A phylogenetic analysis of all the intrageneric taxa of the genus Dioon Lindley (Zamiaceae) was undertaken by 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. mejiae, 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. tomasellii var. sonorense, and D. tomasellii var. tomasellii. A phenetic analysis of the same data showed results broadly congruent with the cladistic analysis. This resulting phylogeny is partially congruent with morphological data and is also compatible with the biogeography of the genus. Modem 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.
Chromosome numbers and karyotypes of species from four American Zamiaceae (Cycadales) are reported. Zamia shows interspecific and intraspecific chromosome variation, whereas Microcycas, Ceratozamia, and Dioon have constant karyotypes within each genus. In Zamia, all karyotypes have the same number of submetacentric and acrocentric chromosomes, but they differ in the number of metacentric and telocentric chromosomes. Centric fission ofmetacentric chromosomes is proposed to explain the karyotypic variation in this genus. Zamia shows karyological relationships with Microeyeas and Ceratozamia, whereas Dioon appears very distinct from the other American cycad genera. Affinity among Zamia, Ceratozamia, and Mieroeyeas karyotypes and distinctiveness of Dioon karyotypes are supported by comparative analysis of phenotypic characters in the four genera.
The preliminary results of a phylogenetic analysis of genus Zamia, based on the sequences of the internal transcribed spacer 2 of the nuclear ribosomal DNA combined with a morphological data set, are reported. The consensus tree among the six equally parsimonious cladograms obtained shows Zamia divided in various, often poorly resolved, clades. The most inclusive dichotomy defines two groups: one composed of mainly Central American species, i.e. Z. acuminata, Z. neurophyllidia, Z. obliqua, Z. pseudoparasitica and Z. skinneri; and the other including the rest of the investigated species. The latter clade basally shows Z. inermis and, more internally, a small unit composed of one Mexican and two West Indian taxa (Z. fischeri, Z. portoricensis and Z. pumila). This group is in a sister group relationship with a large, poorly resolved clade which has Z. standleyi at the base. The internal clade includes Z. soconuscensis and two groups, one made of North American species and the other of mainly South American species. The first group includes Z. furfuracea, Z. loddigesii, Z. paucijuga, Z. polymorpha and Z. verschaffeltii. The other group includes mainly South American taxa (i.e. Chiqua restrepoi, Zamia muricata, Z. boliviana, Z. leicontei, Z. manicata, Z. roezlii and Z. wallisii). The results indicate that morphological resemblance in the genus does not correspond to the pattern of phylogenetic relationships, whereas the latter pattern is broadly congruent with geographical distribution.
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