I have prepared pages for 6 species:
However, the last time I checked, the inventory at The Cycad Society website was up to 15 species, and I refer you there for more current information.
Due to the leaflike character of the sporophylls, it has been suggested that Dioon is among the most primitive of cycad genera, warranting placement in a distinct family (Dehgan and Dehgan 1988). Chloroplast DNA analysis suggests Dioon is the most primitive of American cycad genera (De Luca et al. 1995).
The genus is commonly divided into two groups of distinct morphology. The first includes D. spinulosum, D. meijiae and D. rzedowskii, "characterized generally by large fronds, well-developed trunks, and massive cones." The second group contains the remaining species here recognized (D. edule, merolae, holmgrenii, purpusii, califanoi, caputoi and tomasellii), which are "less robust, with generally shorter trunks, considerably shorter fronds, and smaller cones" (Norstog and Nichols 1997).
These two groups have also been identified by "[a] phylogenetic analysis of all the intrageneric taxa of the genus Dioon ... using chloroplast DNA restriction fragment length polymorphism analysis. Wagner parsimony analysis on a 187 character matrix yielded two equally parsimonious trees, differing only for the position of D. caputoi. The consensus tree has two well-defined major clades. The first is composed of D. mejae, D. rzedowskii, and D. spinulosum; the second is composed of D. califanoi, D. caputoi, D. edule var. angustifolium, D. edule var. edule, D. holmgrenii, D. merolae, D. purpusii, D. tomaselli var. sonorense, and D. tomaselli var. tomaselli. A phenetic analysis of the same data showed data showed results broadly congruent with the cladistic analysis. This resulting phylogeny is partially congruent with morphological data and is also compartible with the biogeography of the genus. Modern species of Dioon may have evolved as a consequence of a very fast succession of vicariance events that mainly occurred during the early Cenozoic. The short time between each of these events may not have allowed the accumulation of a large number of morphological synapomorphies for the groups of species" (Moretti et al. 1993).
"The stems of Dioon are aerial and typically unbranched... [T]ypically, individuals range 3-6 meters tall, and other species are lower... The leaves are usually numerous and sweeping in [the large species], but stiff and erect in others, and the leaflets are nonarticulated... The cones, comparatively large and terminal, are solitary, although females may display cones for more than one year, since their development is prolonged... Sporophylls of both sexes end in upturned, flattened shields that are closely overlapping (i.e., imbricated)." 2n=18 (Norstog and Nichols 1997).
Mexico and Central America, south to Nicaragua. Habitats include evergreen and semi-evergreen tropical moist forest, tropical deciduous forest, pine-oak forest; and dry, rocky hillsides, canyons and coastal dunes. Much of the genus' former range has been lost to agricultural and residential development. "Today, species of Dioon are found mainly in undisturbed habitats, and are restricted to ravines and steep, rocky hillside areas that are considered unsuitable or inaccessible for agriculture or lumbering" (Norstog and Nichols 1997).
Dioon spinulosum grows to a height of 16 m and diameter of 40 cm. It is among the tallest of cycads, but several other species of Dioon achieve the same diameter (Jones 1993). It also has one of the largest female cones known among cycads, attaining 80 cm long and 30 cm diameter (Norstog and Nichols 1997).
Dioon dates to the Jurassic or perhaps earlier. Dioon species have been described from the Eocene of Kupreanof Island, Alaska (Jones 1993).
Dehgan, B. and N.B. Dehgan. 1988. Comparative pollen morphology and taxonomic affinities in Cycadales. American Journal of Botany 75: 1501-1516.
De Luca et al. 1995. Molecular systematics of cycads. P. 131-137 in P. Vorster, ed., Proceedings of the Third International Conference on Cycad Biology. Stellenbosch: Cycad Society of South Africa.
Moretti, A., P. Caputo, S. Cozzolino, P. De Luca, L. Gaudio, G. Gigliano Siniscalco, and D.W. Stevenson. 1993. A phylogenetic analysis of Dioon (Zamiaceae). American Journal of Botany 80: 204-214.
Last Modified 2012-11-23