Showing papers by "National Autonomous University of Mexico published in 1991"

Rhizobium tropici, a novel species nodulating Phaseolus vulgaris L. beans and Leucaena sp. trees.

Abstract: A new Rhizobium species that nodulates Phaseolus vulgaris L. and Leucaena spp. is proposed on the basis of the results of multilocus enzyme electrophoresis, DNA-DNA hybridization, an analysis of ribosomal DNA organization, a sequence analysis of 16S rDNA, and an analysis of phenotypic characteristics. This taxon, Rhizobium tropici sp. nov., was previously named Rhizobium leguminosarum biovar phaseoli (type II strains) and was recognized by its host range (which includes Leucaena spp.) and nif gene organization. In contrast to R. leguminosarum biovar phaseoli, R. tropici strains tolerate high temperatures and high levels of acidity in culture and are symbiotically more stable. We identified two subgroups within R. tropici and describe them in this paper.

Assembly and sealing of tight junctions: possible participation of G-proteins, phospholipase C, protein kinase C and calmodulin.

Abstract: The making and sealing of a tight junction (TJ) requires cell-cell contacts and Ca2−, and can be gauged through the development of transepithelial electrical resistance (TER) and the accumulation of ZO-1 peptide at the cell borders. We observe that pertussis toxin increases TER, while AIF3 and carbamil choline (carbachol) inhibit it, and 5-guanylylimidodiphosphate (GTPΓs) blocks the development of a cell border pattern of ZO-1, suggesting that G-proteins are involved. Phospholipase C (PLC) and protein kinase C (PKC) probably participate in these processes since (i) activation of PLC by thyrotropin-1 releasing hormone increases TER, and its inhibition by neomycin blocks the development of this resistance; (ii) 1,2-dioctanoylglycerol, an activator of PKC, stimulates TER development, while polymyxin B and 1-(5-isoquinoline sulfonyl)-2-methyl-piperazine dihydrochloride (H7), which inhibit this enzyme, abolish TER. Addition of 3-isobutyl-1-methyl-xanthine, dB-cAMP or forskolin do not enhance the value of TER, but have just the opposite effect. Trifluoperazine and calmidazoline inhibit TER development, suggesting that calmodulin (CaM) also plays a role in junction formation. These results indicate that junction formation may be controlled by a network of reactions where G-proteins, phospholipase C, adenylate cyclase, protein kinase C and CaM are involved.

Genetic structure of a soil population of nonsymbiotic Rhizobium leguminosarum.

Abstract: The genetic structure of a population of nonsymbiotic Rhizobium leguminosarum strains was determined by the electrophoretic mobilities of eight metabolic enzymes Nonsymbiotic strains were isolated from the rhizosphere of bean plants and characterized by growth on differential media and at different temperatures, intrinsic antibiotic resistance, the lack of homology to a nifH probe, and their inability to form nodules on bean roots All the isolates clustered with R leguminosarum bv phaseoli reference strains and did not encompass any other Rhizobium taxa Their rRNA operon restriction fragment length polymorphisms and the nucleotide sequence of a fragment of the 16S rRNA gene were also found to be identical to those of R leguminosarum bv phaseoli reference strains When complemented with an R leguminosarum bv phaseoli symbiotic plasmid (p42d), the nonsymbiotic isolates were able to fix nitrogen in symbiosis with bean roots at levels similar to those of the parental strain The symbiotic isolates were found at a relative frequency of 1 in 40 nonsymbiotic R leguminosarum strains

Star formation and the nature of bipolar outflows

Abstract: This paper presents a simple physical model for the bipolar molecular outflows that frequently accompany star formation. The model forges an intrinsic link between the bipolar flow phenomenon and the process of star formation, and it helps to explain many of the systematics known for existing sources.

Spatial relationships between cacti and nurse shrubs in a semi‐arid environment in central Mexico

Abstract: . The Zapotitlan de las Salinas valley, central Mexico, harbours a high diversity of cacti. Pattern analysis indicated that the establishment of two columnar cacti, Neobuxbaumia tetetzo and Cephalocereus hoppenstedtii, and of three small globose cacti, Coryphantapallida, Mammillaria colina andM casoi, is aggregated and associated with perennial nurse shrubs. Some nurse species, Castela tortuosa, Caesalpinia melanadenia and Eupatorium spinosarum have a higher number of cacti beneath their canopies than would be expected by chance. A replacement pattern was found between the columnar cacti and their nurses, an aspect which was not found with the globose cacti. Following the assumption that protection against excessive radiation is the main factor determining the nurse effect, the azimuth orientation of the cacti with respect to their nurses was evaluated. Only Coryphanta pallida presented a non-random distribution with a tendency towards the North and West. The difference in maximum temperature between the soil surface under the different nurse species and of open spaces, which is reached at midday, was 16 °C. No significant differences were found in beneath-canopy temperatures for the three nurse species considered. Soil nitrogen levels were significantly lower beneath the different nurse plants than in open spaces. This result suggests that soil fertility is not an important factor in the nurse-plant phenomenon in Zapotitlan.

Soil emissions of nitric oxide in a seasonally dry tropical forest of México

Abstract: Soil emissions of NO were measured at the Chamela Biological Station, Mexico, using soil covers and a field apparatus of NO detection based on CrO3 conversion of NO to NO2 and detection of NO2 by chemiluminescence with Luminol. Mean NO fluxes from forest soils ranged from 0.14 to 0.52 ng NO-N/sq cm/hr during the dry season and from 0.73 to 1.27 ng NO-N/sq cm/hr during the wet season. A fertilized floodplain pasture exhibited higher fluxes, but an unfertilized upland pasture, which represents the fastest growing land use in the region, had flux rates similar to the forest sites. Wetting experiments at the end of the dry season caused large pulses of NO flux, equaling 10 percent to 20 percent of the estimated annual NO emissions of 0.5-1.0 kg N/ha from the forest sites. Absence of a forest canopy during the dry season and the first wet season rain probably results in substantial NO(x) export from the forest system that may be important to regional atmospheric chemical processes. Wetting experiments during the wet season and a natural rain event had little or no stimulatory effect on NO flux rates.

Production of Fish Protein Hydrolysates with Bacterial Proteases; Yield and Nutritional Value

Abstract: Three bacterial proteases were selected for their activity on casein per unit cost. Conditions for inactivation were determined. Fish protein was hydrolyzed without water addition. Protein was solubilized faster with Pescalase 560 than with HT-200 or Protease N. Increase of protease concentration was associated with a quadratic increase in soluble nitrogen. Yields of 13 to 15% in terms of whole fish were obtained at the pilot plant. Products contained 83 to 86% protein of which 70 to 80% was soluble. Reactive lysine was 70 to 85%. Nutritional evaluation with rats indicated hydrolysates had 10 to 15% lower nutritional value than casein. Results suggest fish protein hydrolysates can be used in the food industry for their solubility and nutritional value.

Gully erosion: processes and models

Abstract: Confusing and sometimes contradictory results and reports have led to a sizeable body of literature on, but unfortunately not to a clear understanding of, gully erosion processes. In the following ...

Interaction between the cactus Neobuxbaumia tetetzo and the nurse shrub Mimosa luisana

Abstract: . Pattern analysis and association analysis showed that recruitment of the giant columnar cactus Neobuxbaumia tetetzo in the semi-arid valley of Zapotitlan is largely limited to areas beneath the canopies of perennial shrubs, acting as nurse plants. Chi-square and Haberman tests revealed that young cacti were more frequently found beneath canopies of leguminous shrubs, especially Mimosa luisana, than were older cacti. Segregation analysis indicated a positive association (= negative segregation) of young cacti and M. luisana canopies, but older cacti were randomly distributed with respect to M. luisana. This, and the height class distributions of cacti associated withM luisana or not, suggested the replacement of M. luisana by N. tetetzo. Mimosa luisana plants in association withiV. tetetzo had greater amounts of dead basal area thanM luisana lacking associated N tetetzo. This suggested competition from JV. tetetzo promotes the replacement process. The roots of N tetetzo are ≤ 30 cm deep. Interception of soil water before penetration to deeper roots of M luisana may be the mode of competition between the two species.

Isotopic studies of the Acatlan Complex, southern Mexico: implications for Paleozoic North American tectonics

Abstract: The Paleozoic Acatlan complex and Grenville-age Oaxaca terrane of southern Mexico have been suggested to be the southern continuation of North American orogenic systems. The Oaxaca terrane yields interpreted ages of ∼1.0 Ga and has many of the characteristics typical of Grenville belt rocks. The Paleozoic Acatlan complex consists of multiply deformed metasedimentary rocks, schists, granitoids, and eclogites that have been compared to rocks of the Appalachian belt. The northward extension of both the Oaxaca terrane and Acatlan complex, how-ever, are obscured by younger rock cover as they enter central Mexico. Furthermore, the configuration of these orogenic systems in southern Mexico is the reverse of that of the rest of North America, with the Grenville Oaxaca terrane to the east of the Paleozoic Acatlan complex. Isotopic studies show that the Acatlan complex records three tectonothermal events. The Sm-Nd whole-rock/mineral isochrons from schists as well as eclogites yield metamorphic ages of 410-380 Ma. This age of metamorphism is supported by U-Pb zircon data from a granitoid which yields an age of 370 ± 34 Ma. A later intrusion of a large stock in the Late Pennsylvanian (287 ± 2 Ma) was probably closely followed by a less significant deformational event. Small granitic intrusions and migmatites were later emplaced at 205-170 Ma (Rb-Sr and Sm-Nd mineral/ whole rock). The metasedimentary rocks and Paleozoic granitoids of the Acatlan complex have present-day e Nd of -9 to -11, with crustal residence ages (T DM ) of 1.3-1.6 Ga. These rocks must have been derived at least in part from a Proterozoic source area, and it is significant that crustal residence ages are identical to those of the Oaxaca terrane. The Oaxaca terrane, along with some South American Precambrian complexes, of which the Oaxaca terrane was probably once a part, may be considered the most likely source areas for the Acatlan complex. The less extensive eclogite, trondjhemite, and amphibolite bodies in the Acatlan complex yield model ages that show them to be juvenile additions to the crust. Neodymium model ages of the Acatlan complex are unlike those of some accreted crustal blocks of the Pacific margin, such as the Alexander, Stikine, and Wrangellia terranes, which have little signs of crustal recycling. Similarities between the Acatlan complex and the Acadian belt, as well as current Paleozoic paleogeographic and paleomagnetic reconstructions, suggest that the Devonian metamorphic event that affected these two areas was the result of a Laurentia-Gondwana collision. A later collision in late Carboniferous time caused deformation in the Acatlan complex, plutonic activity in southern Mexico, and deformation in the Ouachita, Marathon, and Appalachian belts. Both the Acatlan and Oaxaca terranes would have been continuous with South America until the break-up of Pangea in the Mesozoic era.

Enhancement of T-cell activation by the CD43 molecule whose expression is defective in Wiskott-Aldrich syndrome.

Abstract: CD43 (sialophorin, leukosialin, leukocyte large sialoglyco-protein), a heavily sialylated molecule found on most leukocytes and platelets, was initially identified as a major glycoprotein of mouse, rat and human T cells1–8. CD43 expression is defective on the T cells of males with the Wiskott–Aldrich syndrome, an X chromosome-linked recessive immunodeficiency disorder9. Affected males are susceptible to opportunistic infections and do not respond to polysaccharide antigens, reflecting defects in cytotoxic and helper T-cell functions. Anti-CD43 monoclonal antibodies have a modest costimulatory effect on T cells, natural killer cells, B cells and monocytes10–14, and one such antibody has been shown to activate T cells directly15. To investigate a possible physiological role for CD43, a complementary DNA encoding the human protein16,17 was introduced into an antigen-responsive murine T-cell hybridoma18. We observed that CD43 enhances the antigen-specific activation of T cells and that the intracellular domain of CD43, which is hyperphosphorylated during T-cell activation19–21, is required for this function. We also found that antigen-presenting cells can bind specifically to immobilized purified CD43 and that the binding can be inhibited by liposomes containing CD43 as well as by anti-CD43 monoclonal antibodies.

Lymphocyte proliferation kinetics and genotoxic findings in a pilot study on individuals chronically exposed to arsenic in Mexico

Abstract: In the search for early biological markers to detect genetic damage, a pilot study on a hydroarsenicism-exposed group was designed. Blood and urine samples were taken from 11 individuals chronically exposed and from 13 individuals with lower exposure to the metal. Lymphocyte cultures for cytogenetic studies and HGPRT assay were done with coded peripheral blood samples, while arsenic levels and the "rec assay" in B. subtilis were determined in urine samples. The highly exposed group excreted greater amounts of As, nevertheless the rec assay showed negative results. An interesting finding is that the cell-cycle kinetics exhibited a significant difference between the groups studied, the average generation time (AGT) was longer in the highly exposed group. The percentages of chromosomal aberrations and the frequencies of sister-chromatid exchanges were similar in both populations, although complex aberrations were more frequent in the highly exposed group, which also showed a higher average variation frequency in the HGPRT assay, but the 2 latter observations were not statistically significant. The lag in lymphocyte proliferation could mean an impairment of the immune response due to arsenic exposure.

Poisson-distributed patterns of explosive eruptive activity

Abstract: The study of patterns of eruption occurrence could lead to a better understanding of the physics behind the volcanic process. However, various attempts to find a single statistical distribution that describes the occurrences of volcanic eruptions have not been successful. Global data show that, if the energies of point events in time (eruptions) are properly accounted above a certain “noise level”, the stochastic process — whose realization consists of explosive volcanic events — can be well represened by a Poisson point process, though not necessarily stationary. Many previous attempts to describe patterns of eruption occurrences were hampered by counting events with all levels of explosivity in the same category. When eruptions are separated by their sizes, the occurrence patterns of the higher magnitude eruptions become clearly Poissonian. In this study eruptions are classified by size using the Volcanic Explosivity Index (Newhall and Self 1982). Further analysis of the magnitude-characterized eruption data shows direct relations among the energy of eruptions, mean rate of occurrences and distribution of repose intervals between eruptions. An important result from the analysis of energy and mean rate of occurrence data is that, for global data, the product of those parameters is a constant. Simple load-and-discharge models provide an explanation of the random features of the volcanic processes. These considerations lead to the definition of a constinuous magnitude scale for volcanic eruptions which can consistently measure the energy and the rate-of-occurrence of eruptions over a wide range of values.

Nitrogen transformations and nitrous oxide flux in a tropical deciduous forest in México.

Abstract: Emissions of nitrous oxide and soil nitrogen pools and transformations were measured over an annual cycle in two forests and one pasture in tropical deciduous forest near Chamela, Mexico. Nitrous oxide flux was moderately high (0.5–2.5 ng cm−2 h−1) during the wet season and low (<0.3 ng cm−2 h−1) during the dry season. Annual emissions of nitrogen as nitrous oxide were calculated to be 0.5–0.7 kg ha−1 y−1, with no substantial difference between the forests and pasture. Wetting of dry soil caused a large but short-lived pulse of N2O flux that accounted for <2% of annual flux. Variation in soil water through the season was the primary controlling factor for pool sizes of ammonium and nitrate, nitrogen transformations, and N2O flux.

The Ethnoecology of Maize Variety Management: A Case Study from Mexico

Abstract: This paper presents a case study of the relationship between farmers' knowledge of maize varieties and their selection and management of these varieties under conditions of technological change. Research for this paper was done among Spanish-speaking small farmers in an ejidoof central Chiapas, Mexico. This ejido is well integrated into the market, and the use of modem technologies is widespread. This research demonstrates that farmers have an extensive and widely shared knowledge of their maize varieties. This knowledge reflects objective maize characteristics. Variation occurs in the farmers' selection and management of maize varieties, but on average the variation deviates from a random pattern in the direction predicted by the farmers' knowledge base. They have incorporated the technological changes brought about by development into their knowledge base. Farmers maintain maize varieties with contrasting traits, and their knowledge base provides important information about which traits and constraints are important to them.

Sexual size dimorphism and sibling competition: implications for avian sex ratios

Abstract: The blue-footed booby's (Sula nebouxii) first-hatched chick aggressively dominates and outgrows its sib and sometimes kills it when food is in short supply. Differential male mortality is expected since females grow to be 27% heavier than males and should have an advantage in sibling competition in mixed-sex broods. Surprisingly, when the male hatches first, even though his sister outgrows him, he usually sustains dominance over her throughout the nestling phase, and he grows and survives normally. When the sister hatches first, despite a large, persistent sibling size disparity the male grows and survives normally. These observations contradict the prevailing hypothesis that in sexually dimorphic birds that practice siblicidal brood reduction the smaller sex suffers differential mortality as the result of its disadvantage in sibling conflict in mixed-sex broods. Such species consequently are not expected to modify their primary sex ratio or associate sex with hatching order in response to biased mortalit...

Effects of prenatal protein deprivation on postnatal development of granule cells in the fascia dentata

Abstract: The effect of prenatal protein deprivation on the postnatal development of granule cells in the fascia dentata in the rat was studied at 15, 30, 90, and 220 days of age. The granule cells showed a significant reduction in cell size, decreased number of synaptic spines throughout their dendritic extent, and reduced complexity of dendritic branching in the outer two-thirds of the molecular layer. All of these deficits were present at 15 days and persisted throughout the study (220 days). The least deficits in synaptic spine density occurred at 90 days and in dendritic branching at 30 days. Partial restitution of earlier, more severe deficits was associated primarily with maturational events occurring in the protein deprived rats, whereas later increases in deficits were related primarily to a failure of the protein deprived rats to keep pace with neuronal development occurring in the controls. The present results are similar to those noted in our previous study in this journal of the effect of a low protein diet (8% casein) on these neurons that extended from pregnancy until the time of sacrifice at 30, 90, and 220 days of age (Cintra et al., '90; 532:271-277). Taken together, these two studies suggest that the postnatal adaptation of the granule cells to prenatal protein deprivation is primarily due to events that occur during pregnancy and that the site of predilection for the deficit is their dendrites in the outer two-thirds of the molecular layer of the fascia dentata.

On the evolution of supernova remnants – II. Two-dimensional calculations of explosions inside pre-existing wind-driven bubbles

Abstract: Supernova explosions within wind-driven bubbles are studied with 2D hydrodynamical calculations. Two different density distributions for the ejecta are considered : a smooth, unfragmented power-law stratification, and a fragmented distribution. As in 1D models, the presence of the shell of interstellar swept-up matter causes the rapid evolution of the remnant to the radiative phase. The main 2D effects, for both fragmented and unfragmented ejecta, include : substantial chaotic deviations from a purely radial flow in the remnant interior, efficient turbulent mixing between the ejecta and the shocked wind, resulting in homogenization of the former wind cavity, and severe distortion of the wind-driven shell by cooling and Rayleigh-Taylor instabilities

Root biomass of a dry deciduous tropical forest in Mexico.

Abstract: The deciduous tropical dry forest at Chamela (Jalisco, Mexico) occurs in a seasonal climate with eight rainless (November through June) and four wet months (700 mm annual precipitation) The forest reaches a mean height of 10 m Tree density in the research area was 4700 trees per ha with a basal area at breast height of 23 m2 per ha The above-and below-ground biomass of trees, shrubs, and lianas was 736 Mg ha−1 and 31 Mg ha−1, respectively A root:shoot biomass ratio of 042 was calculated Nearly two thirds of all roots occur in the 0–20 cm soil layer and 29% of all roots have a diameter of less than 5 mm

$uv\,by$ and JHKLʹ photometry of OB stars in the association Cygnus OB2⋆

Abstract: uvby and JHK photometry of about 80 stars in the association Cygnus OB2 (VI Cygni), including some foreground stars and three infrared sources, are presented. From two-colors and color-magnitude diagrams, the interstellar absorption and approximate spectral types are derived. It is confirmed that the interstellar extinction law in the direction of Cyg OB2 is similar to the galactic mean. The ratio of total-to-selective extinction and the true distance modulus are determined. A minimum age is estimated for the association.

Air temperature and humidity and human comfort index of some city parks of Mexico City.

Abstract: A series of air temperature (Ta) and relative humidity (HR) measurements were carried out in five urban parks and their surroundings in Mexico City. It was found thatTa is lower andRH is higher inside the parks. There were differences (P<0.05) between the parks and surroundings in air temperature, relative humidity, vapour pressure (e), vapour pressure deficit (VPD) and the thermohygrometric human comfort index (THI).Ta differences were more marked during the early afternoon. Maximum differences ofTa, e, andDPV were 5.6°C, 0.6 and 1.2 mbar. There is strong evidence that the magnitude of differences inTa are directly proportional to the surface of urban parks.

Amplification and deletion of a nod-nif region in the symbiotic plasmid of Rhizobium phaseoli.

Abstract: One remarkable characteristic of the genomes of some Rhizobium species is the frequent occurrence of rearrangements. In some instances these rearrangements alter the symbiotic properties of the strains. However, no detailed molecular mechanisms have been proposed for the generation of these rearrangements. To understand the mechanisms involved in the formation of rearrangements in the genome of Rhizobium phaseoli, we have designed a system which allows the positive selection for amplification and deletion events. We have applied this system to investigate the stability of the symbiotic plasmid of R. phaseoli. High-frequency amplification events were detected which increase the copy number of a 120-kb region carrying nodulation and nitrogen fixation genes two to eight times. Deletion events that affect the same region were also found, albeit at a lower frequency. Both kinds of rearrangements are generated by recombination between reiterated nitrogenase (nifHDK) operons flanking the 120-kb region. Images

Reversible inactivation of the insular cortex by tetrodotoxin produces retrograde and anterograde amnesia for inhibitory avoidance and spatial learning.

Abstract: Tetrodotoxin (TTX; a voltage-sensitive sodium channel blocker) was microinjected bilaterally into the insular (IC), frontal (FC), or parietal (PC) cortex or the ventral caudate nucleus of rats either before or after they were trained in an inhibitory avoidance task. When administered either before or after training, injections of TTX into the IC impaired performance on a 48-hr retention test. Injections of TTX into the PC also impaired retention when administered before training. One week later, rats with cannulae in the IC, FC, and PC received microinjections of TTX either before or after training in a water maze (Morris) spatial learning task and retention was tested 24 hr later. TTX impaired retention when administered to the IC either before or after training. These findings indicate that a functionally intact IC during and after training in these tasks appears to be essential for the storage of long-term memory.

Maximum-entropy model for quantum-mechanical interference effects in metallic conductors

Abstract: We show that all the known quantum-mechanical interference effects characteristic of disordered conductors in the metallic limit can be obtained from a ``maximum-entropy model,'' based on a transfer-matrix formulation, which is independent of any particular form of the disordered microscopic Hamiltonian. In particular, we have derived the weak-localization effect and the associated backscattering peak, as well as the universal conductance fluctuations and the associated long-range correlations in transmission probabilities. We find precise quantitative agreement with microscopic Green-function calculations evaluated in the quasi-one-dimensional limit. We define two random-matrix ensembles characterizing systems with and without time-reversal symmetry (the analogs of the well-known orthogonal and unitary ensembles), and show that, within the model, breaking of time-reversal symmetry has the expected effect on these phenomena. The model has not been shown to yield behavior characteristic of the two- or three-dimensional limits, which appear to be outside its current range of validity.

Murine Taenia crassiceps cysticercosis: H-2 complex and sex influence on susceptibility.

Abstract: Several inbred strains of mice were infected by intraperitoneal injection of tenTaenia crassiceps cysticerci per mouse. Genes linked with the major histocompatibility complex (H-2) were found to influence parasite growth greatly, as demonstrated by the different parasite loads of H-2 congenic mice with BALB background: BALB/c (H-2d) mice were the most susceptible, whereas BALB/k (H-2k) and BALB/b (H-2b) animals were comparatively resistant. Non-H-2 genes had no significant effect on susceptibility in H-2d strains, as reflected by the similar parasite loads in BALB/c, DBA/2, and (BALB/cxDBA/2)F1 mice. Using the H-2b (BALB/b, C57BL/6J) and H-2k (C3H/HeJ, BALB/k, and C3HeB/FeJ) strains, we found that non-H-2 background genes caused a small but significant influence on parasite load. A recombinant mouse strain alleles (Kk, Ik, Sd, Dd) was also susceptible, indicating that S and/or D regions of the H-2d complex are probably involved in the control of resistance to murine cysticercosis. Females of all mouse strains were more susceptible than males. The same effects were observed for H-2 genes and sex, with two strains ofT. crassiceps differing in their rate of growth.

Optical properties of two-dimensional disordered systems on a substrate

Abstract: We calculate the dielectric response of free-standing and supported two-dimensional layers of polarizable entities, such as metallic particles or adsorbed molecules. We take into account dipole-dipole and the image interaction and investigate the effects of disorder within a two-dimensional renormalized polarizability theory. The behavior of the resonances arising from both the single particle's and the substrate's surface plasmon is studied.