Population genetics of Dioon edule Lindl. (Zamiaceae, Cycadales): biogeographical and evolutionary implications

Dioon eduleLindl. (Zamiaceae) is a cycad endemic to Mexico, that occurs as one speciesD. eduleand the geographical varietyD. edulevar.angustifolium(Miq.) Miq.Dioon edulehas a north to south distribution in eastern Mexico. In this study, we analysed 14 allozymic loci in eight populations ofD. edulefrom 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 (HO) and expected heterozygosity (HE) were 0.27 and 0.24, respectively.F‐statistics revealed an excess of heterozygous genotypes, locally and globally (F = −0.17 andf = −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 inD. eduleappear 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 recognizeDioon angustifoliumMiq. for the northernmost disjunct populations. © 2003 The Linnean Society of London,Biological Journal of the Linnean Society, 2003,80 , 457–467