant synthesis and storage of mycolaminarans which are subsequently utilized for the formation of asexual and sexual sporulating structures (sporangia, chlamydospores, oospores) whose thick walls are made principally of insoluble β-glucans.
Small spheroidal structures (mostly 4070 nm in diameter) similar in shape, size, and chitin-synt... more Small spheroidal structures (mostly 4070 nm in diameter) similar in shape, size, and chitin-synthesizing ability to the chitosomes described earlier for yeast cells of Mucor rouxii were isolated from four other fungi: Allomyces macrogynus (Emerson) Emerson and Wilson, Saccharomyces ...
Video microscopy was used to examine a genetically tagged mutant (T30) of Glomerella graminicola ... more Video microscopy was used to examine a genetically tagged mutant (T30) of Glomerella graminicola whose conidia have a propensity to burst during the germination process. Before the germ tube is produced, the cell wall ruptures and the cytoplasm is extruded. The bursting takes place in less than 0.1 second. Bursting can be prevented by adding osmotica to the germination medium. By phasecontrast microscopy of alkali-washed conidia, we found that each ruptured conidium had a single gaping hole that was oriented parallel to the long axis of the spore. The holes were more frequent in the middle region of the conidia than at the apices. A mathematical model for stress based on conidial shape indicated that conidia ruptured where the stress on the wall was greatest. The microscopic observations on the orientation and distribution of the rupture sites are consistent with the hypothesis that T30 conidia have weaker walls than the wild-type. Comparative tests of resistance to mechanical breakage support this hypothesis.
Laminaran-like polysaccharides were isolated from zoospores and mycelium of Phytophthora palmivor... more Laminaran-like polysaccharides were isolated from zoospores and mycelium of Phytophthora palmivora. These glucans, which we have termed mycolaminarans, are readily soluble in cold water, appear highly homogeneous by gel filtration, and contain exclusively D-glucose residues_ Methylation analyses and enzymic digestions revealed that mycolaminarans are linear chains of (1+3)-linked B-Dglucose residues, with one or two branches joined by (1-4)~B-D linkages. The zoospore mycolaminaran has one branching residue and d.p. 29. The mycelial mycolaminaran has two branching residues and d.p. 36. The possibility is considered that mycolaminarans may be distinctive storage-polysaccharldes of the oomycetous fungi. IiWRODUCTION In previous studies' s2, we showed that the cytoplasm of the oomycetous fungus Phytophthora palmivora contains substantial proportions of water-soluble B-Dglucans hereinafter referred to as mycolaminarans. In the mycelium stage of P. palmivora, only a neutral-type mycolaminaran is present, but in other stages of the life cycle, there are also phosphorylated forms' (mycolaminaran phosphates). We now report on the structure of neutral mycolaminaran isolated from mycelium and zoospores of P. palmivora. Glycogen, the usual storage-polysaccharide of fungi, is absent in Phyfophthora and related organisms. Zevenhuizen and Bartnicki-Garcia3 found that a soluble (1+3)-/I-(145)~/I-linked D-glucan (mycohuninaran) constituted the storage polysaccharide of Phytophthora cinnamomi. MATERIALS AND METHODS Microbiological techniques.-A papaya isolate of Phytophthora palmivora (P113) was obtained from M. Aragaki of the University of Hawaii, The organism was
By computer-enhanced videomicroscopy, we mapped the trajectory of external and internal cell surf... more By computer-enhanced videomicroscopy, we mapped the trajectory of external and internal cell surface markers in growing fungal hyphae to determine the pattern of cell wall expansion during apical growth. Carbon particles (India ink) were chosen as external markers for tip expansion of Rhizoctonia solani hyphae. Irregularities in the growing apical walls of R. solani served as internal markers. Marker movement was traced in captured frames from the videotaped sequences. External and internal markers both followed orthogonal trajectories; i.e., they moved perpendicular to the cell surface regardless of their initial position in the hyphal apex. We found no evidence that the tip rotates during elongation. The discovery that the cell wall of a growing hypha expands orthogonally has major repercussions on two fronts: 1) It supports the long-held view that turgor pressure is the main force driving cell wall expansion. 2) It provides crucial information to complete the mathematical derivation of a three-dimensional model of hyphal morphogenesis based on the vesicle supply center concept. In three dimensions, the vesicle gradient generated by the vesicle supply center is insufficient to explain shape; it is also necessary to know the manner in which the existing surface is displaced during wall expansion.
Cell-free extracts from the wall-less slime mutant of Neurospora crassa and the mycelium of wild ... more Cell-free extracts from the wall-less slime mutant of Neurospora crassa and the mycelium of wild type exhibit similar chitin synthetase properties in specific activity, zymogenicity and a preferential intracellular localization of chitosomes. The yield of chitosomal chitin synthetase from slime cells was essentially the same irrespective of cell breakage procedure (osmotic lysis or ballistic disruption)--an indication that chitosomes are not fragments of larger membranes produced by harsh (ballistic) disruption procedures. The plasma membrane fraction, isolated from slime cells treated with concanavalin A, contained only a minute portion of the total chitin synthetase of the fungus. Most of the activity was in the cytoplasmic fraction; isopycnic sedimentation of this fraction on a sucrose gradient yielded a sharp band of chitosomes with a buoyant density = 1.125 g/cm3. Approximately 76% of the total chitin synthetase activity of the slime mutant was recovered in the chitosome band. Because of their low density, chitosomes could be cleanly separated from the rest of the membranous organelles of the fungus. Apparently, the lack of a cell wall in the slime mutant is not due to the absence of either chitosomes or zymogenic chitin synthetase.
We study school choice within the family, analyzing how birth order, gender, innate talent, and f... more We study school choice within the family, analyzing how birth order, gender, innate talent, and family financial restrictions impact the parents´ decision to prioritize the education of one or more of the children over the rest. We find that parents, particularly from lower income homes, are more likely to select more prestigious, higher cost schools for their eldest child, male children and the most talented children. This behavior may explain part of the positive "male bias" in learning and may have a relevant impact on income distribution among family members.
Density gradient centrifugation has been utilized to characterize the subcellular distribution of... more Density gradient centrifugation has been utilized to characterize the subcellular distribution of physiologically relevant enzymes in yeasts and filamentous fungi (Leal-Morales
The 2008-2012 Country Program Evaluation for Guyana concludes that the Bank's strategy was al... more The 2008-2012 Country Program Evaluation for Guyana concludes that the Bank's strategy was aligned with the development objectives and economic priorities of the government. Nevertheless, its program has produced mixed results. In infrastructure, progress was achieved by consolidating the primary road network and extending the electrical grid to unserved areas; however, efforts to reduce losses in the electricity sector did not meet their target. In competitiveness, the financial and judicial sectors benefitted from institutional and structural reforms, and the business environment was strengthened through the adoption of streamlined procedures. However, key initiatives to deepen the financial market and increase access to credit have met with less success. Gains have also been made towards the achievement of the Millennium Development Goals; however, the absence of updated data in the social sectors hinders the evaluation of results. The average age of the loan portfolio is 4.56 years, the oldest in the Bank. With the launch of the Low Carbon Development Strategy, Guyana has an opportunity to benefit from global commitments for rainforest conservation and climate change. The Bank should continue to support new initiatives that lie within the LCDS, and should work closely with Guyana to enhance its national capacity to monitor and report the results. Although Guyana has shown the greatest improvement among D-2 countries in terms of its Country Institutional and Policy Evaluation score, FSO resources available to the country under the current program were 32% less than the allocation under the previous program. The sustainability of the Bank?s work and its relevance in Guyana could be affected by a further reduction in future FSO allocations. The CS emphasized the strengthening and use of national systems by Bank projects. Several operations use the Integrated Financial Management System for accounting, and the intention is to use this for all new Bank-funded operations. Given the findings in this CPE, OVE recommends that the Administration: (i) take advantage of the current period of stability and growth to support Guyana on its path toward becoming a green economy; (ii) seek to leverage declining FSO resources; and (iii) continue strengthening Guyana's national systems.
The 2008-2012 Country Program Evaluation (CPE) concludes that the Bank's program was programm... more The 2008-2012 Country Program Evaluation (CPE) concludes that the Bank's program was programmatically aligned with Government's National Plan for Human Development and addressed five areas essential for growth and poverty reduction. The one significant omission in the Bank's program from a development perspective was the lack of attention to governance an area identified as a priority for future strategies by the previous CPE. The Bank played a central role in financing the country's fiscal gap over the CPE period, reflecting increased country demand in the context of the global financial crisis. Nevertheless, time constraints limited the Bank's analytical efforts to identify important policy reforms in the first series of three programmatic policy-based loans, resulting in relatively weak policy content in those loans. Even with significant increases in the level of concessional resources allocated to Nicaragua and greater complexity in programming, the efficiency of program execution improved. In terms of developmental effectiveness, project-level results have generally been poorly documented. Furthermore, the Bank has at times overlooked the issue of recurrent cost financing for services initiated under investment loans, leading to the untimely suspension of programs and undermining the effectiveness of the initial investment. In light of CPE findings, OVE recommends that the Bank: (i) undertake further diagnostic and analytic work to delineate the Bank?s potential role and value-added; (ii) continue to support improvements in the efficiency of public expenditure, including in the electricity sector; (iii) strengthen the measurement and reporting of results at the project level; (iv) identify potential sources of financing for the continued provision of services beyond project completion; and (v) consider allowing D-2 countries access to emergency lending facilities to meet fiscal needs in times of crisis.
La Evaluacion del Programa de Pais de Guyana 2008-2012 concluye que la estrategia del Banco estab... more La Evaluacion del Programa de Pais de Guyana 2008-2012 concluye que la estrategia del Banco estaba alineada con los objetivos economicos y las prioridades economicas del gobierno. Sin embargo, los resultados del programa de dicha estrategia fueron desiguales. Con respecto a la infraestructura, aunque se lograron avances mediante la consolidacion de la red vial primaria y la ampliacion de la red electrica para atender a zonas sin servicio, las medidas adoptadas para reducir las perdidas tecnicas y no tecnicas en el sector de la energia electrica no alcanzaron la meta fijada. En cuanto a la competitividad, los sectores financiero y juridico se beneficiaron de reformas institucionales y estructurales, y la adopcion de procedimientos simplificados ha fortalecido el entorno empresarial del pais. Las iniciativas mas importantes emprendidas para profundizar el mercado financiero e incrementar el acceso al credito, sin embargo, han sido menos fructiferas. Se ha seguido avanzando en la consecucion de los Objetivos de Desarrollo del Milenio; no obstante, la falta de datos actualizados sobre el sector social limita la evaluacion de los resultados. La duracion media de la cartera de prestamos es de 4,56 anos, la mas antigua del Banco. Gracias a la implantacion de la estrategia de desarrollo con baja emision de carbono, el pais tiene la posibilidad de beneficiarse de los compromisos asumidos a nivel mundial de conservar las selvas tropicales y hacer frente al cambio climatico. El Banco debe seguir respaldando iniciativas nuevas que se encuadren dentro de dicha estrategia, y debe colaborar estrechamente con Guyana a fin de incrementar la capacidad del pais para realizar el seguimiento de los resultados y divulgar informacion al respecto. Aunque Guyana ha registrado el avance mas destacado entre los paises del grupo D2, en cuanto a su puntaje en la evaluacion de politicas e instituciones nacionales, el monto de recursos del FOE que tuvo a su disposicion fue de un 32% menos que los asignados en la estrategia anterior. La sostenibilidad de la labor del Banco y su pertinencia en Guyana podrian verse afectadas por una nueva reduccion de la futura asignacion de recursos del FOE. La estrategia de pais hizo hincapie en el fortalecimiento y el uso de los sistemas nacionales en los proyectos del Banco. Varias operaciones se valen del Sistema contable integrado de administracion financiera para realizar la contabilidad, y se tiene la intencion de utilizar este sistema en todas las operaciones nuevas. Dadas las conclusiones de la presente evaluacion del programa de pais, OVE recomienda que la Administracion (i) aproveche el periodo actual de estabilidad y crecimiento para apoyar a Guyana en sus esfuerzos por convertirse en una economia verde; (ii) procure apalancar los recursos del FOE que van en disminucion, y (iii) siga fortaleciendo los sistemas nacionales de Guyana.
The hyphoid equation y = x cot (xV/N) provides a mathematical foundation for the cellular basis f... more The hyphoid equation y = x cot (xV/N) provides a mathematical foundation for the cellular basis for dimorphism. Its parameters, N and V, define two morphogenetically critical factors: the amount of wall-building vesicles produced per unit time and the rate of advance of the VSC (vesicle supply center). The V/N ratio describes the essence of vesicle dynamics of the cell. High values determine a hyphal morphology; low values, a yeast cell morphology. Manipulation of this ratio by computer graphics produces a full spectrum of morphologies seen in dimorphic fungi: from hyphal tubes to budding yeast cells plus other variations including pseudomycelium formation and triangular cell morphogenesis. In this manner, the specific dimorphic features exhibited by fungi of the genera Mucor, Candida, Saccharomyces, and Trigonopsis have been duplicated on the computer screen. The ultimate validity of the present mathematical analysis of dimorphism, which is centered mainly on the rate of displacement of the VSC, depends on the existence of a VSC in living fungal cells. There is good circumstantial evidence to believe that the Spitzenkorper found in the growing hyphal tips of higher fungi functions as a VSC, and to extrapolate that other similar vesicle accumulations seen in regions of wall formation, represent VSCs. Accordingly, one may predict that the key molecular event(s) regulating dimorphism would be those responsible for moving the VSC.
Proceedings of the National Academy of Sciences, 1976
The fine structure of isolated chitin synthetase (UDP-2-acetamido-2-deoxy-D-glucose:chitin 4-beta... more The fine structure of isolated chitin synthetase (UDP-2-acetamido-2-deoxy-D-glucose:chitin 4-beta-acetamido-deoxyglucosyltransferase; EC 2-4-1-16) particles (chitosomes) from Mucor rouxii and the elaboration of chitin microfibrils were studied by electron microscopy. Chitosomes are spheroidal, but often polymorphic, structures, mostly 40-70 nm in diameter. Their appearance after negative staining varies. Some reveal internal granular structure enclosed by a shell measuring 6-12 nm thick; others do not show internal structure but have a pronounced depression of the external surface. In thin sections, isolated chitosomes appear as microvesicular structures with a tripartite shell 6.5-7.0 nm thick. Morphologically similar structures can be seen in intact cells of M. rouxii. Isolated chitosomes undergo a seemingly irreversible series of transformations when substrate and activators are added. The internal structure changes, and a coiled microfibril (fibroid) appears inside the chitosome...
GS-1 (ncu04189) is a protein required for the synthesis of b-1,3-glucan in Neurospora crassa. As ... more GS-1 (ncu04189) is a protein required for the synthesis of b-1,3-glucan in Neurospora crassa. As chitin, b-1,3-glucan is a morphogenetically relevant component of the fungal cell wall. Previously, we showed that chitin synthases are delivered to the growing hyphal tip of N. crassa by secretory microvesicles that follow an unconventional route and accumulate in the core of the Spitzenkörper (Spk). Tagged with the green fluorescent protein (GFP), GS-1 accumulated in the hyphal apex forming a dynamic and pleomorphic ring-like structure ('Spitzenring') that corresponded to the Spk outer macrovesicular stratum and surrounded the inner core of chitin synthase-containing microvesicles. TIRF microscopy revealed that GS-1-GFP reached the hyphal apex as a population of heterogeneous-size particles that moved along defined paths. On sucrose density gradients, GS-1-associated particles mainly sedimented in a high density range 1.1272-1.2124 g ml-1. Clearly, GS-1 and chitin synthases of N. crassa are contained in two different types of secretory vesicles that accumulate in different strata of the Spk, a differentiation presumably related to the spatial control of cell-wall synthesis.
We examined in fine detail growth kinetics and intracellular events during lateral and apical bra... more We examined in fine detail growth kinetics and intracellular events during lateral and apical branching in hyphae of Neurospora crassa. By high-resolution video-enhanced light microscopy, we found remarkable differences in the events preceding lateral vs apical branching. While apical branching involved a significant disturbance in the apical growth of the parental hypha, lateral branching occurred without any detectable alterations in the growth of the parental hypha. Prior to the emergence of a lateral branch, an incipient Spitzenk€ orper was formed about 12-29 lm behind the apex. Lateral branch formation did not interfere with the elongation rate of the primary hypha, the shape of its apex or the behavior of its Spitzenk€ orper. In sharp contrast, apical branching was preceded by marked changes in physiology and morphology of the parental hypha and by a sharp drop in elongation rate. The sequence involved a cytoplasmic contraction, followed by a retraction, dislocation, and disappearance of the Spitzenk€ orper; hyphal elongation decreased sharply and a transient phase of isotropic growth caused the hyphal apex to round up. Growth resumed with the formation of two or more apical branches, each one with a Spitzenk€ orper formed by gradual condensation of phase-dark material (vesicles) around an invisible nucleation site. The observed dissimilarities between lateral and apical branching suggest that these morphogenetic pathways are triggered differently. Whereas apical branching may be traced to a sudden discrete disruption in cytoplasmic organization (cytoplasmic contraction), the trigger of lateral branching probably stems from the subapical accumulation of wall precursors (presumably vesicles) reaching a critical concentration.
Abstract Zoospores ofPhytophthora palmivora, induced to encyst by vigorous agitation, synthesize ... more Abstract Zoospores ofPhytophthora palmivora, induced to encyst by vigorous agitation, synthesize alkali-insoluble β-glucans at a linear rate for ca. 10 min. Cell-free extracts of zoospores and cysts ofP. palmivora contained particulate enzymes capable of synthesizing alkali-insoluble, noncellulosic β-1,3-, β-1,6-glucans from uridine diphosphate glucose. Similar levels of activity were detected in zoospores and cysts. Enzymatic activity was stimulated by Mg2+; Ca3+ and Mn2+ were less effective. Cellobiose and other β-linked glucose disaccharides were strong activators. Evidently, the swimming zoospore is equipped with the enzymatic ability to make insoluble, noncellulosic β-glucan—a major component of the cyst wall; hence, a mechanism must exist which prevents the operation of glucan synthetase(s) until the time of encystment. The cell-free extracts also had the ability to synthesize soluble β-1,3-glucan. Most of this material was characterized as neutral mycolaminaran and mycolaminaran phosphate.
Abstract Mycolaminarans, soluble β-1,3-glucans, are the storage polysaccharides in mycelium, spor... more Abstract Mycolaminarans, soluble β-1,3-glucans, are the storage polysaccharides in mycelium, sporangia, zoospores, cysts, and chlamydospores of diverse species of Phytophthora . Glycogen is absent. Two types of mycolaminaran have been recognized: neutral mycolaminaran and mycolaminaran phosphate. In the mycelium of all six species studied, neutral mycolaminaran is the sole reserve polysaccharide. Mycolaminaran phosphate accompanied by a smaller proportion of neutral mycolaminaran constitutes the reserve polysaccharides of sporangia, zoospores, and cysts. In chlamydospores, the reserve polysaccharide is neutral mycolaminaran with smaller amounts of mycolaminaran phosphate. In contrast to other life-cycle stages, oospores store very little, if any, mycolaminaran; only minute amounts of neutral mycolaminaran and no mycolaminaran phosphate were found in mature sexual structures (oogonium-oospore-antheridium complex). Large quantities of β-glucans are produced and metabolized during morphogenesis in Phytophthora . During the development of vegetative mycelium there is abundant synthesis and storage of mycolaminarans which are subsequently utilized for the formation of asexual and sexual sporulating structures (sporangia, chlamydospores, oospores) whose thick walls are made principally of insoluble β-glucans.
Small spheroidal structures (mostly 4070 nm in diameter) similar in shape, size, and chitin-synt... more Small spheroidal structures (mostly 4070 nm in diameter) similar in shape, size, and chitin-synthesizing ability to the chitosomes described earlier for yeast cells of Mucor rouxii were isolated from four other fungi: Allomyces macrogynus (Emerson) Emerson and Wilson, Saccharomyces ...
Video microscopy was used to examine a genetically tagged mutant (T30) of Glomerella graminicola ... more Video microscopy was used to examine a genetically tagged mutant (T30) of Glomerella graminicola whose conidia have a propensity to burst during the germination process. Before the germ tube is produced, the cell wall ruptures and the cytoplasm is extruded. The bursting takes place in less than 0.1 second. Bursting can be prevented by adding osmotica to the germination medium. By phasecontrast microscopy of alkali-washed conidia, we found that each ruptured conidium had a single gaping hole that was oriented parallel to the long axis of the spore. The holes were more frequent in the middle region of the conidia than at the apices. A mathematical model for stress based on conidial shape indicated that conidia ruptured where the stress on the wall was greatest. The microscopic observations on the orientation and distribution of the rupture sites are consistent with the hypothesis that T30 conidia have weaker walls than the wild-type. Comparative tests of resistance to mechanical breakage support this hypothesis.
Laminaran-like polysaccharides were isolated from zoospores and mycelium of Phytophthora palmivor... more Laminaran-like polysaccharides were isolated from zoospores and mycelium of Phytophthora palmivora. These glucans, which we have termed mycolaminarans, are readily soluble in cold water, appear highly homogeneous by gel filtration, and contain exclusively D-glucose residues_ Methylation analyses and enzymic digestions revealed that mycolaminarans are linear chains of (1+3)-linked B-Dglucose residues, with one or two branches joined by (1-4)~B-D linkages. The zoospore mycolaminaran has one branching residue and d.p. 29. The mycelial mycolaminaran has two branching residues and d.p. 36. The possibility is considered that mycolaminarans may be distinctive storage-polysaccharldes of the oomycetous fungi. IiWRODUCTION In previous studies' s2, we showed that the cytoplasm of the oomycetous fungus Phytophthora palmivora contains substantial proportions of water-soluble B-Dglucans hereinafter referred to as mycolaminarans. In the mycelium stage of P. palmivora, only a neutral-type mycolaminaran is present, but in other stages of the life cycle, there are also phosphorylated forms' (mycolaminaran phosphates). We now report on the structure of neutral mycolaminaran isolated from mycelium and zoospores of P. palmivora. Glycogen, the usual storage-polysaccharide of fungi, is absent in Phyfophthora and related organisms. Zevenhuizen and Bartnicki-Garcia3 found that a soluble (1+3)-/I-(145)~/I-linked D-glucan (mycohuninaran) constituted the storage polysaccharide of Phytophthora cinnamomi. MATERIALS AND METHODS Microbiological techniques.-A papaya isolate of Phytophthora palmivora (P113) was obtained from M. Aragaki of the University of Hawaii, The organism was
By computer-enhanced videomicroscopy, we mapped the trajectory of external and internal cell surf... more By computer-enhanced videomicroscopy, we mapped the trajectory of external and internal cell surface markers in growing fungal hyphae to determine the pattern of cell wall expansion during apical growth. Carbon particles (India ink) were chosen as external markers for tip expansion of Rhizoctonia solani hyphae. Irregularities in the growing apical walls of R. solani served as internal markers. Marker movement was traced in captured frames from the videotaped sequences. External and internal markers both followed orthogonal trajectories; i.e., they moved perpendicular to the cell surface regardless of their initial position in the hyphal apex. We found no evidence that the tip rotates during elongation. The discovery that the cell wall of a growing hypha expands orthogonally has major repercussions on two fronts: 1) It supports the long-held view that turgor pressure is the main force driving cell wall expansion. 2) It provides crucial information to complete the mathematical derivation of a three-dimensional model of hyphal morphogenesis based on the vesicle supply center concept. In three dimensions, the vesicle gradient generated by the vesicle supply center is insufficient to explain shape; it is also necessary to know the manner in which the existing surface is displaced during wall expansion.
Cell-free extracts from the wall-less slime mutant of Neurospora crassa and the mycelium of wild ... more Cell-free extracts from the wall-less slime mutant of Neurospora crassa and the mycelium of wild type exhibit similar chitin synthetase properties in specific activity, zymogenicity and a preferential intracellular localization of chitosomes. The yield of chitosomal chitin synthetase from slime cells was essentially the same irrespective of cell breakage procedure (osmotic lysis or ballistic disruption)--an indication that chitosomes are not fragments of larger membranes produced by harsh (ballistic) disruption procedures. The plasma membrane fraction, isolated from slime cells treated with concanavalin A, contained only a minute portion of the total chitin synthetase of the fungus. Most of the activity was in the cytoplasmic fraction; isopycnic sedimentation of this fraction on a sucrose gradient yielded a sharp band of chitosomes with a buoyant density = 1.125 g/cm3. Approximately 76% of the total chitin synthetase activity of the slime mutant was recovered in the chitosome band. Because of their low density, chitosomes could be cleanly separated from the rest of the membranous organelles of the fungus. Apparently, the lack of a cell wall in the slime mutant is not due to the absence of either chitosomes or zymogenic chitin synthetase.
We study school choice within the family, analyzing how birth order, gender, innate talent, and f... more We study school choice within the family, analyzing how birth order, gender, innate talent, and family financial restrictions impact the parents´ decision to prioritize the education of one or more of the children over the rest. We find that parents, particularly from lower income homes, are more likely to select more prestigious, higher cost schools for their eldest child, male children and the most talented children. This behavior may explain part of the positive "male bias" in learning and may have a relevant impact on income distribution among family members.
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