Esquema local de precondicionamiento para resolver la ecuación de Poisson mediante funciones de base radial

En este trabajo se explora la construcción de un precondicionador basado en la aproximación de una función cardinal mediante una combinación lineal de funciones multicuádricas, con el objetivo de reducir el tiempo de cómputo y aminorar los efectos del mal condicionamiento en la solución numérica del sistema lineal asociado a la aproximación de la solución de la ecuación de Poisson mediante un esquema de colocación asimétrico. Se aproxima la función cardinal usando un esquema local, que permite aproximarla tomando en cuenta solo un pequeño subconjunto de nodos del dominio. Varios investigadores proponen reducir el error que ocurre en los nodos del dominio que se encuentran más distantes, mediante la introducción de un conjunto de nodos especiales. En este artículo también se propone una distribución de puntos en el cuadrado unitario, que permiten seleccionar el conjunto de nodos especiales destinados a complementar el esquema local. Finalmente, se hace una comparación entre el número de flops necesarios para calcular el procondicionador y para calcular la pseudoinversa de Moore Penrose

Twisted Covariant Noncommutative Self-dual Gravity

A twisted covariant formulation of noncommutative self-dual gravity is presented. The formulation for constructing twisted noncommutative Yang-Mills theories is used. It is shown that the noncommutative torsion is solved at any order of the $\theta$-expansion in terms of the tetrad and some extra fields of the theory. In the process the first order expansion in $\theta$ for the Pleba\'nski action is explicitly obtained.Comment: 23+1 pages, no figures, corrected typos, references and Appendix B is adde

Beneficios y costos de la rehabilitación de pasturas degradadas en Honduras

The impact of pasture degradation on animal productivity in six regions of Honduras was assessed, and pasture rehabilitation costs were estimated. The study, carried out in March 2004, involved the interviewing of 25 livestock producers and eight extension agents. The milk and beef production of cows grazing pastures with different levels of degradation was calculated, as well as the losses in income also attributable to the process of degradation. The proportion of pastures found at each level of degradation was determined for each of the six administrative regions of Honduras, and the different strategies to rehabilitate degraded pastures were identified and their cost calculated. A 4-level degradation gradient was defined, level 1 being non-apparent degradation and level 4, severe degradation. Based on personal and descriptive information provided by the interviewees, regressions were generated that better explained the loss in animal productivity at each level of pasture degradation. According to producers, 29% of the area under pastures in Honduras ranked as level 1 of degradation, whereas extension agents ranked 19% of the pastures as level 1. Similarly, livestock producers considered that 27% of the pastures were level 4 of degradation, while extension agents estimated that 31% ranked as level 4. According to producer estimates, each year Honduras ceases to produce 284,106 MT of fluid milk and animal live weight gains equivalent to 48,271 MT of beef due exclusively to the loss of productivity of pasture in level 4 of degradation, which is equivalent to 48% of the country’s annual milk production and 37% of its beef production. In economic terms, losses in milk production totaled US$63 million per year and those of beef production, US$48 million. The cost of rehabilitating pastures of level 4 degradation in Honduras was estimated at US$57.1 million, according to producers, and at US$83.6 million, according to extension agents. These figures represented, respectively, 51% of the US$111.2 million in annual income for sale of milk and beef that were not received and 52$159.8 million in lower annual income per concept of lower sale of milk and beef. Livestock producers indicated that the average useful life of improved pastures was approximately 10 years, ranging from 9 years for B. humidicola and Digitaria swazilandensis to 12 years for star grass (Cynodon nlemfuensis). Extension agents, on the other hand, considered that grasses have a useful life of 8.4 years, ranging from 6 years for D. swazilandensis to 12 years for B. brizantha cv. Marandú. According to study results, the annual rate of degradation of pastures in Honduras varied from 10% to 12%. To eliminate pasture areas of level 4 of degradation nationwide, it would be necessary to make a one-time investment of US$57 million. However, the annual profit in increased milk and beef production would amount to 156,000 liters milk per day and 26,500 kg liveweight that amount to an additional income of US$22.2 million per year. This should serve as both economic and production incentive for the private and public sectors to jointly develop and execute an action plan that allows paddocks in advanced state of degradation to be recovered

Efficient plant regeneration from leaf explants of Solanum americanum

A very efficient system for direct plant regeneration from in vitro–derived leaf explants of Solanum americanum was developed. S. americanum is a tropical plant with important medical properties. The in vitro procedure that was established consists of (i) induction of shoots from leaf tissue, (ii) elongation of shoots, and (iii) rooting of plantlets. The induction of shoots was achieved on Murashige and Skoog solid medium supplemented with different combinations of zeatin riboside and 1–naphthalene acetic acid or 6–benzylaminopurine and 1–naphthalene acetic acid. The best combination for plant regeneration was MS with 5.7 mM zeatin riboside and 0.11 mM 1–naphthalene acetic acid. In the second step, the shoot clumps were transferred to MS basal medium without plant growth regulators, resulting in the differentiation of most of the shoot initials into well developed shoots. In the third stage, plantlets were efficiently rooted on half-strength MS basal medium supplemented with 58.5 mM sucrose. The rooted plants were established in soil with a 100% success rate. This system can be useful to perform further experiments to obtain transgenic plants of this species as well as for other biotechnological approaches.Keywords: American nightshade, organogenesis, shoots regeneration, in vitro culture, zeatin riboside, 1–naphthalene acetic acid, medicinal plant speciesAfrican Journal of Biotechnology Vol. 9(36), pp. 5830-5835, 6 September, 201

El elemento endémico de la flora vascular del Desierto Chihuahuense

Resumen: Antecedentes y Objetivos: El endemismo es un fenómeno común de los organismos en las regiones áridas. En el presente trabajo se revisó la distribución de las plantas endémicas del Desierto Chihuahuense (DCH), cuya superficie abarca parte de los estados de Arizona, Chihuahua, Coahuila, Durango, Nuevo León, Nuevo México, San Luis Potosí, Tamaulipas, Texas y Zacatecas. Métodos: A través de la recopilación de información se elaboró una base de datos de especies con distribución restringida al DCH. Se consideraron los siguientes tipos de endemismo: 1) endémicos estrictos: aquellos elementos con localización conocida solo para el área de estudio, 2) cuasi-endémicos: aquellos presentes principalmente en el DCH y algunas poblaciones en localidades vecinas, 3) micro-endémicos: los que tienen una distribución restringida a una sola localidad y 4) endémicos a la región, pero de ecosistemas no áridos. Resultados clave: Se identificaron 826 taxa (incluye especies, taxa infraespecíficos adicionales e híbridos) que representan 24.87% de la flora total, 560 elementos de distribución endémica, 165 cuasi-endémicos, 176 micro-endémicos y 116 de ecosistemas no áridos. Las familias con mayor cantidad de endemismos son Cactaceae (141 especies), Asteraceae (106), Boraginaceae (34) y Brassicaceae (31) que en conjunto representan 46.2% del total. Los géneros con mayor número de taxa son Coryphantha (24), Echinocereus (14), Mammillaria (15) y Turbinicarpus (15), así como nueve géneros exclusivos al DCH. Las formas biológicas dominantes son las hierbas perennes y arbustivas, presentándose frecuentemente en el matorral xerófilo. La distribución endémica es de forma heterogénea, con una densidad de 0.128 especies/km2 . Coahuila concentra la mayor proporción de elementos (31.15%). Conclusiones: Se sugiere continuar con la protección de Cuatro Ciénegas, Brewster, las Sierras La Madera y El Carmen y proponer como áreas de conservación a las Sierras de Parras, La Paila y Jimulco, y Concepción del Oro y Presidio, por presentar una alta concentración de endemismos. Palabras clave: conservación, desiertos, fitogeografía, zonas áridas. Abstract: Background and Aims: Endemism is a common phenomenon in arid regions. In this study we reviewed the distribution of endemic plants in the Chihuahuan Desert (DCH), which includes portions of the states of Arizona, Chihuahua, Coahuila, Durango, Nuevo Leon, Nuevo Mexico, San Luis Potosi, Tamaulipas, Texas and Zacatecas. Methods: A database was set up summarizing the information on the restricted species from the DCH. The following types of endemism were considered: 1) endemics which are known from the study region only, 2) quasi-endemics with distribution mainly in the DCH and some populations in the neighboring areas, 3) micro-endemics which are restricted to one location, and 4) endemics of non-arid habitats in the area. Key results: A total of 826 taxa (including species, additional infraspecific taxa and hybrids) were determined to be endemics or quasi-endemics, representing 24.87% of the total flora. Of this, 560 are endemics, 165 quasi-endemics, 176 micro-endemics and 116 are from non arid habitats. The families with the most endemics are Cactaceae (141 species), Asteraceae (106), Boraginaceae (34) and Brassicaceae (31), that represent 46.2% of the total. The genera with the highest taxa numbers are Coryphantha (24), Echinocereus (14), Mammillaria (15), Turbinicarpus (15). Nine genera are exclusive to the DCH. The dominant biological forms are perennial herbs and shrubs, frequently distributed in the xeric shrubland. The endemic distribution is heterogeneous, with a density index of 0.128 species/km2 . Coahuila has the largest number of elements (31.15%). Conclusions: It is suggested to continue the protection of Cuatro Ciénegas, and portions of Brewster, as well as the Sierras La Madera and El Carmen. Sierras de Parras, La Paila and Jimulco, and Concepción del Oro and Presidio, are proposed as conservation areas, as they concentrate a large number of endemic species. Keys words: arids zones, conservation, deserts, phytogeography

A preliminary study of genetic factors that influence susceptibility to bovine tuberculosis in the British cattle herd

Associations between specific host genes and susceptibility to Mycobacterial infections such as tuberculosis have been reported in several species. Bovine tuberculosis (bTB) impacts greatly the UK cattle industry, yet genetic predispositions have yet to be identified. We therefore used a candidate gene approach to study 384 cattle of which 160 had reacted positively to an antigenic skin test (‘reactors’). Our approach was unusual in that it used microsatellite markers, embraced high breed diversity and focused particularly on detecting genes showing heterozygote advantage, a mode of action often overlooked in SNP-based studies. A panel of neutral markers was used to control for population substructure and using a general linear model-based approach we were also able to control for age. We found that substructure was surprisingly weak and identified two genomic regions that were strongly associated with reactor status, identified by markers INRA111 and BMS2753. In general the strength of association detected tended to vary depending on whether age was included in the model. At INRA111 a single genotype appears strongly protective with an overall odds ratio of 2.2, the effect being consistent across nine diverse breeds. Our results suggest that breeding strategies could be devised that would appreciably increase genetic resistance of cattle to bTB (strictly, reduce the frequency of incidence of reactors) with implications for the current debate concerning badger-culling