Genetic diversity, conservation and sustainable use of wild Agave cupreata and Agave potatorum extracted for mezcal production in Mexico
Highlights
► Population genetics of Agave extracted from tropical dry forests for making mezcal. ► We found high levels of genetic diversity, but contrasting structuration patterns. ► We identified three genetically distinct populations. ► Propagation based on seeds from genetically similar populations is recommended. ► In vitro propagation should be avoided due to inbreeding depression.
Introduction
Mezcal, a generic term that comes from the nahuatl mexcalli (‘baked agave’), is the name applied to traditional distilled alcoholic drinks produced in many rural areas of Mexico, from the northern states of Tamaulipas and Sonora to Oaxaca in the south. These spirits are obtained from the fermented sugars of the cooked stems of more than fourteen species of the genus Agave, commonly named ‘maguey’. Two of them are cultivated in monospecific agroindustrial plantations: Agave tequilana (for tequila, which can be considered a kind of mezcal) and Agave angustifolia. The latter is widely cultivated in the state of Oaxaca and it is the source of most of the commercial mezcal produced in Mexico (Conabio, 2006; Gentry, 1982). The remaining species are found in different productive contexts ranging from entirely wild populations to agroecosystems associated to maize, grazing and homegardens. These systems therefore embody most of the biodiversity and traditional knowledge associated to agave spirits. They also exhibit production and conservation issues that are very different from those of the well studied cultivated species A. tequilana and A. angustifolia.
Production of the spirit from wild populations requires the harvest of reproductive individuals selected when the inflorescence begins to develop. This practice implies the extraction of the individual and leads to the total suppression of pollen and seed production in these semelparous plants. In exploited sites, populations persist via asexual offshoots or, in the case of species reproducing only by seeds, through individuals located in sites hard to reach, as well as early and late-flowering individuals that “escape” harvesting periods.
Constant and extensive extraction of reproductive individuals and the subsequent reduction of effective population size can modify the levels and distribution of genetic diversity or allele richness (Sebbenn et al., 2008). Low genetic diversity can prevent species to respond to environmental changes (through adaptation or diversification) and a reduced number of individuals can lead to genetic drift and inbreeding that usually results in lower population fitness (Hedrick, 2000). The subsequent recruitment based on seeds from few early and late-flowering individuals can also diminish the female effective population size and act as a mechanism that might displace phenological patterns and reduce gene flow between populations.
Agave cupreata (called ‘papalometl’ in nahuatl, which means butterfly agave, or ‘maguey papalote’) and Agave potatorum (‘maguey tobalá’) are the most representative wild species used for making mezcals in southwestern Mexico. A. cupreata is endemic to the Balsas River basin, and A. potatorum to the inner slopes of the Sierra Madre del Sur and Tehuacán Valley (Fig. 1). They occur on dry ecosystems such as oak forests and deciduous tropical forests considered among the most endangered vegetation types in Mexico on account of fragmentation and disturbance caused by clearing, grazing activities and deforestation (Trejo and Dirzo, 2000).
Both species have remarkably similar morphologies and ecological niches; A. potatorum can be considered to replace A. cupreata in the same vegetation types when moving southward and in some places is referred to by the same common name ‘papalometl’ (L.E. Eguiarte, pers. obs.; Gentry, 1982). It has been observed that these species do not produce neither bulbils nor offshoots as other members of Agave do (Gentry, 1982) and depend entirely on outcrossing for seed production (García-Meneses, 2004). Seeds are wind dispersed from oblong capsules located on the upper half of the three to six meters tall inflorescence. Both seed number and germination rate are high (an individual of A. cupreata can produce 50 000 viable seeds with a germination rate of ca. 95%; one of A. potatorum can produce ca. 5700 seeds) (García-Meneses, 2004; Torres-García, 2008).
The diverse age structure attained by wild populations where seedlings are recruited every year allows producers to extract reproductive individuals on an annual basis following a forestry approach, which contrasts with five to eight year periods between harvests in even aged plantations of A. angustifolia and A. tequilana. Accordingly, A. cupreata and A. potatorum are important non-timber forest species relevant for the economy of the inhabitants of these dry tropical regions.
Traditional mezcal production in small distilleries offers employment and supports a local cash crop that complements rain fed agriculture: Agave flowering and the subsequent harvesting begins in October and productive activities continue throughout all the dry season. When the rainy season arrives, peasants transfer their activities to sowing maize. Finally, mezcal commercialization is important to regional economies: in 2002 in the Chilapa region alone (Guerrero state, see Fig. 1) 60 000 L of mezcal were produced with a local value of more than 2 million Mexican pesos (C. Illsley, unpubl. res.). However, despite such economic and cultural relevance, comments have been gathered from inhabitants in all their distribution range about sites where maguey used to be abundant and that nowadays are depleted (X. Aguirre-Dugua, pers. obs.; Delgado-Lemus, 2008), which may suggest that local extinctions have occurred recently.
We consider that A. cupreata and A. potatorum populations and their genetic diversity levels may be endangered by the extraction of reproductive individuals, since adult survival rate has been described as the most important factor contributing to the growth rate of populations (lambda) (Illsley et al., 2007). Moreover, previous reports on cultivated species of Agave suggest that the genus is very sensitive to strong selection pressures and genetic bottlenecks. For instance, the blue variety of the species A. tequilana, propagated by asexual offshoots and micropropagation, has very low levels of genetic diversity (96% of similarity between individuals with AFLPs, Gil-Vega et al., 2006) and a diminished capacity of forming seeds on account of low pollen viability associated to structural chromosome aberrations (Ruvalcaba-Ruiz and Rodríguez-Garay, 2002). Agave fourcroydes, known as sisal, has also been genetically eroded because of vegetative propagation and a strong selection towards thicker fibers since the XIXth century (all individuals analyzed within three different cultivars have the same isozyme electrophenotype; Colunga-GarcíaMarín et al., 1999).
The aim of the present study is to estimate the amount and distribution of genetic diversity in the wild native species A. cupreata and A. potatorum as a contribution to the design of conservation strategies and in situ management plans that assure their sustainable long term use for mezcal production. With the help of neutral markers we assess the degree of genetic erosion of the species and analyze basic parameters regarding genetic variability for the identification of population units with genetic distinctiveness (evolutionary significant units sensu Waples, 1991) that may merit specific management. Our results are also used to discuss two previously proposed strategies of reforestation based on seeds collecting. We also aim to explore if their morphological and ecological resemblance extends to the molecular level.
Section snippets
Populations sampling
Five populations of each species were collected throughout their range (Table 1). Localities were selected according to the distributions reported by Gentry (1982) and specimens deposited at the National Herbarium MEXU. Tissue from the youngest leaf of each rosette was collected from 50 adult individuals in each population. Plant materials were immediately frozen in liquid nitrogen for transportation and kept at −80 °C in the laboratory before DNA isolation. In all populations there were
Genetic variation
A total of 34 bands (“loci”) were amplified from 400 to 2000 base pairs (bp) in both species: 14 loci with primer 846 and 20 loci with primer 857; two additional bands from primer 846 (1900 and 1800 bp) were only present in A. cupreata, so all A. potatorum individuals were regarded as recessive homozygous for these loci. In contrast, no exclusive loci were found in A. potatorum. On average, 33.2 individuals per population were successfully analyzed. All analyzed individuals, considering both
Genetic diversity
It has been observed that in plants of economic importance, direct extraction, together with propagation and management practices, can create a selection pressure (not necessarily intentional) that diminishes effective population size and modifies the genetic composition of populations (Sebbenn et al., 2008). In the case of wild A. cupreata and A. potatorum populations exploited for mezcal production, reduction of population size is caused mainly by the direct extraction of reproductive
Conclusions
The data obtained in this study provide evidence for high levels of genetic diversity in wild A. cupreata and A. potatorum despite their long history of extraction, although the former species has a higher degree of structuring. To face the needs for local mezcal production based on wild stands, and considering the genetic structuration patterns we detected, we suggest that A. cupreata populations could be reinforced using regional stands as “seed zones”, while the weaker genetic structure in
Acknowledgments
This work was financed by the Collective Biological Resources Program at the National Commission for the Knowledge and Use of Biodiversity (Conabio) [projects V038, CS016]; Consejo Mexicano de Ciencia y Tecnología-Secretaría de Educación Pública [grant number 2004-C01-46475-Q]; Secretaría de Medio Ambiente y Recursos Naturales-Consejo Mexicano de Ciencia y Tecnología [2002 C01-0246]; and Dirección General de Asuntos de Personal Académico (DGAPA), UNAM, Programa de Apoyo a Proyectos de
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