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Magnolia rzedowskiana es una especie recien descrita, con pocas poblaciones relictas, de escasa densidad y endemica del bosque de niebla de la sierra Madre Oriental, Mexico. Con el objetivo de determinar su variacion morfologica en su area de distribucion, se seleccionaron 40 individuos de 4 localidades. En cada arbol se recolectaron 5 flores y 10 hojas, para analizar 26 caracteres morfologicos, mediante estadistica descriptiva, analisis de agrupamiento (AA), componentes principales (ACP) y discriminante. En el AA, tanto en las caracteristicas florales como foliares se definieron 3 grupos, que corresponden, en general, a diferentes zonas geograficas. En el ACP, los primeros 4 componentes explicaron 75% de la variacion floral y los 2 primeros componentes, 78% de la variacion foliar. Las primeras 2 funciones discriminantes explicaron 93.37% y 97.67 % de la variacion floral y foliar, respectivamente. Los caracteres con mas contribucion a la diferenciacion poblacional fueron: numero y longitud de carpelos y estambres, diametro del peciolo y ancho maximo de la lamina. Los datos sobre distribucion, caracteristicas generales y variacion morfologica de M. rzedowskiana, son fundamentales para definir estrategias de manejo y para proponer su inclusion en alguna categoria de riesgo en la legislacion nacional e internacional.

The geographical distribution is an important concept in biological sciences, however in some cases it has been interpreted inadequately. We consider that the unified species concept can help to clarify the geographical distribution concept, since incorporate the spatial dimension because the species are a separately evolving metapopulation lineage. The geographical distribution is defined as that fraction of the geographical space where a species is present and interacts in unephemeral way with the ecosystem. The presence or absence of the species in the geographic space is defined by biogeography, physiology, and ecological factors. Geographical distribution is an attribute difficult to estimate, nevertheless, it has described some patterns concerning to size, shape and boundaries. Currently, there are some methods that allow to carry out spatially explicit accurate hypothesis about the geographical distribution of the species.

Magnolia vovidesii has been found in a few small patches in central Veracruz, Mexico. Previous ecological studies have suggested high reproductive potential and stable population growth; however, in the last 20 years, there have been severe anthropogenic environmental impacts on the species. The main objectives of this study were (a) to document the current trend of the population, (b) to determine its spatial structure, and (c) to identify the main threats to the species in order to propose conservation and management strategies. Our results show a population in decline, caused by high mortality during early establishment stages (seeds and seedlings) and the removal of young sick trees during the reproductive stage. We found a strong spatial dependence between seedlings and reproductive individuals (young and old adults) and a weak spatial association between reproductive stages, suggesting a nursing effect, inbreeding, and pollen dependence. The main threats to the population are frequent plant removal, trails used by people, land-use change, and parasitism. These data show the urgent need to carry out prompt conservation action for the species, with special emphasis on ex situ propagation. The results of this study suggest that M. vovidesii should be transferred to the International Union for Conservation of Nature Red List category for critically endangered species.

Las poblaciones humanas obedecen a los mismos supuestos evolutivos que el resto de los organismos, aunque mezclados con elementos sociales y culturales que pueden promover la expresion de ciertas enfermedades en grupos etnicos especificos, causadas principalmente por la frecuente endogamia. En este trabajo se analiza el principio de Hardy-Weinberg desde un enfoque medico, social y biologico, para entender los procesos evolutivos que dan lugar a las enfermedades autosomicas recesivas. A manera de conclusion se puede senalar que la incidencia de estas enfermedades esta inversamente relacionada con los niveles de la variabilidad genetica en las poblaciones, variabilidad que depende de eventos de colonizacion, recolonizacion y migracion, asi como de convenciones sociales como el racismo, la estratificacion social y la segregacion.

The American tree fern Dicksonia sellowiana reaches its northern distribution limit in Mexico. This species grows only in the highly fragmented remnants of the montane cloud forest (MCF). Given the lack of data on ecological aspects of this endangered tree fern species, considered under special protection by Mexican legislation, the objectives of the present research were to investigate the structure of D. sellowiana populations and to characterize its habitats. The study was carried out in seven populations located at different elevations and latitudes in the MCF of the Sierra Madre Oriental in Mexico. In each locality, three sampling plots were established to characterize the habitat and to record density, height, stem diameter, number of leaves, and individual fertility. The resulting values were analyzed with univariate and multivariate statistics. In four of the D. sellowiana populations, individuals of low height predominated; these were classified as dynamic populations. However, the density of individuals by unit area was low compared with some South American populations, probably because of the high degree of fragmentation of the MCF. The variation in population density was mainly explained by the vegetation cover. Environmental factors such as elevation and slope orientation, however, explained better the variation of the basal area of tree fern populations. These results constitute the first reference on the current state of Mexican D. sellowiana populations, which is important for the management and conservation of this endangered species, and for monitoring changes in the environmental conditions of its natural habitat, the MCF of Mexico.

Zamia furfuracea L. f. es endemica de Veracruz, Mexico. La especie es una cicada dioica que se encuentra en peligro de extincion. No se ha registrado la existencia de un dimorfismo sexual en esta especie, por lo que no es posible efectuar una identificacion del sexo en estadios tempranos o sin estructuras reproductivas. La determinacion de las diferencias asociadas con el sexo en Z. furfuracea se hizo a traves de la evaluacion de nueve variables morfologicas en 31 individuos machos y 26 hembras de una poblacion natural de Veracruz. El numero de pares de foliolos, el ancho de la hoja, el promedio del ancho del foliolo y el diametro del peciolo fueron significativamente diferentes entre sexos (P = 0.043, P = 0.032, P = 0.041 y P = 0.002, respectivamente). Estos caracteres pueden constituir marcadores morfometricos para determinar, con mayor certeza, la proporcion de sexos en la especie

Zamia inermis (Zamiaceae) es una cícada rara, endémica y actualmente listada en Peligro Crítico de Extinción. De la misma, sólo se conoce una población compuesta de tres subpoblaciones, que se encuentra distribuido en tan sólo 2,5 km2 de pastizales y bosques estacionalmente secos y fragmentados. Durante la década de los 70's y 80s se sustrajeron ilegalmente muchas plantas para comercio, por lo que su población actual estimada está compuesta entre 300 y 700 ejemplares. Además, debido a la ausencia de su polinizador natural, un escarabajo, esta especie presenta un reclutamiento extremadamente bajo y su diversidad genética está insuficientemente representada en esta singular población. Por todas estas razones se le considera como las cícada mexicana que se encuentra en mayor riesgo de extinción. Zamia inermis (Zamiaceae) is a rare endemic and critically threatened species of cycad, with only one known population, composed of three subpopulations. The total population is distributed in only 2.5 km2 on grassland and fragmented seasonally dry forest. Many plants have been illegally removed for the ornamental plant trade during the 1970´s and 80s and its current population is estimated between 300 and 700 individuals, with extremely poor recruitment due to absence of its natural beetle pollinator. Additionally, its genetic diversity is under represented in this unique population, and for these reasons this species should be considered as the Mexican cycad at greatest extinction risk. A CONACYT por el apoyo financiero brindado al Proyecto de Ciencia Básica 152073.

La mayoría de las cícadas contienen altas concentraciones de aceites esenciales, flavonoides, polifenoles y polisacáridos que interfieren en la extracción de ADN, causando productos de amplificación errados o inhibiendo la PCR. La optimización del aislamiento del ADN y el empleo de iniciadores de secuencias intergénicas repetidas simples (ISSRs) se investigaron en Ceratozamia mexicana Brongn., una cícada mexicana en peligro de extinción. El ADN obtenido de tejido foliar fresco, con un amortiguador modificado de cetil trimetil amonio, nos permitió obtener un ADN de buena calidad, sin pigmentos coloridos o contaminantes. La modificación al protocolo de extracción de ADN, basado en CTAB, fue un prelavado por 1 h, del tejido foliar, con una solución de 0.7 M de NaCl, para facilitar la lisis celular. El ADN extraído exitosamente se amplificó por PCR, usando seis iniciadores arbitrarios ISSR. Se observaron productos de amplificación reproducibles en todas las reacciones de PCR. Nuestros resultados muestran que la implementación mejora significativamente la calidad del ADN obtenido, usando una concentración baja de iniciadores (25 pM). Se detectaron 23 bandas fuertes, nueve de las cuales fueron polimórficas. Los resultados indican que el protocolo de optimización del aislamiento del ADN y en el sistema de PCR es viable para futuros trabajos en esta especie. Este trabajo es el primer protocolo de extracción de ADN y de ISSR reportado para esta especie ornamental en peligro de extinción.

Plants are organisms that offer unique reproductive systems to study sex determination, in other words, the expression of masculinity or femininity. Within the plant kingdom, the separation of sex has emerged over the evolutionary history in more than one hundred occasions. With each new phylogenetically-independent event there is a new reproductive system which may be similar or completely different from the previous system, so in plants there is not an only one mechanism to explain the dioecism. Cycads are a group of primitive dioecious plants that offers an extraordinary opportunity to study the emergence of dioecism in plant species. Several studies have shown that sex in cycads may be determined by homomorphic or heteromorphic sexual chromosomes; although the possibility of epigenetic factors influencing sex determination in these plants cannot be ruled out. The possibility to anticipate the sex of the cycads would allow us to establish management plans focused to the production of quality seeds in sufficient quantity to meet the demand for these ornamental plants.

El aprovechamiento forestal basado en la extracción de individuos fenotípicamente superiores, al igual que otros factores, podría hacer degenerar el bosque al dejar árboles de menor calidad y disminuir la diversidad genética. La población de Juglans pyriformis en San José Buenavista, Veracruz (SJB), ha sido reducida por actividades agropecuarias y aprovechamiento forestal con fines maderables bajo el método silvícola de Selección Individual, mientras que la población de Coacoatzintla, Veracruz (COA), se ha reducido por cambios de uso de suelo y explotación clandestina. El objetivo fue determinar los niveles de variación fenotípica de J. pyriformis en SJB y contrastarla con la variación encontrada en la población de COA en un estudio previo, utilizando la misma metodología. En SJB el análisis de componentes principales identificó tres indicadores, FUSTE, COPA y ALT-DIAM, con un coeficiente de variación alto 26, 30 y 42 % respectivamente. El análisis de conglomerados mostró dos grupos de árboles, y el estadístico de Levene fue significativo (P

The distribution of genetic diversity and structure for three populations of Dioon edule Lindley (Zamiaceae) at Monte Oscuro (MO), El Farallon (EF) and Rancho del Nino (RN) in Veracruz, Mexico was studied using 20 allo- zyme loci, considering four life history classes: seeds, seedlings, juveniles and adults. The MO population is genetically less diverse than the EF and RN populations. Total and local inbreeding differ significantly between life history classes. An increment of inbreeding among all classes was observed, and genetic differentiation among populations was higher in seeds and seed- lings than in juveniles and adults. In terms of percentage of polymorphic loci, the MO seeds showed least (80%), followed by RN (95%) and EF had the highest values (100%), probably because of a reduction in effective pop- ulation size and habitat fragmentation processes. In this context, the mean effective population size was 23.2 ± 11.3 for all populations. We conclude that seed cohorts in EF and RN represent a reservoir of genetic diversity within these two populations. Also, preservation of adult plants is an essen- tial aspect to consider in management and conservation efforts for popula- tions of Dioon edule in natural conditions.

The demographic dynamics of three populations of Dioon edule Lindl. (Zamiaceae) were studied in a fragmented landscape using projection matrix modelling. Compared with other plant species, D. edule behaves like a tree life-form species. Density and spatial distribution patterns varied among populations according to models for animal-dispersed tree species. In all scenarios, l was most sensitive to changes in abundance of adult plants. The elasticity reproductive component (F) for the three populations was zero and stasis values (L) were higher, this being a function of the permanence of non-reproductive individuals. It was detected that disturbance influences the population dynamics of D. edule as a function of adult plant persistence. This observation suggests that the conservation of adult plants is critical for D. edule and perhaps for all cycads species. Adult plant decapitation should be halted at the ‘Monte Oscuro’ population, subjected to sustainable management since 1990, if higher seed production is needed in rural nurseries. © 2008 The Linnean Society of London, Botanical Journal of the Linnean Society, 2008, 157, 381–391. ADDITIONAL KEYWORDS: conservation biology – demography – elasticity analysis – leaf-crown removal – projection modelling – survival.

The genetic diversity and structure of four populations of the cycad Zamia loddigesii were studied throughout its range in Mexico. Allozyme electrophoresis of 15 loci was conducted. The mean number of alleles per locus was 1.80 ± 0.09, the percentage of polymorphic loci was 66.6 ± 5.4, and the expected heterozygosity was 0.266 ± 0.02. The results indicated that the genetic diversity was relatively higher, with respect to tropical tree species and other cycads. The genetic variation explained by differences among populations was 18%. On average, gene flow between paired populations was similar ( Nm = 1.6) to other tropical forest trees and cycad species. Our results indicated that the geographical isolation among populations of Z. loddigesii generated allele loss, as well as a clinal variation in the frequencies of two loci ( MDH and MNR2 ), in relation to the latitudinal distribution of populations. The populations have become fragmented due to increasingly higher pressure of habitat conversion and disturbance. The importance of the establishment of sanctuaries and protected areas and a reduction in deforestation is highlighted in this research as a way of preserving the high genetic diversity of this and other endemic species. © 2006 The Linnean Society of London, Botanical Journal of the Linnean Society , 2006, 152 , 533‐544. ADDITIONAL KEYWORDS: allozymic electrophoresis ‐ conservation genetics ‐ fragmentation ‐ Mexico ‐ population genetics ‐ threatened species.

The cycad Ceratozamia mirandae is endemic to Chiapas, Mexico. Demographic studies were made in two of its populations in the Sepultura Biosphere Reserve under different conservation con- ditions; in the nucleus zone ''Tres Picos'' (conserved) and buffer zone ''La Sombra'' (disturbed and under management). Spatial distribution of C. mirandae was aggregated, showed a clumped local distribution on shallow soils on steep slopes and male and female cones appear to be synchronous in both populations. The population structure was of type I (Bongers) for both sites. Individuals between the sites showed dif- ferences in growth pattern. The oldest plants (80- 90 cm tall) were estimated to be about 490 years at ''La Sombra''. The finite growth rate (k) in the buffer zone population showed a tendency for decrease whilst in the nucleus zone this estimate remained stable. The highest elasticity values lied in the transition of the first three classes of the ''La Sombra'' population, in ''Tres Picos'' this corresponded to adult plants between 20 and 30 cm tall. Given the above, it is proposed that in the nucleus zone, reproductive adults should be of highest conservation priority, whereas in the buffer zone seedling reintroduction should be carried out regularly until the population increases. We recom- mend an IUCN Red List category of Vulnerable (VU C, 2a), largely due to difficult-to-control destructive annual forest fires that occur in this Reserve.

Human populations follow the same evolutionary principles as other organisms, although mixed with social and cultural elements, which can result in a high prevalence of certain diseases within specific ethnic groups. In this work, the Hardy-Weinberg principle is analyzed from a medical, social and biological viewpoint to understand the evolutionary processes of autosomal recessive diseases. It can be concluded that the incidence of these diseases is inversely related to the levels of genetic variability within populations, which depends on colonization, recolonization and migration events, as well as on social conventions such as racism, social stratification and segregation.

It is well known that ionizing radiation is currently a very important way to create genetic variability that is not exists in nature or that is not available to the breeder (Ahloowalia & Maluszynski, 2001; Lemus et al., 2002). Therefore, there are many papers aimed to determine the best radiation dose to applied in plant breeding work. As a result it has been defined intervals gamma radiation useful for many cultivated species, though the determination of the radiosensitivity of tissues by exposure to different intensities of radiation (De la Fe et al., 1996; Castillo et al., 1997; Fuchs et al., 2002; Lemus et al., 2002; Fuentes et al., 2004; Ramirez et al., 2006). However, most studies have been conducted have been designed to evaluate the biological response to high doses of radiation, while in relatively few studies have used low doses to stimulate physiological processes (radiostimulation) although the ionizing radiation hormesis has been widely supported (Luckey, 1980). Hormesis is the excitation, or stimulation, by small doses of any agent in any sistem (Luckey, 2003). The beneficial effect of hormesis has been well documented in species of agricultural importance (Zaka et al., 2004; Kim et al., 2005). However, there is not enough information about its use in forestry. Although little is known about the basic nature of this phenomenon, Vaiserman (2010) had indicated the possible relationship between the hormesis and epigenetic effects. The application of low-dose ionizing radiation could produce in coniferous species hormetics radiostimulants effects through genetic and epigenetic changes that manifest as adaptive responses.

• Background and Aims Dioon angustifolium was considered within D. edule. Recent morphometric and allozyme studies on D. edule have shown that D. angustifolium has originated from geographic isolation and is therefore considered to be a separate species. This cycad is endemic to north-eastern Mexico and is known only from three populations in the Sierra Madre Oriental mountain chain. Its populations are small when compared with its southern relative D. edule. In this study, genetic variation was determined within and between populations of D. angustifolium and the genetic consequences of habitat fragmentation and isolation of populations of this species were assessed. • Methods Allozyme electrophoresis of 14 presumptive loci was used. The data were analysed with statistical approximations for estimating genetic diversity, structure, gene flow and recent genetic bottlenecks. • Key Results Means and standard deviations of genetic diversity estimators were: number of alleles per locus (A = 1·67 ± 0·23), percentage of polymorphic loci (P = 52·4 ± 23 %) and expected heterozygosity (HE = 0·218 ± 0·093). The genetic variation attributable to differences among populations was 16·7 %. Mean gene flow between paired populations was Nm = 1·55 ± 0·67, which is similar to that reported for endemic plant species of narrow geographical distribution and species with gravity-dispersed seed. A recent bottleneck is detected in the populations studied. • Conclusions Dioon angustifolium presents high levels of genetic diversity compared with other cycad species, in spite of small population sizes. The recent bottleneck effect did not effectively reduce the genetic variation to the extent of eliminating these populations. The distribution of D. angustifolium appears to be the result of historical biogeographical effects related to the Pleistocene glaciations. It is recommended that this species be catalogued in the IUCN Red List of Threatened Species and conservation efforts be made to preserve it.

Information on the taxonomic composition and conservation status of the semi-evergreen tropical forest in the state of Hidalgo is limited and descriptive, and disturbances caused by human activities are drastically affecting its structure and original physiognomy. For these reasons, the objectives of this study were: 1) to conduct an inventory of the tree flora, and 2) to provide quantitative data on the structure of the different semi-evergreen tropical forest successional stages. The floristic inventory and structural characterization were conducted in 9 localities of Huautla municipality, in the “Huasteca hidalguense”. In each locality, the basal area, frequency, density of individuals (normal diameter > 10 cm) and relative importance value of each species were calculated. A total of 92 species, 76 genera and 31 families of trees were identified. The families Fabaceae, Malvaceae and Moraceae were the most representative for their species richness. The species with highest relative importance value were Adelia barbinervis and Cedrela odorata in early stages of succession, Bursera simaruba and Guazuma ulmifolia in intermediate stages and Brosimum alicastrum in mature stages. In few localities, mature vegetation was observed, and total tree species richness was comparable with that recorded in the same type of vegetation in other entities of Mexico.