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Abstract
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.
Highlights
Human management consists of a series of deliberate actions undertaken to transform ecological systems into socioecological systems, following the requirements and interests of the societies to which they belong (Lindig and Casas 2013; Casas et al 2014, 2015)
Factorial analyses of variance of total damage indicated that damage levels were significantly higher in cultivated populations; between regions, populations from Valle de Tehuacan had a significantly higher incidence of damage compared to populations from the Mixteca Baja region
Our results showed that cultivated populations of S. stellatus exhibited a significant decrease in levels of genetic diversity and resistance, as well as an increase in damage levels, which means that these nontargeted attributes have changed during human selection for fruit production, which is the target attribute (Casas et al 2006; Cruse-Sanders et al 2013)
Summary
Human management consists of a series of deliberate actions undertaken to transform ecological systems into socioecological systems, following the requirements and interests of the societies to which they belong (Lindig and Casas 2013; Casas et al 2014, 2015). Management means manipulating populations of target species to ensure their availability by selecting for attributes desired by humans (Casas et al 1997, 2007; Blancas et al 2013, 2014) This process, called domestication, generates morphological, physiological, and genetic divergences between populations of managed organisms and their wild relatives, known as domestication syndromes (Schwanitz 1966; Harlan 1975; Casas et al 1999, 2015; Gepts 2003). Domesticated populations exhibit lower levels of genetic diversity as a result of selection by humans over hundreds or thousands of years of domestication (Maxted et al 2013) This reduction in genetic diversity in domesticated populations makes them vulnerable to herbivore attack and pathogen infestation because of the loss of the chemical and structural mechanisms that protect them (Pickersgill 2007; Casas and Parra 2016; Bravo-Avilez 2017). It has been argued that this effect is correlated with increases in productivity, such as the increased size of flowers, fruits, or seeds (Benrey et al 1998), at the expense of a reduction in the distribution of energy to other functions, such as the synthesis of secondary
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