Showing papers by "National Autonomous University of Mexico published in 1992"

A double-sided radio jet from the compact Galactic Centre annihilator 1E1740.7-2942

Abstract: RECENT observations1,2 with the γ-ray telescope SIGMA, on the GRANAT satellite, indicated that the hard X-ray source 1E1740.7 – 2942 may be the source of the strongest outbursts of 511-keV electron-positron annihilation radiation from the Galactic Centre region3. We have observed this source using the Very Large Array, and find that its radio structure is that of a doublesided jet emanating from a compact and variable core. The changes in flux density and spectral index of the core are correlated with variations in the hard X-ray output. The jets are symmetrical about the core, and end in edge-brightened radio lobes; they are probably a result of synchrotron emission of electrons and positrons from the compact core. Our observation suggest that 1E1740.7 – 2942 is a 'microquasar' stellar remnant near the Galactic Centre, which ejects positrons that travel more than a parsec before slowing and annihilating in the interstellar gas.

Rates of Deforestation in Los Tuxtlas, a Neotropical Area in Southeast Mexico

Abstract: On the basis of ground surveys, aerial photographs (for 1967 and 1976), and satellite imagery (for 1986), maps of rain forest distribution were developed for the northern part of the Sierra de Los Tuxtlas (Veracruz, southeast Mexico). Forest coverage maps for 1967, 1976, and 1986 show dramatic deforestation proceeding up from the lowlands, with the remaining natural vegetation now increasingly restricted to the high, steep, and less accessible parts of the Sierra. From the digitizing of the maps it was estimated that (1) during the nearly 20 years of this analysis the vegetation was reduced by 56%, and (2) by early 1986, approximately 84% of the original forest area had been lost. The derived annual deforestation rates (expressed as the percentage of remaining forest that is cleared per year) were 4.2% for the interval 1967–1976 and 4.3% for 1976–1986. Were these trends and rates to continue, only 8.7% of the original vegetation would persist by the year 2000, and then only in the form of an archipelago of very small forest fragments of which the largest would be the now-protected areas (e.g, the Los Tuxtlas Biological Station with only 640 ha), and the most inaccessible tracts. Immediate action is urgently needed to protect these small but rich remnants and preserve them as foci of potential biological inocula for future ecorestoration of these, the northernmost tropical forests on the continent.

New links between the Chicxulub impact structure and the Cretaceous/Tertiary boundary

Abstract: THE 200-km-diameter Chicxulub structure1–3 in northern Yucatan, Mexico has emerged as the prime candidate for the Cretaceous/Tertiary (K/T) boundary impact crater3–6. Concentric geophysical anomalies associated with enigmatic occurrences of Upper Cretaceous breccias and andesitic rocks led Penfield and Camargo1 to suspect that this structure was a buried impact basin. More recently, the discovery of shocked quartz grains in a Chicxulub breccia3, and chemical similarities between Chicxulub rocks and K/T tektite-like glasses3–6 have been advanced as evidence that the Chicxulub structure is a K/T impact site. Here we present evidence from core samples that Chicxulub is indeed a K/T source crater, and can apparently account for all the evidence of impact distributed globally at the K/T boundary without the need for simultaneous multiple impacts or comet showers. Shocked breccia clasts found in the cores are similar to shocked lithic fragments found worldwide in the K/T boundary ejecta layer7,8. The Chicxulub melt rocks that we studied contain anomalously high levels of iridium (up to 13.5 parts per 109), also consistent with the iridium-enriched K/T boundary layer9. Our best estimate of the crystallization age of these melt rocks, as determined by 40Ar/<39Ar analyses, is 65.2 ±0.4 (1σ) Myr, in good agreement with the mean plateau age of 64.98 ± 0.05 Myr recently reported10. Furthermore, these melt rocks acquired a remanent magnetization indicating that they cooled during an episode of reversed geomagnetic polarity. The only such episode consistent with40Ar/<39Ar constraints is chron 29R, which includes the K/T boundary.

The starburst model for active galactic nuclei : the broad-line region as supernova remnants evolving in a high-density medium

Abstract: It is shown that the broad permitted and semipermitted emission lines of the broad-line region (BLR) of radio-quiet active galactic nuclei (AGN) can be generated by strongly radiative (rapidly cooling) supernova remnants expected to occur in the central regions of early-type galaxies undergoing a starburst. Supernova remnants interacting with a circumstellar medium with a density of about n ∼ 10 7 cm -3 become strongly radiative while still expanding at several thousand km/s, and miss the adiabatic Sedov track. Radiative cooling becomes important well before the thermalization of the ejecta is completed, and the shocked matter undergoes a fast condensation behind both the outgoing forward shock and the reverse shock. Two concentric, high-density, and fast-moving thin shells are then formed

Luminescent properties of ZnS:Mn films deposited by spray pyrolysis

Abstract: The room temperature, photoluminescent properties of manganese‐doped zinc sulfide films deposited by spray pyrolysis are reported. These films were deposited on Pyrex glass substrates at atmospheric pressure using air as a carrier gas. All films were polycrystalline with a wurtzite (hexagonal) structure. The manganese doping was achieved by mixing MnCl3 with the starting solution to deposit ZnS. The photoluminescence spectra was measured at room temperature as a function of the different deposition parameters and the Mn concentration. Besides the characteristic light emission associated with Mn impurities in a ZnS matrix, a peak associated with the self‐activated emission was also observed under certain deposition conditions (low substrate temperatures and/or long deposition times). The presence of chlorine impurities in the films is suggested to be associated with this emission. The Mn luminescence presents a quenching effect with the Mn concentration. This quenching effect is similar to the one reported on films deposited by other techniques. The light emission at this center has an activation energy of 0.71±0.05 eV with the deposition temperature. This energy is proposed to be related with the energy required by the Mn atoms to find a proper site during the growth process to form a Mn2+ center.

Floristic composition and structure of vegetation under isolated trees in neotropical pastures

Abstract: . Large isolated trees are a common feature of the agricultural landscape in humid tropical regions originally covered by rain forest. These isolated trees are primarily used as a source of shade for cattle and people. 13 pastures (totalling ca. 80 ha) currently used as cattle pasture were studied. In them, we registered 265 isolated trees belonging to 57 species. 50 trees of the most frequent species (Ficus spp. n = 30 and Nectandra ambigens n = 20) were selected to examine the influence of isolated trees on floristic composition and vegetation structure in the pastures. At each tree, three 4–m2 quadrats were sampled: under the canopy, directly under the canopy perimeter, and beyond the canopy in the open pasture. Under-canopy vegetation was structurally and floristically different from the other two sampling sites. Mean species richness per quadrat was significantly higher under the canopy (17.8 ± 4.3 SD) than at the canopy perimeter (11.2 ± 3.4) and in the open pasture (10.6 ± 3.6) sites. Stem density was higher at under-canopy sites, where greater proportions of endozoochorous and rain-forest species were found. Isolated trees function as nursery plants for rain-forest species by facilitating the establishment of zoochorous species whose seeds are deposited under the tree canopies by frugivorous birds or bats. Our results imply that isolated trees may play a major role in seed dispersal and establishment of native species, which is of consequence for the preservation of rain-forest species in these fragmented landscapes.

Hierarchical analysis of linkage disequilibrium in Rhizobium populations: evidence for sex?

Abstract: Many bacterial species exhibit strong linkage disequilibrium of their chromosomal genes, which apparently indicates restricted recombination between alleles at different loci. The extent to which restricted recombination reflects limited migration between geographically isolated populations versus infrequent mixis of genotypes within populations is more difficult to determine. We examined the genetic structure of Rhizobium leguminosarum biovar phaseoli populations associated with wild and cultivated beans (Phaseolus spp.) over several spatial scales, ranging from individual host plants to throughout the Western Hemisphere. We observed significant linkage disequilibrium at scales at least as small as a cultivated plot. However, the amount of disequilibrium was much greater among isolates collected throughout the Western Hemisphere than among isolates from one area of Mexico, even when disequilibrium was quantified using an index that scales for allelic diversity. This finding suggests that limited migration between populations contributes substantially to linkage disequilibrium in Rhizobium. We also compared the genetic structure for R. leguminosarum bv. phaseoli taken from a cultivated plot with that for Escherichia coli obtained from one human host in an earlier study. Even at this fine scale, linkage disequilibrium in E. coli was very near the theoretical maximum level, whereas it was much less extreme in the local population of Rhizobium. Thus, the genetic structure for R. leguminosarum bv. phaseoli does not exclude the possibility of frequent mixis within local populations.

The real southern Basin and Range: Mid- to late Cenozoic extension in Mexico

Abstract: Much of northern and central Mexico underwent east-northeast extension in the mid- to late Cenozoic; this area constitutes a major but little-recognized part of the Basin and Range province. The extended region is bounded on the east by the Laramide thrust front of the Sierra Madre Oriental. On the west, the relatively unfaulted Sierra Madre Occidental separates the extended area in central Mexico from that around the Gulf of California. Extension occurred as far south as what is now the Trans-Mexican volcanic belt, and in Oaxaca, south of the belt. The Basin and Range province in Mexico constitutes approximately half of the 19 x 10 5 km 2 of western North America that underwent mid- to late Cenozoic extension. North-northwest orientations of numerous epithermal vein systems indicate that east-northeast extension began as early as 30 Ma in areas north of the volcanic belt. Major episodes of faulting began at about 23 to 24 Ma and 12 to 13 Ma, both in Mexico and in the southwestern United States. Faulting was commonly accompanied by eruption of alkali basalts typical of intraplate rifting. Widespread Quaternary fault scarps and alkali basalts indicate that extension continues to the present in the region north of the Trans-Mexican volcanic belt. In contrast, the tectonics in and south of the belt are now probably related to subduction of the Rivera plate. Contemporaneity of the 12 Ma episode with early extension around the Gulf of California attributed to Pacific-North American plate boundary reorganization suggests that extension is related predominantly to plate boundary effects. The beginning of extension at ∼30 Ma may be related to initial encroachment of the East Pacific Rise upon the trench that lay off western North America.

Different plasmids of Rhizobium leguminosarum bv. phaseoli are required for optimal symbiotic performance.

Abstract: Rhizobium leguminosarum bv. phaseoli CFN42 contains six plasmids (pa to pf), and pd has been shown to be the symbiotic plasmid. To determine the participation of the other plasmids in cellular functions, we used a positive selection scheme to isolate derivatives cured of each plasmid. These were obtained for all except one (pe), of which only deleted derivatives were recovered. In regard to symbiosis, we found that in addition to pd, pb is also indispensable for nodulation, partly owing to the presence of genes involved in lipopolysaccharide synthesis. The positive contribution of pb, pc, pe, and pf to the symbiotic capacity of the strain was revealed in competition experiments. The strains that were cured (or deleted for pe) were significantly less competitive than the wild type. Analysis of the growth capacity of the cured strains showed the participation of the plasmids in free-living conditions: the pf- strain was unable to grow on minimal medium, while strains cured of any other plasmid had significantly reduced growth capacity in this medium. Even on rich medium, strains lacking pb or pc or deleted for pe had a diminished growth rate compared with the wild type. Complementation of the cured strains with the corresponding wild-type plasmid restored their original phenotypes, thus confirming that the effects seen were due only to loss of plasmids. The results indicate global participation of the Rhizobium genome in symbiotic and free-living functions.

Hartree-Fock and Roothaan-Hartree-Fock energies for the ground states of He through Xe

Abstract: We report a compilation of Roothaan-Hartree-Fock (RHF) wave functions for the ground states of He through Xe, with atomic orbitals expressed in terms of Slater-type functions. Slight increases in the size of the basis set with respect to those used in Clementi and Roetti's tables [At. Data Nucl. Data Tables 14, 177 (1974)] turn out to yield total energies to better than eight figures, reducing between 21 and 2770 times the energy errors in the above tables, and also improving over the previous numerical HF energies computed by Froese Fischer $[---The Hartree-Fock Method for Atoms (Wiley, New York, 1977)]. We also report 10-digit numerical HF energies with different results for Cr, Cu, Nb, Mo, Ru, Rh, Pd, and Ag.

Genetic structure, outcrossing rate and heterosis in Astrocaryum mexicanum (tropical palm): Implications for evolution and conservation

Abstract: The population genetics of the understory tropical rain forest palm Astrocaryum mexicanum were studied in Los Tuxtlas, Veracruz, Mexico, using enzyme electrophoresis. The percentage of polymorphic loci was 31.8 and the mean expected heterozygosity was 0.153. Segregation patterns for five polymorphic loci met Mendelian expectations. Outcrossing rates were estimated using single and multilocus methods, and in most cases were not statistically different from 1.0. An excess of heterozygotes, both for seeds and adults, was found, as shown by the fixation indices estimated in 1987 (mean F for adults = −0.41, mean F for seeds = −0.19). Low but significant levels of genetic differentiation were found, especially for adults (mean Fst for adults = 0.040, mean Fst for seeds = 0.009). There was a positive relationship for adults between trunk growth and an individual's heterozygosity. No significant correlation was found between heterozygosity and fecundity. The genetic structure of A. mexicanum appears to be the result of a balance between cross-pollination and long distance movement of pollen by pollinators (beetles) that reduce genetic differentiation among plots, and natural selection that could operate during the long life cycle of this palm, and may increase the genetic differentiation among sites and the proportion of heterozygotes. The relatively high level of genetic variation, low genetic spatial differentiation, excess of heterozygotes, high outcrossing rate and heterosis found in A. mexicanum seem to be common in tropical trees. These population genetics characteristics appear not to satisfy the conditions necessary for non-adaptive evolution, a hypothesis commonly invoked to explain high tropical tree diversity. Management and conservation strategies aimed at preserving tropical tree's high intrapopulation genetic variation will probably require the maintenance of large tree populations.

Early persistent activation of sperm K+ channels by the egg peptide speract.

Abstract: Transduction by sperm of the instructive signal provided by the egg peptide speract involves rapid, complex changes in internal ion and cyclic nucleotide content. Here, investigations of hypotonically swollen sperm provide insight into the underlying processes and identify K+ channel activation as an initial ionic event in gamete recognition. A sustained hyperpolarization of swollen sperm is promoted by less than 2.5 pM speract and is followed (with greater than 100 pM speract) by transient repolarization and (with greater than 10 nM speract) by depolarization that is dependent on external Ca2+. Monophasic increases in pHi are produced only by greater than 25 pM speract, indicating that hyperpolarization may not directly promote alkalinization. Increased K(+)-selective (K+ greater than Rb+ greater than Cs+ greater than Na+) membrane permeability is found after all speract greater than 2.5 pM, suggesting that hyperpolarization results from persistent activation of K+ channels and that repolarization has a different ionic basis. Supporting this contention, the K+ channel blocker tetraethylammonium (20 mM) inhibits the increased K+ permeability that follows treatment of swollen sperm (and of sperm in seawater) with 2.5 pM speract. Such induced activation of K+ channels is observed in patch-clamped swollen sperm examined in the cell-attached configuration, upon application of 5-50 pM speract to the bath medium. The efficacy of externally applied speract and its potency indicate that activation is indirect and probably involves an as yet unidentified diffusible mediator whose production is promoted by speract at concentrations 0.01-0.001 times those predicted from reported estimates of the Kd for the known speract receptor.

Control of Ascites Syndrome By Feed Restriction Techniques

Abstract: Feed restriction has been suggested as a means of reducing the incidence of ascites syndrome (AS) in broilers A series of experiments investigated the use of several restriction techniques to reduce ascites while minimizing weight depression In a preliminary experiment, feed was provided either ad libitum (ie, full-fed), eight hours per day, or at a level 10% below that of full-fed controls In both restriction programs, incidence of ascites syndrome was reduced significantly, but body weight gains were reduced severely In three additional experiments, skip-a-day feeding programs implemented at various times during the starter period were compared In two studies conducted at 1940 meters (6363 ft) above sea level, skip-a-day programs from 7–13 or 15–21 days of age significantly reduced the incidence of ascites syndrome without severe reduction in body weight gain or feed conversion In another study at 2500 meters (8200 ft), a more severe restriction was necessary to reduce the incidence of ascites syndrome It was concluded that skip-a-day feed restriction programs during the starting period can effectively reduce the incidence of ascites syndrome without severely compromising body weight gain or feed conversion

Air Pollution and School Absenteeism among Children in Mexico City

Abstract: To determine the acute effects of ozone exposure, the authors conducted a short follow-up study of respiratory illness in a population of 111 preschool children frequently exposed to ozone levels that regularly exceed 0.120 parts per million (ppm). The children attended a private kindergarten in the southwestern part of Mexico City. Parents completed a questionnaire on demographic data, medical history, and potential sources of indoor air pollution. To determine the relation of ozone and respiratory-related school absenteeism, the authors used a logistic regression model for longitudinal data. During the 3-month follow-up, 50% of the children had at least one respiratory-related absenteeism period, and 11.7% had two or more. Children exposed for 2 consecutive days to high ozone levels (> or = 0.13 ppm) had a 20% increment in the risk of respiratory illness. For children exposed for 2 consecutive days to a high ozone level and the previous day to low temperature (< or = 5.1 degrees C), the risk reached 40% (odds ratio = 1.44, 95% confidence interval 1.37-1.52). This study suggests that ozone exposure might be positively associated with the risk of respiratory illness in children and that it may have an interactive effect with low temperature exposure.

Metal sulphide thin film photography with lead sulphide thin films

Abstract: Metal sulphide thin film (MSTF) photography based on photo-accelerated chemical deposition (PACD) of PbS thin films is described. Here an intensity distribution over the surface of a growing PbS thin film produces a thickness variation (0.06–0.15 μm) of the film which, when viewed under daylight, yields a specularly reflective image. Under 800 W m−2 of solar radiation a bluish MSTF photographic image (0.15 μm film thickness) on a coppery-bronze background (0.08 μm) is obtained in the PACD of PbS at the end of 25 min deposition when a high-contrast photographic negative is used as the object. The best contrast of 0.46 in the PbS MSTF photography in the reflection mode is obtained under the above condition of exposure when the optical transmission in a photographic negative in the image area is ∽30% and that in the background is ∽1%. The contrast available in the transmission mode in the MSTF photographic image is considerably less: ∽0.28 (maximum) for optical transmission of 1% and 100% in the background and image areas of the photographic negative respectively.

[Epidemiology of mycetoma in Mexico: study of 2105 cases].

Abstract: A survey was carried out in Mexico to determine the incidence and epidemiological characteristics of mycetoma. Data was collected from a total of 2105 cases of mycetoma throughout a 30 year period (1956-1985), with an average incidence of 70 cases per year. Results showed a sex distribution of 76.1% male and 23.9% females. Age distribution indicated a 35% between 16 to 30 and 23% between 31 to 40 year old population. Most cases occurred in land-workers (60.2%) and in housewives with rural residence (21.3%). Lesions occurred most frequently in lower limbs (64.1%), trunk (17.4%) and upper limbs (13.6%). The geographic distribution within Mexico revealed that the States with the highest incidence were: Jalisco, Nuevo Leon, San Luis Potosi, Morelos and Guerrero The predominant etiologic agents found 97.8% corresponded to actinomycetes, from which Nocardia brasiliensis (86.6%) and Actinomadura madurae (10.2%) showed the higher frequency. Eumycetoma (2.2%) was due to Madurella grisea and M. mycetomatis in most cases.

The effects of gap size and age on the understorey herb community of a tropical Mexican rain forest

Abstract: 1. The effects of gap age and size on understorey herb communities and their light environments (indirect skylight and direct sunlight estimated from hemispherical canopy photographs) were investigated in a tropical rain forest in Los Tuxtlas, Veracruz, Mexico. Thirteen gaps, ranging in size from 70 to 700 m 2 and ranging in age from young (1-2 years old) to old (>>5 years old), were studied. 2. The mean amounts of relative indirect skylight and relative direct sunlight were 0.083 and 0.116, respectively

Altitudinal distribution of birds in the Sierra Madre del Sur Guerrero, Mexico

Abstract: Altitudinal patterns of distribution, diversity, and species turnover in the avifauna of the Sierra de Atoyac, Sierra Madre del Sur, Guerrero, Mexico, were studied along a transect from the tropical coastal plain to high-altitude coniferous forests. Species richness is highest at low elevations and declines with increasing altitude. Richness is relatively uniform within habitat types, but high levels of species turnover are found at habitat ecotones. Altitudinal ranges of congener species pairs overlap more frequently than predicted by null models, suggesting that competition may not have played a dominant role in structuring the community. Patterns of diversity, endemism, and habitat destruction are discussed in terms of conservation priorities.

Effects of litter from a tropical rainforest on tree seed germination and establishment under controlled conditions

Abstract: The effects of litter cover on the germination of seeds of three tropical rainforest, pioneer tree species (Cecropia obtusifolia, Heliocarpus appendiculatus and Piper auritum) was investigated Germination of seeds of all tree species was partially or totally inhibited when the seeds were covered with litter, whereas uncovered seeds or seeds covered with paper instead of litter germinated Seed size was directly correlated with the capacity of the germinated seeds to emerge through the litter In soil experiments in the greenhouse, seedling emergence from the soil was inhibited by the presence of litter on the soil surface

Amino acid sequence and immunological characterization with monoclonal antibodies of two toxins from the venom of the scorpion Centruroides noxius Hoffmann.

Abstract: Two toxins, which we propose to call toxins 2 and 3, were purified to homogeneity from the venom of the scorpion Centruroides noxius Hoffmann. The full primary structures of both peptides (66 amino acid residues each) was determined. Sequence comparison indicates that the two new toxins display 79% identity and present a high similarity to previously characterized Centruroides toxins, the most similar toxins being Centruroides-suffusus toxin 2 and Centruroides limpidus tecomanus toxin 1. Six monoclonal antibodies (mAb) directed against purified fraction II–9.2 (which contains toxins 2 and 3) were isolated in order to carry out the immunochemical characterization of these toxins. mAb BCF2, BCF3, BCF7 and BCF9 reacted only with toxin 2, whereas BCF1 and BCF8 reacted with both toxins 2 and 3 with the same affinity. Simultaneous binding of mAb pairs to the toxin and cross-reactivity of the venoms of different scorpions with the mAb were examined. The results of these experiments showed that the mAb define four different epitopes (A–D). Epitope A (BCF8) is topographically unrelated to epitopes B (BCF2 and BCF7), C (BCF3) and D (BCF9) but the latter three appear to be more closely related or in close proximity to each other. Epitope A was found in all Centruroides venoms tested as well as on four different purified toxins of C. noxius, and thus seems to correspond to a highly conserved structure. Based on the cross-reactivity of their venoms with the mAb, Centruroides species could be classified in the following order: Centruroides elegans, Centruroides suffusus suffusus = Centruroides infamatus infamatus, Centruroides limpidus tecomanus, Centruroides limpidus limpidus, and Centruroides limpidus acatlanensis, according to increasing immunochemical relatedness of their toxins to those of Centruroides noxius. All six mAb inhibited the binding of toxin 2 to rat brain synaptosomal membranes, but only mAb BCF2, which belongs to the IgG2a subclass, displayed a clear neutralizing activity in vivo.

Mangrove Ecosystems of the Pacific Coast of Mexico: Distribution, Structure, Litterfall, and Detritus Dynamics

Abstract: Publisher Summary This chapter discusses mangrove systems—also known as tidal forest or mangrove swamps—that are among the most productive ecosystems under optimal conditions. Their productivity is higher than that of tropical rain forests and similar to that of sugarcane fields, which are considered one of the most productive crops. Mangrove trees are facultative halophytes, found between sub- and supratidal areas on tropical and subtropical coast. They grow in salinities ranging from fresh to hypersaline water but reach their maximal growth in brackish water. Delta plains and estuarine conditions, which are preferred by mangroves, are less common. The Pacific coast is classified as a collision coast with a narrow or nonexistent continental shelf. The Gulf of Mexico corresponds to a marginal sea coast with a typical low-relief extensive coastal plain and shelf. Mangrove forests are among the most productive ecosystems in the world and, together with marshes, are one of the principal sources of detritus for tropical coastal lagoons. It is well known that only a small part of the mangrove production is directly consumed by organisms, and energy and materials flow mostly through detritus food webs.