Diversity and genetic structure of three species of Dioon Lindl. (Zamiaceae, Cycadales) from the Pacific seaboard of Mexico

We have estimated levels of genetic diversity and partitioning in the Mexican endemic cycad species Dioon sonorense, Dioon tomasellii, and Dioon holmgrenii, whose populations are exclusively distributed along the Pacific seaboard. For the three species, the patterns of variation at 19 allozyme loci in a total of 11 populations were evaluated. The average number of alleles per locus was in the range 2.05-1.68, corresponding to the northernmost population of D. sonorense (Mazatán), and the southernmost population of Dioon holmgrenii (Loxicha), respectively. In turn, the percentage of polymorphic loci peaked (94.73) in the El Higueral and Altamirano populations of Dioon tomasellii, and was estimated to be lowest (57.89) in the Loxicha population of D. holmgrenii. The mean expected heterozygosis varied markedly between taxa, with relatively high indices for D. sonorense and D. tomasellii (HE = 0.314 and 0.295, respectively) and substantially lower values for D. holmgrenii (HE = 0.170). Comparison of the inferred genetic structure based on F-statistics for the three species also indicated differences along the north-south Pacific seaboard axis. For D. sonorense and D. tomasellii, local inbreeding (FIS) was zero but global inbreeding (FIT) values were positive and significantly different from zero (0.130 and 0.116, respectively). By contrast, values of both FIT and FIS were negative and significantly different from zero (-0.116 and -0.201, respectively) for D. holmgrenii. The genetic differentiation between populations (FST) had positive values in all taxa and corresponded with their geographic location along the north-south axis: according to this statistic, D. sonorense was the most differentiated species (FST = 0.151), D. tomasellii had intermediate values (FST = 0.145), and D. holmgrenii was the less differentiated taxon (FST = 0.069). Finally, a phenogram representing Nei's genetic distances among populations displayed three major groups, each one corresponding to each of the studied species. Within D. tomasellii (of intermediate geographic distribution), a further division into two clusters corresponded precisely to the pair of populations that are geographically divided by the Trans Mexican Neovolcanic Mountains.