Dioon edule Lindl. (Zamiaceae) is a cycad endemic to Mexico, that occurs as one species D. edule and the geographical variety D. edule var. angustifolium (Miq.) Miq. Dioon edule has a north to south distribution in eastern Mexico. In this study, we analysed 14 allozymic loci in eight populations of D. edule from its total distribution range by sampling all known populations. Patterns of diversity and genetic variability, within and among populations, were obtained. The mean number of alleles per locus ( A ) was 1.44 and the percentage of polymorphic loci was relatively high ( P  = 54.78). The mean observed ( H O ) and expected heterozygosity ( H E ) were 0.27 and 0.24, respectively. F ‐statistics revealed an excess of heterozygous genotypes, locally and globally ( F  = −0.17 and f  = −0.27, respectively). The genetic variation explained by differences among populations was only 7.5%. We also detected a negative relationship between genetic diversity and latitude. On average, the gene flow between population pairs was relatively high ( Nm  = 2.98); furthermore, gene flow between population pairs was significantly correlated with geographical distances ( r  = −0.38, P  = 0.025). Therefore, patterns of genetic diversity in D. edule appear to be associated with the post‐Pleistocene spread of the species, from its southerly (origin) to its northerly range (derived populations, including its central distribution). The biogeographical and evolutionary aspects of the results of this study are discussed. We recognize Dioon angustifolium Miq. for the northernmost disjunct populations. © 2003 The Linnean Society of London, Biological Journal of the Linnean Society , 2003, 80 , 457–467

Dioon angustifolium presents high levels of genetic diversity compared with other cycad species, in spite of small population sizes. The recent bottleneck effect did not effectively reduce the genetic variation to the extent of eliminating these populations. The distribution of D. angustifolium appears to be the result of historical biogeographical effects related to the Pleistocene glaciations. It is recommended that this species be catalogued in the IUCN Red List of Threatened Species and conservation efforts be made to preserve it.

Propagation of the cycad Dioon edule aimed at sustainable management in the state of Veracruz, Mexico has been ongoing since 1990 under the supervision of staff at Francisco Javier Botanic Garden, with the principal objectives of addressing illegal trafficking and habitat destruction. Plant sales have been limited. Nevertheless these, along with the sale of other managed forest products, have given the cycad producers and other villagers enough incentive to conserve 80 hectares of cycad habitat and to discourage illegal collecting. This model was taken up by four similar nurseries in the buffer zones of two biosphere reserves in Chiapas for the propagation of four additional cycad species and two endangered Chamaedorea palms. A further biosphere reserve in Puebla hosts a similar nursery for the critically endangered D. caputoi. Here the producers are paid through the Reserve authority for cultivation and reintroduction of the cycad. All species were studied at the population level prior to and during nursery establishment. Cultivation knowledge has been passed on to the farmers as well as limited help in marketing. Seedling reintroduction experiments have been carried out but further demographic studies of D. edule and C. mirandae have given reason to re-think reintroduction strategies. There is great potential for these nurseries to act as shelter for rescued plants during civil engineering projects. The marketing problem is still an issue and has been approached by the involvement of conservation authorities in Chiapas to assist the producers with permit paperwork and to seek markets. This experience is an important example of botanic garden extension to rural communities in Mexico that covers several articles of the Convention on Biological Diversity.

The cycad Dioon edule Lindl. inhabits a seasonally‐dry tropical forest along with associated CAM plants such as bromeliads and cacti. To test the hypothesis that D. edule might also be a CAM plant, diel total‐acid fluctuation was measured through the dry to wet seasons of 4 consecutive years on adult D. edule plants in their natural forest habitat in Veracruz, Mexico. Correlations between acid fluctuation index and climatic data, and also soil water potential were determined over this period. Laboratory trials were followed up to estimate diel patterns of CO2 exchange and estimation of δ13C value. A comparison of stomatal density cm−2 with other C3, CAM and CAM‐facultative plants was made. The diel total titratable‐acid fluctuation values, although variable, were found to be consistent and significant for the dry season. Carbon dioxide exchange was found to be typical of C3 plants when hydrated but when the plant was stressed by withholding water, although the leaf remained healthy, there was no significant dark‐period CO2 output. Stomatal density was comparable to other CAM and CAM‐facultative plants. It was concluded that D. edule is a C3 plant that shows CAM‐cycling metabolism when water stressed. © 2002 The Linnean Society of London, Botanical Journal of the Linnean Society, 138, 155–161.

The relationship in geographical distribution and morphological variation of leaflet width and length (diagnostic trait), between and within populations of Dioon edule Lindl., has been investigated throughout its known range in eastern Mexico (from the states of Nuevo León to Veracruz, north to south, respectively). A total of 1832 leaflets were measured for width and length from 154 plants distributed amongst five populations using four leaflet replicas from each of three leaves per plant. For leaflet width and length the variation among populations indicated significant stat‐istical differences (F4,147 = 125.83; P < 0.0001; R2 = 92.17% and F4,147 = 9.04; P < 0.001; R2 = 26.8%), respectively. With respect to leaflet width, the multiple range test showed three groups with a north to south distributional relationship along the range of the species. The correlation coefficient among paired populations, respect to geographical distance and the absolute value of the mean difference of leaflet width in each population, was positive, and different from zero (r = 0.82; P = 0.013). A great variation of important ecological and evolutionary parameters was shown. © 2003 The Linnean Society of London, Botanical Journal of the Linnean Society, 2003, 141, 465–470.

The genetic diversity and structure of four populations of the cycad Zamia loddigesii were studied throughout its range in Mexico. Allozyme electrophoresis of 15 loci was conducted. The mean number of alleles per locus was 1.80 ± 0.09, the percentage of polymorphic loci was 66.6 ± 5.4, and the expected heterozygosity was 0.266 ± 0.02. The results indicated that the genetic diversity was relatively higher, with respect to tropical tree species and other cycads. The genetic variation explained by differences among populations was 18%. On average, gene flow between paired populations was similar ( Nm = 1.6) to other tropical forest trees and cycad species. Our results indicated that the geographical isolation among populations of Z. loddigesii generated allele loss, as well as a clinal variation in the frequencies of two loci ( MDH and MNR2 ), in relation to the latitudinal distribution of populations. The populations have become fragmented due to increasingly higher pressure of habitat conversion and disturbance. The importance of the establishment of sanctuaries and protected areas and a reduction in deforestation is highlighted in this research as a way of preserving the high genetic diversity of this and other endemic species. AA. 1994. Population genetic structure of Cecropia obtusifolia, a tropical pioneer tree species. Evolution 48: 437-453. Alvarez-Buylla ER, García-Barrios R, Lara-Moreno C, Martínez-Ramos M. 1996. Demographic and genetic models in conservation biology. Applications and perspectives for tropical rain forest tree species.

The paper reviews an attempt to establish wild populations of the cycad Dioon edule Lindl. as a locally valued economic resource. Areas of wild habitat have been protected to act as a source of propagation material. The cycads are being grown as a village based industry to provide revenue to supplement traditional agriculture.

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