Diversity and genetic structure of the endangered cycad Dioon sonorense (Zamiaceae) from Sonora, Mexico: Evolutionary and conservation implications

The diversity and genetic structure of the cycad Dioon sonorense (De Luca, Sabato & Vázq. Torres) Chemnick, T.J. Greg. & S. Salas-Mor. were evaluated from the four known populations in tropical dry forests of Sonora, Mexico. The estimates of genetic diversity in the 19 loci analysed were: mean number of alleles per locus (A = 2.0), percentage of polymorphic loci (P = 81.6) and expected heterozygosity (H_E = 0.314). The subpopulations were found to be in Hardy–Weinberg equilibrium at the local level (F_is), even though we found a global excess of homozygotes (F_it = 0.13). Genetic variation attributed to differences among populations was 15%. Our results indicate that the geographical isolation caused by the historical effects of the Pleistocene among populations has probably generated clinal variation of the allelic frequencies at two loci, in relation to their latitudinal distribution. We compare our results with the long-lived southern hemisphere trees Fitzroya and Nothofagus in relationship to the Pleistocene glaciations. We also infer that the populations have become fragmented recently due to increasing pressure of habitat conversion, disturbance and illegal extraction of plants and seeds for the international horticultural trade. We therefore recommend three aspects: (i) propagation of this species for reintroduction, (ii) declare the localities of its northern distribution as a protected area, and (iii) an amendment of the current International Union for Conservation of Nature Red List categorisation for this species to ‘Critically Endangered’.