Jaime Sahagún-Castellanos

Variability of three regional sources of germplasm of Tejocote (Crataegus spp.) from central and southern Mexico

Tejocote (Crataegus spp.) is a genus of fruit-bearing trees distributed widely throughout Mexico; 13 species are reported for the north and central zones and two or more species may be present in southern Mexico. Accessions of this genus are safeguarded in the Germplasm Bank of Tejocote at the Autonomous University of Chapingo, mainly from three regional sources, i.e. the states of Puebla, Mexico, and Chiapas, including five different species that belong to series Mexicanae and series Crus-galli. They can be morphologically characterized by leaves from different shoot types, flowers and fruits. Multivariate statistical methods were used to elucidate patterns of variation in each of these regional sources. The sources displayed very low intra-source variability. The source from Chiapas showed significant statistical differences in all morphological variables evaluated, as a result, this genetic pool is considered as different from the other two sources. The sources from the states of Puebla and Mexico only differed by 22.79% (with Pxa0≤xa00.05), and thus they could be considered as components of a single genetic pool. The most highly discriminant variables were from the leaf, such as basal angle, petiole length/major axis length ratio, minor axis length/major axis length ratio, and number of veins.

A Restricted Selection Index Method Based on Eigenanalysis

Selection indices, used in animal and plant breeding to select the best individuals for the next breeding cycle, are based on phenotypic observations of traits recorded in candidate individuals. The restrictive selection index (RSI) facilitates maximizing the genetic progress of some characters, while leaving others unchanged. Recently a selection index (SI) was proposed based on the eigen analysis method (ESIM), in which the first eigenvector (from the largest eigenvalue) is used as the SI criterion, and its elements determine the proportion of the trait that contributes to the SI. However, the current ESIM, which has two main limitations, is based on the assumption that the vector of coefficients of the index is equal to the genotypic variance-covariance matrix among the traits multiplied by the vector of economic weights, and does not allow one to restrict the number of traits. In this study, we develop a more general ESIM that has two main features, namely, it makes no assumption concerning the coefficients of the index and it can be generalized to a restrictive ESIM (RESIM). We use two datasets to illustrate the theoretical results and practical use of ESIM and RESIM, and to compare them with standard unrestrictive and restrictive selection indices. The main advantages of RESIM over traditional unrestrictive and restrictive SIs are that its statistical sampling properties are known; its selection responses are equal to or greater than those estimated from the traditional restrictive SI; and it does not require economic weights and thus can be used in practical applications when all or some of the traits need to be improved simultaneously (traditional SIs cannot improve several traits simultaneously if economic weights are not available). Finally, we prove that the coefficients of the traditional RSI belong to the space generated by the eigenvectors of RESIM.

A Molecular Selection Index Method Based on Eigenanalysis

The traditional molecular selection index (MSI) employed in marker-assisted selection maximizes the selection response by combining information on molecular markers linked to quantitative trait loci (QTL) and phenotypic values of the traits of the individuals of interest. This study proposes an MSI based on an eigenanalysis method (molecular eigen selection index method, MESIM), where the first eigenvector is used as a selection index criterion, and its elements determine the proportion of the traits contribution to the selection index. This article develops the theoretical framework of MESIM. Simulation results show that the genotypic means and the expected selection response from MESIM for each trait are equal to or greater than those from the traditional MSI. When several traits are simultaneously selected, MESIM performs well for traits with relatively low heritability. The main advantages of MESIM over the traditional molecular selection index are that its statistical sampling properties are known and that it does not require economic weights and thus can be used in practical applications when all or some of the traits need to be improved simultaneously.

Interspecific Differences in Postharvest Quality on Mexican Christmas Trees

Abstract There are no comparative studies in postharvest quality of Mexican Christmas trees. The objective of this study was to identify the best postharvest performing Mexican cultivated species. The experiment was done in the 2004-2005 season with six replications (trees) of Abies religiosa, Cupressus lindleyi, Pinus ayacahuite, and Pseudotsuga menziesii; from two provenances (Tlaxcala and Veracruz) for the last two species. Cultural management was similar. Each tree was placed under dry conditions according to a completely randomized design. Secondary branches, twig diameter and density, initial and final weight, biomass allocation, areas and volumes, total and twig moisture content, foliage density, color, chlorophyll a/b ratio, CO2 and ethylene production were evaluated. Analyses of variance, comparisons of means, correlation, and simple regression were performed. The four studied species displayed undesirable characteristics. Genetic improvement is required. P. menziesii showed values nearer ideotype breeding in more variables. The best provenance was Tlaxcala. Several correlations between variables showed tradeoffs in selecting the best species.

The Pathosystem Solanum tuberosum L.-Phytophthora infestans (Mont.) de Bary in Chapingo, Mexico. Expected, Observed, and Simulated

The 1:1 ratio of the Phytophthora infestans A1 and A2 mating types favors the sexually derived genetic diversity of the late blight pathogen of potatoes, which is widespread in the central highlands of México. This ratio guarantees the successful infection of this pathogen, even in resistant potato hosts. However, wild Solanum species present in the region serve as alternative hosts to the pathogen. Knowledge about the external factors that influence the dynamics of this disease facilitates the assessment and selection for genetically resistant potato cultivars to late blight, in addition to providing the capacity to predict epidemics. This work aimed to assess the expected, observed, and simulated progress of natural P. infestans infection of potatoes during two epiphytotic field seasons (2009 and 2010) at Chapingo, México. Using 8-years of weather datasets, six ideal situations were predicted with four to eight infection cycles of 6 to 12xa0h each. In comparison to the predictions, the observed effect of the area under the disease progress curve, and its components (AUDPC, RAUDPC, RaRAUDPC), was highly significant, with a low coefficient of variation among the potato cultivars used in the study. In conclusion, we confirm that the LATEBLIGHT-LB2004 model is useful for simulating and predicting late blight epidemics based on the weather conditions of Chapingo, except for the magnitude of the relative humidity threshold variable (RH_threshold), which requires calibration for each cultivar.ResumenLa proporción 1:1 de los grupos de compatibilidad A1 y A2 de Phytophthora infestans favorece la diversidad genética derivada sexualmente del patógeno del tizón tardío de la papa, que está ampliamente distribuido en los altiplanos centrales de México. Esta proporción garantiza la infección exitosa del patógeno, aún en hospedantes de papa resistentes. No obstante, las especies silvestres de Solanum presentes en la región sirven como hospederos alternantes del patógeno. El conocimiento de los factores externos que influencian la dinámica de esta enfermedad facilita el análisis y la selección para la resistencia genética de variedades de papa al tizón tardío, además de proporcionar la capacidad de predecir epidemias. El propósito de este trabajo fue analizar el progreso de la infección natural de P. infestans respecto a lo esperado, observado y simulado en papa durante dos ciclos de campo epifíticos (2009 y 2010) en Chapingo, México. Con el registro de ocho años de datos meteorológicos, se predijeron seis situaciones ideales con cuatro a ocho ciclos de infección de 6 a 12 hs cada uno. En comparación con las predicciones, el efecto observado de área bajo la curva de progreso de la enfermedad, y sus componentes (AUDPC, RAUDPC, RaRAUDPC), fue altamente significativo, con bajo coeficiente de variación entre las variedades de papa usadas en este estudio. En conclusión, confirmamos que el modelo LATEBLIGHT-LB2004 es útil para simular y predecir la epidemia de tizón tardío, con base a las condiciones atmosféricas de Chapingo, excepto por la magnitud del umbral de la variable de humedad relativa (RH_threshold), que requiere de calibración para cada variedad.