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a Centro de Investigación Científica de Yucatán, Apartado Postal 87, Cordemex, Mérida, Yucatán, México 97310; and b Instituto de Ecología, UNAM. Apartado Postal 70-275. Ciudad Universitaria, México, D.F.
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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Key Words: Agavaceae • Agave angustifolia; • Agave fourcroydes • domestication • germplasm diversity • henequén • isozymes • phylogeny
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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The present paper analyzes the isozymatic variation and the phylogenetic relationships among existing henequén varieties and wild populations of A. angustifolia that grow in the Yucatan Peninsula. Results are discussed in the context of the hypotheses postulated in previous papers (Colunga-GarcíaMarín and May-Pat, 1993; Colunga-GarcíaMarín et al., 1993; Colunga-GarcíaMarín, Estrada-Loera and May-Pat, 1996; Colunga-GarcíaMarín and May-Pat, 1997), and in terms of alternatives for conservation of wild and cultivated germplasm.
Henequén is a crop plant whose successful cultivation has been restricted to the Yucatan Peninsula, some regions of the Mexican states of Tamaulipas and Veracruz, and Cuba. In these last three areas, the germplasm was introduced from Yucatan. Propagation is strictly vegetative, by means of rhizomatous suckers. A prevailing agricultural practice in henequén cultivation is the severing of the inflorescence peduncle soon after it begins to emerge, making it unusual to encounter flowering plants. When flowers do develop, they seldom form seeds, and even then the seeds are never used for propagation. Germination percentage of henequén seeds is ~9%, while that of wild plants is ~73% (P. Colunga-GarcíaMarín and F. May-Pat, unpublished data).
Previous studies were made of the ethnobotanical evidence on past germplasm diversity of this cultigen (Colunga-GarcíaMarín and May-Pat, 1993) and morphological variation in natural and uniform conditions of both henequén and its putative wild ancestor (Colunga-GarcíaMarín, Estrada-Loera, and May-Pat, 1996; Colunga-GarcíaMarín and May-Pat, 1997). These studies indicated the lack of evidence about the diversity of henequén during the Prehispanic Mayan era, but we assume that this diversity was at least equal or larger than that published in agronomic manuals around the beginning of the 20th century (de Echánove, 1814; Regil and Peón, 1853; Espinosa, 1860; Barba 1895–1896; Bolio, 1914). Those manuals report on seven varieties of henequén and the experimental cultivation of wild plants. At present, only three of these varieties can be found: Sac Ki (SK), Yaax Ki (YK), and Kitam Ki (KK) (abbreviations also shown in Table 1). Variety SK is preferred for making cordage, because it has large, coarse, and abundant fibers. YK is morphologically very similar to SK, but it produces slightly shorter fibers and in slightly lesser amounts. KK is practically unknown to producers, and despite the fact that it is always found cultivated, it is often confused with a wild plant. It has a considerably different morphology from the other two varieties. It produces significantly less fiber, which is soft and short. Both YK and KK are found only in very small populations, particularly KK, which is only cultivated for craft-manufacturing.
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Morphological evidence (Colunga-GarcíaMarín, Estrada-Loera, and May-Pat, 1996; Colunga-GarcíaMarín and May-Pat, 1997) suggested that SK and YK both differ from wild plants in similar direction and magnitude, a trend that may be described as four domestication syndromes (domestication syndrome refers to a combination of characteristics with anthropocentric interest or related to the process of artificial selection). These syndromes are: gigantism, enhanced fiber content, less thorniness, and diminished reproductive capacity. There is a conspicuous correspondence among these syndromes and the anthropocentric interests that guided the process of artificial selection of henequén, at least during the former century. KK is the cultivated variety that most resembles wild plants. Ethnobotanical and morphological evidence suggests that KK was recently introduced to the Yucatan Peninsula and/or that it went through a selection process having different direction and intensity with respect to that in SK and YK (Colunga-GarcíaMarín and May-Pat, 1997).
Variety SK clearly matches Gentry's (1982) diagnosis of A. fourcroydes. Given that valid and complete taxonomic descriptions for varieties YK and KK are lacking, in this paper we refer to them by their Mayan horticultural designations.
With respect to the A. angustifolia populations, ethnobotanical exploration (Colunga-GarcíaMarín and May-Pat, 1993) suggested the existence of three variants of A. angustifolia, which correspond to the different habitats in which the populations grow: coastal dunes (D); Tropical deciduous forests (DF); and Tropical subdeciduous forests (SF). According to the morphological evidence (Colunga-GarcíaMarín and May-Pat, 1997), these three variants belong to two ecotypes: one including D and DF populations and the other corresponding to SF populations. This conclusion is derived from the fact that under uniform growing conditions no morphological differences were found between D and DF populations. Artisans who use wild plant fibers distinguish three variants within SF populations according to fiber quality. Of these, Chelem White (CHW) is considered more similar to cultivated variants, while both Chelem Green (CHG) and Chelem Yellow (CHY) are seen as of lower quality (in that order). This artisan classification agrees with morphological evidence (Colunga-GarcíaMarín and May-Pat, 1997). Based on ethnobotanical and morphological evidence, three hypotheses were postulated with respect to these wild variants used by artisans (Colunga-GarcíaMarín and May-Pat, 1993, 1997): (1) SF populations gave rise to cultivated plants; (2) CHW represents ancestral henequén populations; or (3) CHW is a clone introduced to cultivation and later abandoned during the turn of the century, a period during which attempts were made to cultivate wild populations for the textile industry.
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MATERIALS AND METHODS |
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Isozyme analysis
Tissue was collected from the leaf most recently detached from the bud in the center of the rosette and consistently sampled between 0800 and 0900. The terminal spine from this leaf was excised and 100 mg of tissue were taken from the adjacent area. Tissue samples were macerated in 100 mL of extraction buffer. The macerate was absorbed onto filter paper wicks, which were stored for 24 h at -10°C. After this, wicks were placed in starch gels (12%, containing 3% sacarose).
Starch gels were run horizontally following the Wendel and Weeden (1989) protocols. In a preliminary study, 24 enzymes systems in 11 gel systems were assayed using four different extraction buffers (results available from first author). Only three of these 24 enzyme systems gave satisfactory activity, good band resolution, and consistent results and were thus selected for this study. These isozymes were acid phosphatase (ACP) (E.C. [Enzyme Commission number] 3.1.3.2), cathodic peroxidase (PRX) (E.C. 1.11.1.7), and malate dehidrogenase (MDH) (E.C. 1.1.1.37).
Gel systems found optimal for MDH were system D (pH 6.5) designed by Stuber et al. (1988) for maize and for ACP and PRX system A (pH 5.0), also designed by Stuber et al. (1988) for maize. The best extraction buffer was that of Soltis et al. (1983). Staining protocols were for MDH that of Stuber et al. (1988) and for ACP and PRX those of Wendel and Weeden (1989), modified as follows: (1) ACP, 4 mL of 1 mol/L sodium acetate, pH 5.0, 50 mL distilled water, 100 mg magnesium chloride, 20 mg 
-naphtyl acid phosphate, and 75 mg of Fast Garnet (GBC) Salt, (2) PRX, 3.5 mL N-N dimethyl formamide, 35 mL distilled water, 2.5 mL 1 mol/L sodium acetate pH 5.0, 100 mg calcium chloride, 0.5 mL 3% hydrogen peroxide, and 50 mg 3-amino-9-ethyl carbazole.
Due to polyploidy, activity bands could not be assigned to specific loci, given that they probably represented product combinations from different loci on homologous chromosomes. As a consequence, banding patterns were recorded as the presence-absence of individual bands: 14 bands for ACP, four bands for PRX, and nine for MDH (Fig. 2
). For each isozyme system, bands were numbered consecutively, beginning with the band running closer to the origin (bottom in Fig. 2). A number was also assigned to each pattern of presence-absence of bands, or isozyme electrophenotype, and their frequency was recorded for (1) each variant analyzed, (2) pooled wild variants, and (3) pooled cultivated variants (Table 2).
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= 0.05). Besides these 55 originally quantitative characters, another 11 qualitative characters were added. Of the 66 morphological characters, order was declared for 44 of these, based on the analysis of henequén domestication syndromes of Colunga-GarcíaMarín, Estrada-Loera, and May-Pat (1996) and Colunga-GarcíaMarín and May-Pat (1997), assuming that the ancestral character state was equal to 0. Characters used, their states and order are also available in Colunga-GarcíaMarín (1996). Wild variant from Dunes (D) was considered the outgroup, because its populations are perhaps the oldest in the Yucatan Peninsula. Character changes were mapped using Mac Clade 3.0 (Maddison and Maddison, 1992) to find apomorphies. Results were evaluated with a bootstrap analysis based on 1000 replicates with two random input orders and a decay analysis using the total number of possible trees, considering trees up to 10 steps longer than the most parsimonious tree. |
RESULTS |
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The three cultivars of henequén were distinct from each other, but no differences in enzyme activity band presence were found among individuals. This means that when genetic variation is estimated by means of the three isozyme systems analyzed all genetic variation is found among populations. The differences between SK and YK were found only on one of the PRX bands, while KK differs from them in both MDH and ACP electrophenotypes.
Within A. angustifolia variants, the highest variation level was recorded in D (21 different electrophenotypes) and the lowest in CHW (five different electrophenotypes), which is the variant considered by producers to be the most similar to SK. In wild A. angustifolia populations most of the genetic variation is found within variants (Table 2). Neverthless, a significant amount of variation is present among variants, as all of them (except CHW) have electrophenotypes not present in the others.
The most abundant ACP electrophenotype in A. angustifolia populations was the same as that of the SK and YK. KK had a unique electrophenotype, as did 11 A. angustifolia plants whose ACP electrophenotype was not repeated in other plants under study. Most of the A. angustifolia plants growing outside the Yucatan Peninsula also showed unique electrophenotypes.
With respect to PRX, the most abundant electrophenotype within A. angustifolia populations was also present in SK and KK. YK had an electrophenotype with intermediate abundance in A. angustifolia. Only two wild plants from the Yucatan Peninsula had unique electrophenotypes (one of D and one of CHY). All A. angustifolia plants growing outside of the Yucatan Peninsula had similar electrophenotypes to those growing in this region.
For MDH, all plants growing in the Peninsula had the same electrophenotype, with the exception of KK plants, which have an electrophenotype also found in two plants from Veracruz and two from Oaxaca.
Phylogenetic relations
The parsimony analysis evaluated 13 135 trees in the exhaustive search. One single most parsimonious tree (total length 112 steps) was found (Fig. 2). The mean ± SD length of the total set of possible trees was 138.4 ± 5.4 steps, g1= -1.58, which indicates that our total data set is more structured than a random data set (Hillis and Huelsenbeck, 1992). Two large clades may be observed in this tree: (1) one including a monophyletic group formed by the cordage variants SK and YK, which has variant SF as its sister group (the wild variant morphologically most similar to the cultigens), and (2) a clade formed by the remaining wild variants, as well as variant KK (the cultigen most similar to the wild populations). In a bootstrap with 1000 iterations, the only strongly supported clade was that formed by the cordage variants SK and YK. Results of these iterations are consistent with the morphometric analysis previously performed (Colunga-GarcíaMarín and May-Pat, 1997). The same results were obtained with the decay analysis. In all the trees up to ten steps longer than the most parsimonious tree, the clade formed by SK and YK was found. All other clades collapsed with just one step. Apomorphies found for this clade are the same characters that comprise the domestication syndromes found with the morphological variation analysis (Colunga-GarcíaMarín and May-Pat, 1996).
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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The extremely small fraction of total isozymatic variation observed within cultivated varieties is our main finding. Genetic variation losses are common in cultivars (Doebley, 1989), but in none of the reported cases has such reduction been so severe relative to the wild ancestors, both within and between cultivars. Loss of genetic variation in henequén is possibly, to a certain extent, a consequence of the domestication process begun in prehispanic times. As Doebley (1992) noted, although there is little hard evidence concerning the beginnings of plant domestication, it seems a reasonable assumption that the first farmers experimented with only a small fraction of the variation present within the progenitor species of today's crops. Further, as the domestication process proceeded these farmers probably selected only the best phenotypes. For these reasons one expects a significant loss of genetic variation, but competing forces such as introgression from wild relatives and selection against loss of fitness due to inbreeding have probably counterbalanced the expected loss of genetic variation. These competing forces are usually favored by traditional agricultural practices (Colunga-GarcíaMarín, Hernández-Xolocotzi, and Castillo-Morales, 1986; Zizumbo-Villarreal, Hernández-Xolocotzi, and Cuanalo-de la C., 1988) as has been demonstrated for species such as the common bean (Escalante et al., 1994), maize, squash, and tomato (see reviews in Doebley, 1992), but they have not been promoted in henequén cultivation today. In other vegetatively propagated cultigens, such as Opuntia, the occasional seed cultivation has been observed in traditional farming (Colunga-GarcíaMarín, Hernández-Xolocotzi, and Castillo-Morales, 1986), a practice that may result in an increase of genetic variation. Under cultivation conditions of the present century the only remaining source of genetic variation for henequén would be somatic mutations.
The lack of genetic variation of other clonal cultivars has already been reported. Results of DNA oligonucleotide fingerprinting of Malus, Prunus, and Rubus failed to detect genetic variation within clones (Nybom, Rogstad, and Schaal, 1990). Similar results with this technique were obtained in banana, in which the only differences were detected between an induced mutant and its progenitor (Kaemmer et al., 1992). However, even if variation was not observed within clones of these cultigens, the clones belong to species having numerous varieties. This is not the case in henequén, for which the number of cultivated varieties was reduced in the present century from seven to three, two of which are nearly extinct. During the henequén cultivation boom of 1915 in the state of Yucatan, henequén was cultivated over an area of up to 300 000 ha (López and García de F., 1984), and it is possible all estimated 900 million plants had a single genotype, Sac Ki.
Isozymatic variation of wild A. angustifolia populations
Wild populations of A. angustifolia have high levels of isozyme diversity both within and among variants, a situation that contrasts strongly with that of henequén. This is normally expected in long life-cycle outcrossing perennials having wide geographic ranges (Hamrick, Godt, and Sherman-Broyles, 1992). These high levels of genetic variation emphasize the need to preserve wild germplasm in extant populations, based on which an eventual increase in diversity of cultigens may occur through a careful breeding program. The implementation of such program would require basic research on the reproductive biology of these species.
The Coastal dunes area, where we found the largest electrophenotype diversity, is threatened by intense touristic development today, and there is strong human pressure to introduce permanent agriculture to the few remaining zones of Tropical deciduous and subdeciduous forest. A conservation area within the Yucatan Peninsula is urgently needed to preserve the in situ germplasm of wild A. angustifolia. In addition, a seed bank should be established to conserve genetic variation.
Phylogenetic relationships
Phylogenetic relationships within wild and cultivated variants growing in the Yucatan Peninsula inferred in this study are consistent with the ethnobotanical and morphological data previously reported (Colunga-GarcíaMarín and May-Pat, 1993; Colunga-GarcíaMarín, Estrada-Loera, and May-Pat, 1996; Colunga-GarcíaMarín and May-Pat, 1997). Two different domestication trends in present-day henequén variants seem to have occurred: one of SK and YK, selected for thicker and more abundant fibers and better suited for the cordage industry (whose sister group is SF); and another of KK, which is nearly extinct in Yucatan, selected for its softer fibers and better suited for textile applications. Within this last domestication trend, CHW was probably included at the beginning of this century, with its cultivation being abandoned later on. KK is a cultivated variant that morphologically groups together with wild ones, perhaps due to the artificial selection of wild characteristics (e.g., soft fibers). Even if we only obtained a most parsimonious tree, the low bootstrap support and the decay analysis values suggest that there is a lot of homoplasy in the data.
The results obtained in this study agree with the predictions of Doebley (1989) in cases of wild-cultivated derivatives when isozymes are analyzed: (1) the cultigen falls within the variation range of the putative wild progenitor (in this study SK and YK electrophenotypes are the same as those of some plants of the Yucatan wild populations of A. angustifolia), (2) the cultigen has a subset of the allelic diversity found in the wild progenitor (in our case, only one electrophenotype per cultivar per isozyme), and (3) in addition to the cultigen having less genetic variation than the wild population, this genetic variation is distributed in a different way (henequén has more variation among variants than within variants, while this is the opposite in the case of A. angustifolia).
The fact that KK has a different MDH electrophenotype with respect to all other Yucatan plants studied, and similar to some A. angustifolia plants from Oaxaca and Veracruz, seems to support the hypothesis of its recent introduction (Colunga-GarcíaMarín and May-Pat, 1997).
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FOOTNOTES |
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3 Author for correspondence (e-mail:pcolunga{at}cicy.cicy.mx
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