Population Genetics of Agave cocui: Evidence for Low Genetic Diversity at the Southern Geographic Limit of Genus Agave
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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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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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
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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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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Book•
01 Jan 1978
TL;DR: "Wright's views about population genetics and evolution are so fundamental and so comprehensive that every serious student must examine these books firsthand."
Abstract: "Wright's views about population genetics and evolution are so fundamental and so comprehensive that every serious student must examine these books firsthand. . . . Publication of this treatise is a major event in evolutionary biology."-Daniel L. Hartl, "BioScience"
1,537 citations
TL;DR: An attempt to improve methods of analysis of fern enzymes in starch gel electrophoresis by experimenting with modifications of the method of sample preparation outlined by Soltis et al. (1980), and determining gel and electrode buffers that provide clear starch gel enzyme banding for 22 enzyme systems in ferns.
Abstract: The homosporous pteridophytes have been largely uninvestigated by electrophoresis, despite the fact that they offer many exciting research possibilities (Soltis et al., 1980). The paucity of electrophoretic studies of ferns and fern allies may be due in large part to the high concentrations of condensed tannins that many species contain (Cooper-Driver, 1976 and pers. comm.). These compounds render enzymes inactive by binding with them following cellular disruption, thereby frustrating researchers who have attempted electrophoretic analysis utilizing standard methods of sample preparation. The method of sample preparation developed by Kelley and Adams (1977a, b) in their analysis of enzyme variation in Juniperus was an important procedural breakthrough in overcoming the difficulties that result from the liberation of large amounts of phenolic compounds during tissue preparation. Recently, a simplified version of that method was applied by Soltis et al. (1980) to fern leaf tissue, facilitating rapid preparation of active enzyme samples and thereby making electrophoretic analyses of large numbers of individuals more feasible. In an attempt to improve methods of analysis of fern enzymes in starch gel electrophoresis, we have experimented with modifications of the method of sample preparation outlined by Soltis et al. (1980). We also have examined several different methods of sample preparation such as those of Gottlieb (1981a), Mitton et al. (1979), and Werth et al. (1982), and have evaluated the relative merits of each with fern tissue. Finally, during the course of our electrophoretic investigations of ferns we found that standard gel and electrode buffers and staining schedules, such as those of Brewer (1970) and Shaw and Prasad (1970), often provided unsatisfactory results when applied to ferns. We have determined gel and electrode buffers, as well as staining schedules, that provide clear starch gel enzyme banding for 22 enzyme systems in ferns. Requests for advice resulting from the recent surge of interest in fern enzyme electrophoresis have prompted us to compile our procedural data so that other researchers can take advantage of our experimentation. We hope that these data will stimulate more extensive electrophoretic investigation of pteridophytes and other electrophoretically difficult taxa. Gottlieb (1981b) recently reviewed aspects of enzyme electrophoresis primarily in gymnosperms and angiosperms. His discussion is equally relevant to understanding the potential applications and limitations of electrophoretic evidence in pteridophytes. Since homosporous pteridophytes have high chromosome numbers, it is tempting to invoke polyploidy in interpreting their enzyme band patterns. It is well
1,432 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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TL;DR: Despite the large number of studies testing these simple predictions, very few attempted to test possible mechanisms causing reduced peripheral diversity or increased differentiation, which is likely to influence the adaptive potential of populations across the geographical range.
Abstract: There is growing interest in quantifying genetic population structure across the geographical ranges of species to understand why species might exhibit stable range limits and to assess the conservation value of peripheral populations. However, many assertions regarding peripheral populations rest on the long-standing but poorly tested supposition that peripheral populations exhibit low genetic diversity and greater genetic differentiation as a consequence of smaller effective population size and greater geographical isolation relative to geographically central populations. We reviewed 134 studies representing 115 species that tested for declines in within-population genetic diversity and/or increases in among-population differentiation towards range margins using nuclear molecular genetic markers. On average, 64.2% of studies detected the expected decline in diversity, 70.2% of those that tested for it showed increased differentiation and there was a positive association between these trends. In most cases, however, the difference in genetic diversity between central and peripheral population was not large. Although these results were consistent across plants and animals, strong taxonomic and biogeographical biases in the available studies call for a cautious generalization of these results. Despite the large number of studies testing these simple predictions, very few attempted to test possible mechanisms causing reduced peripheral diversity or increased differentiation. Almost no study incorporated a phylogeographical framework to evaluate historical influences on contemporary genetic patterns. Finally, there has been little effort to test whether these geographical trends in putatively neutral variation at marker loci are reflected by quantitative genetic trait variation, which is likely to influence the adaptive potential of populations across the geographical range.
1,366 citations
"Population Genetics of Agave cocui:..." refers background or result in this paper
...This hypothesis explains low levels of genetic diversity in peripheral populations of plants and animals as a consequence of smaller effective population size and greater geographical isolation in comparison with geographically central populations (Lesica and Allendorf 1995; Eckert et al. 2008)....
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...These results are concordant with the central– marginal hypothesis, frequently used to explain why genetic diversity declines from populations located in the center of the geographical range of a species toward the periphery of its distribution (Lesica and Allendorf 1995; Eckert et al. 2008)....
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TL;DR: Wright's F‐statistics can be defined as ratios of gene diversities of heterozygosities rather than as the correlations of uniting gametes, irrespective of the number of alleles involved or whether there is selection or not.
Abstract: It is show that Wright's F-statistics can be defined as ratios of gene diversities of heterozygosities rather than as the correlations of uniting gametes. This definition is applicable irrespective of the number of alleles involved or whether there is selection or not. The relationship between F-statistics and Nei's gene diversity analysis is discussed.
1,194 citations
"Population Genetics of Agave cocui:..." refers background in this paper
...Nei’s parameters of genetic diversity (Nei 1973, 1977) were estimated for each polymorphic locus, including total genetic diversity (HT), mean genetic diversity within populations (HS), and the proportion of genetic diversity due to differences among populations (GST 5 (HT HS)/HT)....
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01 Jan 2002
1,089 citations
"Population Genetics of Agave cocui:..." refers background in this paper
...Small population size has been associated with low genetic variation in plants and animals in general (Ellstrand and Elam 1993; Frankham et al. 2005; Letelier 2007)....
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