Evolution under domestication of correlated characters in populations of Stenocereus stellatus (Pfeiff.) Riccob., under different forms of management in central Mexico: genetic diversity, damage, and defense mechanisms

Across the process of domestication, human selection produces changes in target attributes as well as changes that are not necessarily desired by humans due to pleiotropic or linked genes. In this paper we addressed, correlated changes between genetic diversity, damage level, defense mechanisms (resistance and tolerance), and fitness due to the domestication process of Stenocereus pruinosus (Otto ex Pfeiff.) Buxbaum and Stenocereus stellatus (Pfeiff.) Riccobono, an endemic columnar cactus of south-central Mexico. One hundred eighty individuals of S. stellatus from wild, in situ managed, and cultivated populations of Valle de Tehuacán and Mixteca Baja, Puebla, were sampled, and attributes including damage level, defense mechanisms and fitness (number of fruits) were measured. The DNA of 176 individuals was extracted to amplify and analyze five microsatellites in order to estimate genetic diversity and structure. As expected, cultivated populations showed a significantly higher damage level, as well as lower resistance and genetic diversity. Depending on the form of management, correlations between genetic diversity and the rest of the attributes exhibited different patterns. In wild populations, genetic diversity was positively correlated with damage and negatively with resistance; in situ managed populations exhibited the opposite pattern, and in cultivated populations, no correlations were found between these attributes. We propose a hypothetic model of human selection to explain the variation in these correlations. No differences in genetic diversity and tolerance were detected between regions; however, the populations of Valle de Tehuacán exhibited more damage and more resistance. In both regions, populations showed a positive correlation between fitness and resistance and a negative correlation between damage and resistance, suggesting the existence of a defense mechanism to ensure fitness. Also, non-regional differentiation suggests an eventual gene flow due to pollinators, human movement of branches, or a common ancestry before the domestication process.

EL CULTIVO DE LA PITAYA DE AGOSTO (Stenocereus stellatus Pfeiffer) EN LA MONTAÑA DE GUERRERO

Objetivo: Caracterizar el cultivo de la pitaya de agosto (Stenocereus stellatus Pfeiffer) en dos comunidades de la Montaña de Guerrero, México.Diseño/metodología/aproximación: Se aplicó un cuestionario con variables sobre prácticas de manejode la pitaya de agosto y de su contribución en los ingresos de las familias rurales a una muestra de 29 productores de un total de 105 Unidades de Producción Rural (UPR). La información se analizó conestadísticas descriptivas. Resultados: En las comunidades de estudio el cultivo de la pitaya de agosto es realizada por productores mayores de 40 años, tanto hombres como mujeres, con una experiencia promedio de 9.8 años en la producción del cultivo y realizan pocas prácticas de manejo. El periodo de cosecha es en promedio de 66 días, con una producción estimada de 3375.9 kg por unidad de producción y un precio promediode 7.48 MX por kg. Se estimó un ingreso de $25000.00 MX por año por unidad de producción y un ingreso regional total de $3,044,435.07 MX.Limitaciones del estudio/Implicaciones: El amplio rango del periodo de cosecha y la característica de que la pitaya de agosto es un cultivo altamente perecedero dificulta la cuantificación de cosecha en unamuestra alta de unidades de producción.Hallazgos/conclusiones: Al cultivo de la pitaya de agosto se le invierte poco en la región de la Montaña de Guerrero, tanto en recursos económicos como humanos. Sin embargo, es un cultivo que contribuye, sobre todo estacionalmente, de manera importante en el ingreso económico de las familias de la región.

Manejo de la vegetación, domesticación de plantas y origen de la agricultura en Mesoamérica

A model of domestication of plants in Mesoamerica based on selective management of plant populations and communities by silvicultural practices is analyzed. Archaeological and ethnobotanical information suggests that intentional manipulation of vegetation by Mesoamerican peoples has occurred in past and present times in order to control availability of useful plants. Forms of management of plant communities or populations have included tolerance, protection and enhancement of individual plants of particular species during clearance of vegetation and other ways of perturbation. Processes of artificial selection (selection in situ) may be carried out through these forms of plant management. These processes may cause significant morphological differences between wild and managed populations as illustrated by the cases discussed here of Anoda cristata, Crotalaria pumila, Leucaena esculenta and Stenocereus stellatus. Processes of artificial selection in situ are mechanisms of incipient domestication of plants which appear to have been carried out in Mesoamerica, perhaps since pre-agricultural times, and that could contribute to explain the processes that led to the origins of agriculture in this region.

La domesticación de Stenocereus stellatus (Pfeiffer) Riccobono (Cactaceae)

This study aimed at analyzing processes of domestication by silvicultural forms of management. The effects of artificial selection on morphology and reproductive biology of Stenocereus stellatus are analyzed in wild; wild-managed in situ (silvicultural management) and cultivated populations from the Tehuacan Valley and La Mixteca Baja. People select the phenotypes of this columnar cactus species with relatively larger and sweeter fruits with thinner peel and fewer spines, favoring their numbers in wild populations managed in situ as well as in home gardens. Favorable phenotypes prevail in wild populations managed in situ, indicating that artificial selection has had significant effects under this form of management. These phenotypes are especially abundant in home gardens, which suggests that artificial selection is even stronger under cultivation. Favorable phenotypes are scarce or absent in non-manipulated wild populations. Such morphological divergence is maintained despite the reproductive system of S. stellatus, is self-incompatible in all populations, and even when there were not found spatial and temporal barriers for pollen flow between wild and cultivated populations. Abundance of favorable phenotypes in areas managed by people may be explained by action of artificial selection, while absence of some domesticated phenotypes in wild populations seems to be due to pollen incompatibility between some cultivated phenotypes and wild populations, but it could also be due to pressures of natural selection against cultivated phenotypes.

Consecuencias de la domesticación de Stenocereus stellatus en el tamaño de las semillas y en la germinación en un gradiente de estrés hídrico

Stenocereus stellatus es una cactácea columnar de importancia ecológica y cultural en el Valle de Tehuacán; sus frutos se colectan en poblaciones silvestres, pero también existe manejo silvícola y cultivo de plantas de esta especie en sistemas agroforestales. Investigaciones previas documentaron divergencias morfofi siológicas y genéticas entre poblaciones silvestres y manejadas, ocasionadas por selección artifi cial y limitaciones al fl ujo génico entre poblaciones. En esto último podrían infl uir diferencias en patrones de germinación y establecimiento de semillas en plántulas silvestres y cultivadas en bosques y ambientes manejados. Se probó la hipótesis de que la selección artifi cial por frutos grandes favorece un mayor tamaño de las semillas en plantas cultivadas, las cuales presentarían mayor tasa de germinación en condiciones de alta humedad, pero serían más susceptibles a estrés hídrico que las semillas de plantas silvestres. Se evaluó el peso de frutos y semillas y su tasa de germinación en un gradiente de humedad (0.0, -0.2, -0.4, -0.6, -0.8 y -1.0 Megapascales) en poblaciones silvestres y cultivadas. Los frutos cultivados y sus semillas fueron signifi cativamente más pesados (54.616 ± 1.547 g, 0.190 ± 0.017 g, respectivamente) que los frutos y semillas silvestres (37.023 ± 1.122 g, 0.15 3± 0.005 g, respectivamente). Los análisis de devianza mostraron diferencias signifi cativas en el inicio (X2 = 1,639.0, gl = 4; X2 = 236.31, gl = 1, respectivamente) y la velocidad de germinación (X2 = 20.91, gl = 4), asociadas al tipo de manejo y al potencial hídrico (las semillas cultivadas fueron más susceptibles al estrés que las silvestres). La selección artifi cial que favorece frutos grandes parece haber infl uido indirectamente en el incremento de tamaño de las semillas, y las condiciones ambientales de los sitios manejados podrían haber infl uido en una mayor susceptibilidad a estrés hídrico.

Seed banking in the columnar cactusStenocereus stellatus: distribution, density and longevity of seeds

The soil seed bank is the reserve of viable seeds found in the soil. This reserve contributes to plant population persistence in unpredictable environments; thus, determining its presence is basic to understanding recruitment patterns and population dynamics. Studies of soil seed banks in the Cactaceae are scarce, although these plants are ecologically dominant in American arid and semi-arid environments. Most studies have inferred the presence of seed banks by analysing morphological seed traits or germination of seeds stored in the laboratory for different periods of time. Few studies have determined their presence through evaluation of distribution, density and longevity of seeds in the field. To fill this information gap, we determined the existence of, and studied, the soil seed bank ofStenocereus stellatus, a columnar cactus endemic to central Mexico. This study reports the evaluation of these characteristics in the field and discusses whether this species forms a soil seed bank. We found a higher number of seeds under shrubs than in areas lacking vegetation. Recently dispersed seeds did not germinate because they have primary dormancy. This dormancy was broken after 6 months of burial in the soil. Seeds buried for 10 months entered secondary dormancy and they were not viable at 24 months, probably because of pathogen attack. Considering dormancy and seed longevity, we suggest thatS. stellatushas the potential to form a short-term persistent seed bank. However, this should be confirmed by conducting studies on otherS. stellatuspopulations throughout their geographical distribution.