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Journal ArticleDOI

Population Genetics of Agave cocui: Evidence for Low Genetic Diversity at the Southern Geographic Limit of Genus Agave

01 May 2011-Journal of Heredity (Oxford University Press)-Vol. 102, Iss: 3, pp 306-314
TL;DR: Level of allozyme diversity and population structure of Agave cocui, the species at the southern limit of distribution of the genus, and one of the lowest values of genetic variability are presented, suggesting that human impact represents a significant threat to the available genetic pool that this species possesses in the region.
Abstract: The Agave genus embraces many species with outstanding ecological and economic importance in the arid regions of the Americas. Even though this genus covers a broad geographic distribution, our knowledge on the population genetics of species is concentrated in taxa located in North America. Recently, it has been demonstrated that plant domestication decreases levels of genetic diversity in managed populations and increases population structure with respect to wild populations. We examined levels of allozyme diversity (N = 17 loci) and population structure of Agave cocui, the species at the southern limit of distribution of the genus. We sampled 7 wild populations (N = 30-35 individuals per population) representative of the geographic distribution of the species in Venezuela. Among the agaves studied, A. cocui has some of the lowest estimates of genetic diversity (H(e)[species] = 0.059, H(e)[population] = 0.054) reported until present. We propose that this condition is probably linked to the recent origin of this species in arid and semiarid regions of Colombia and Venezuela, probably through one or a few founder events. The lowest estimates of genetic diversity were associated with small populations in very restricted arid patches; but also with overexploitation of rosettes for production of fermented drinks and fibers. Santa Cruz de Pecaya, one of the 2 centers of economic use of agaves in northwestern Venezuela presented one of the lowest values of genetic variability, a sign suggesting that human impact represents a significant threat to the available genetic pool that this species possesses in the region.

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Citations
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Journal ArticleDOI
TL;DR: This work describes studies in 22 Agave species using different genetic markers and identifies some species that should be subject to special conservation genetic efforts, in particular the endangered A. victoriae-reginae and both wild populations and landraces of A. angustifolia.
Abstract: Mexico is a megadiverse country, but less than 54 % of its original vegetation still remains. In particular Mexican deserts and arid and semiarid ecosystems harbor a large number of endemic taxa, and the genus Agave is an outstanding example. Agave is one of the largest genera of the Mexican flora, including a total estimated number of 200 species, 74 % of them endemic to the country. Agave is also one of the Mexican plant genera with more population genetic studies. We describe here studies in 22 Agave species using different genetic markers. For the genus we found on average a high level of genetic variation, H s = 0.19, and a low genetic differentiation, F st = 0.15. We identify some species that should be subject to special conservation genetic efforts, in particular the endangered A. victoriae-reginae and both wild populations and landraces of A. angustifolia, including the cultivated A. tequilana.

41 citations


Cites background or methods from "Population Genetics of Agave cocui:..."

  • ...An isolation by distance analysis using Fst/(1−Fst) as an estimate of the genetic distance between pairs of populations vs. the ln of the geographic distance, indicated a positive correlation with a r2=0.187 (Figueredo & Nassar, 2011)....

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  • ...The Nei’s genetic distances were accordingly low, with an average of D=0.005 and a range between 0.001 and 0.016 (Figueredo & Nassar, 2011)....

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  • ...The population genetics of the species was described by Figueredo and Nassar (2011) using 17 isoenzymatic loci and seven populations, including 30 to 35 plants per population and covering a large area, of ca. 240,000 km2....

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Journal ArticleDOI
TL;DR: The data indicate that these species are not genetically eroded in general, and that they retain their historical patterns of diversity inside populations, and three genetically distinct populations are identified where conservation efforts should be a priority.

36 citations

Journal ArticleDOI
TL;DR: Bayesian analysis indicated that genetic clusters best fit with the corresponding habitats where populations grow, and natural mechanisms of gene flow and movement of agave propagules among populations by people explain these patterns.
Abstract: Domestication is a continuous evolutionary process guided by humans. This process leads to divergence in characteristics such as behaviour, morphology or genetics, between wild and managed populations. Agaves have been important resources for Mesoamerican peoples since prehistory. Some species are domesticated and others vary in degree of domestication. Agave inaequidens Koch is used in central Mexico to produce mescal, and a management gra- dient from gathered wild and silvicultural populations, as well as cultivated plantations, has been documented. Significant morphological differences were reported among wild and managed populations, and a high phenotypic variation in cultivated populations composed of plants from different populations. We evaluated levels of genetic diversity and structure associated with management, hypothesizing that high morphological variation would be accompanied by high genetic diversity in populations with high gene flow and low genetic structure among managed and unmanaged populations. Wild, silvicultural and cultivated populations were studied, collecting tissue of 19-30 plants per population. Through 10 nuclear microsatellite loci, we compared population genetic parameters. We analysed partition of variation associated with management categories to estimate gene flow among populations. Agave inaequidens exhibits high levels of genetic diversity (He ¼ 0.707) and moderate genetic structure (FST ¼ 0.112). No differences were found in levels of genetic diversity among wild (He ¼ 0.704), silviculturally managed (He ¼ 0.733) and cultivated (He ¼ 0.698) popula- tions. Bayesian analysis indicated that five genetic clusters best fit the data, with genetic groups corresponding to habi- tats where populations grow rather than to management. Migration rates ranged from zero between two populations to markedly high among others (M ¼ 0.73-35.25). Natural mechanisms of gene flow and the dynamic management of agave propagules among populations favour gene flow and the maintenance of high levels of variation within all popu- lations. The slight differentiation associated with management indicates that domestication is in an incipient stage.

20 citations


Cites background from "Population Genetics of Agave cocui:..."

  • ...Often, the highest levels of genetic diversity have been found in the centre of origin, and levels decrease as long as the distance increases until the extremes of the area of distribution (Eckert et al. 2008; Figueredo and Nassar 2011; Parra et al. 2015)....

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Journal ArticleDOI
TL;DR: The study suggests slight effects of management on genetic diversity of A. potatorum, apparently related to alterations of reproductive biology and pollination patterns, and Bayesian analysis indicated two genetic groups.
Abstract: Agave potatorum is a wild endemic species of Mexico. Its stems are used for mescal production, which cancels sexual reproduction. Agaves extraction from forests decreases their reproductive success and demographic performance. We evaluated patterns of genetic diversity and structure of wild populations under and without extraction in order to identify effects of its use and actions required for conserving genetic variation. This study was conducted with seven SSR markers in 12 populations representing the entire distribution area of the species. Standard parameters of genetic diversity, differentiation, structure and genetic similarity of populations were calculated and analyzed. The populations studied showed intermediate to high genetic diversity (H e = 0.36–0.64), compared with other Agave species so far studied. The wild category was the most diverse (H T = 0.87), but without significant differences with respect to populations under extraction (H T = 0.72), and two germplasm banks sampled (H T = 0.69). High structure among populations (Φ PR = 0.38) and inbreeding levels (F IS = 0.26, F IT = 0.55) were identified; a 3 % of genetic diversity being distributed among management status where germplasm banks represent a genetic pool with incipient divergence from the other categories. Bayesian analysis indicated two genetic groups. Our study suggests slight effects of management on genetic diversity of A. potatorum, apparently related to alterations of reproductive biology and pollination patterns.

16 citations


Additional excerpts

  • ...Figueredo and Nassar 2011)....

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References
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Journal ArticleDOI
TL;DR: Under some circumstances, conservation of peripheral populations may be beneficial to the protection of the evolutionary process and the environmental systems that are likely to generate future evolutionary diversity.
Abstract: A great deal of effort is spent protecting geographically peripheral populations of widespread species. We consider under what conditions it is appropriate to expend resources to protect these populations. The conservation value of peripheral populations depends upon their genetic divergence from other conspecific populations. Peripheral populations are expected to diverge from central populations as a result of the interwoven effects of isolation, genetic drift, and natural selection. Available empirical evidence suggests that peripheral populations are often genetically and morphologically divergent from central populations. The long-term conservation of species is likely to depend upon the protection of genetically distinct populations. In addition, peripheral populations are potentially important sites of future speciation events. Under some circumstances, conservation of peripheral populations may be beneficial to the protection of the evolutionary process and the environmental systems that are likely to generate future evolutionary diversity. Se gasta mucho esfuerzo protegiendo poblaciones geograficamente perifericas de especies ampliamente distribuidas. En el presente estudio, consideramos bajo que condiciones es apropiado gastar recursos para proteger estas poblaciones. El valor de conservacion de las poblaciones perifericas depende de su divergencia genetica con respecto a otras poblaciones de la misma especie. Se espera que las poblaciones perifericas diverjan de poblaciones centrales como resultado de los efectos interconectados del aislamiento, la deriva genetica y la seleccion natural. Evidencia empirica disponible sugiere que las poblaciones perifericas son con frecuencia, divergentes genetica y morfologicamente de las poblaciones centrales. La conservacion a largo plazo de las especies depende probablemente de la proteccion de las poblaciones geneticamente distintas. Adicionalmente, las poblaciones perifericas son potencialmente importantes para eventos de especiacion futuros. Bajo algunas circunstancias, la conservacion de poblaciones perifericas podria ser beneficiosa para la proteccion de los procesos evolutivos y sistemas ambientales que probablemente generen la diversidad evolutiva futura.

1,010 citations

Journal ArticleDOI
01 Aug 1995-Genetics
TL;DR: Different exact tests, appropriate for small sample size and large number of alleles, are proposed in this perspective, and their properties are evaluated by power comparisons.
Abstract: Currently used tests of Hardy-Weinberg proportions do not take into account the nature of the alternative hypothesis, which is generally a heterozygote deficiency. Different exact tests, appropriate for small sample size and large number of alleles, are proposed in this perspective, and their properties are evaluated by power comparisons. Some tests are found to be close to optimal for the detection of inbreeding or heterozygote excess, one of which is a score test closely related to Robertson and Hill's estimator of the inbreeding coefficient. This test is also easily applied to multiple samples. Such tests are not always the most appropriate if alternative hypotheses differ from those considered here.

792 citations


"Population Genetics of Agave cocui:..." refers methods in this paper

  • ...Departures from Hardy– Weinberg (H-W) expectations were examined for each locus and population with the H-W exact test proposed by Rousset and Raymond (1995) using the program GENEPOP on the Web (http://genepop.curtin.edu.au/)....

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Journal ArticleDOI
TL;DR: A review of the literature on the analysis of spatial genetic variation within continuously distributed populations, with special emphasis on plant populations and on the detection of structure attributable to isolation by distance (IBD), is presented in this article.
Abstract: The documentation of spatially restricted dispersal in a wide variety of plant species has led to the prediction that plant populations will often be genetically subdivided on a local scale, as the result either of spatial variation in selection (environmental heterogeneity) or of local genetic drift (isolation by distance) (4, 5, 32, 49). The potential for environmental heterogeneity to generate significant spatial structuring of genotypes in plant populations that occupy pronouncedly patchy habitats is well documented (e.g. 38, 88). In contrast, the effects of isolation by distance (IBD) on the internal structuring of plant populations are less well studied. This review critically examines the methodologies currently available for the analysis of spatial genetic variation within continuously distributed populations, with special emphasis on plant populations and on the detection of structure attributable to IBD. Except where explicitly stated otherwise, conclusions will refer to diploid nuclear genes. The extensive literature on the detection of spatial associations between genetic and environmental variables has been reviewed several times (see 34) and is specifically omitted here. Therefore, in reviewing the empirical evidence for local structure, studies deliberately conducted with populations occupying manifestly heterogeneous habitats have been excluded.

450 citations

Journal ArticleDOI

429 citations


"Population Genetics of Agave cocui:..." refers background in this paper

  • ...Buffer recipes and stains are modified from Soltis et al. (1983) and Mitton et al. (1979), with the exception of recipes for Aat and Dia (Cheliak and Pitel 1984)....

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