AOBPreview originally published online on March 10, 2005
Annals of Botany 2005 95(6):999-1007; doi:10.1093/aob/mci106
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Allozyme Variation in the Three Extant Populations of the Narrowly Endemic Cycad Dioon angustifolium Miq. (Zamiaceae) from North-eastern Mexico
1 Laboratorio de Genética de Poblaciones and 2 Laboratorio de Biología Evolutiva de Cycadales, Departamento de Biología Evolutiva, Instituto de Ecología, A. C. Km. 2.5 Antigua Carretera a Coatepec No. 351, Xalapa 91070, Veracruz, Mexico
* For correspondence. E-mail astorga{at}ecologia.edu.mx
Received: 2 July 2004 Returned for revision: 29 October 2004 Accepted: 17 January 2005 Published electronically: 10 March 2005
• Background and Aims Dioon angustifolium was considered within D. edule. Recent morphometric and allozyme studies on D. edule have shown that D. angustifolium has originated from geographic isolation and is therefore considered to be a separate species. This cycad is endemic to north-eastern Mexico and is known only from three populations in the Sierra Madre Oriental mountain chain. Its populations are small when compared with its southern relative D. edule. In this study, genetic variation was determined within and between populations of D. angustifolium and the genetic consequences of habitat fragmentation and isolation of populations of this species were assessed.
• Methods Allozyme electrophoresis of 14 presumptive loci was used. The data were analysed with statistical approximations for estimating genetic diversity, structure, gene flow and recent genetic bottlenecks.
• Key Results Means and standard deviations of genetic diversity estimators were: number of alleles per locus (A = 1·67 ± 0·23), percentage of polymorphic loci (P = 52·4 ± 23 %) and expected heterozygosity (HE = 0·218 ± 0·093). The genetic variation attributable to differences among populations was 16·7 %. Mean gene flow between paired populations was Nm = 1·55 ± 0·67, which is similar to that reported for endemic plant species of narrow geographical distribution and species with gravity-dispersed seed. A recent bottleneck is detected in the populations studied.
• Conclusions 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.
Key words: Conservation, cycads, endemic narrow distribution, genetic bottleneck, genetic structure, Mexico, Pleistocene refugia, population genetics